Monterey City Council
February 7, 2023

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1063, Packet Page 1163 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 55 Figure 4.19. Estimated bedload rating curve for San Jose Creek Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1064, Packet Page 1164 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 56 Figure 4.20. Estimated suspended‐sediment rating curve for San Jose Creek Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1065, Packet Page 1165 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 57

5. FISH BARRIER ANALYSIS

5.1 Introduction
Task 3.0 in the MPRPD Scope of Services indicated Balance/PWA team should evaluate the three major fish passage barriers identified by CDFW (Nelson 2006a, b) and other barriers encountered during field transects. In order to adequately evaluate these barriers, we completed longitudinal creek profiles and cross sections near the fish barriers and calculated impounded sediment volumes. In addition to evaluating the barriers, the Request for Proposals (RFP) requested the consultant present a range of solutions and/or benefits related to restoration options and comment on the potential for steelhead habitat improvement.

5.2 CDFW Barrier and Other Potential Barrier Analyses
In November 2013, after reviewing available literature identifying potential fish barriers, Denis Ruttenberg (Balance) and Danny Hagans (PWA) conducted additional stream surveys to locate and perform a more in depth analysis of site and channel conditions at 1) the 3 major fish barriers in SJC (FB #1 - #3, Map 5) identified by CDFW (Nelson 2006a)19; 2) an abandoned small concrete dam (Site #1, Map 5) located on SJC just upstream of the mouth of Animas Creek, and 3) at the largest log jam identified during the 2013 in-stream sediment source inventory (Site #47, Map 5). Table 5.1 and Map 5 provide the location of each of the 5 surveyed in-stream features along San Jose Creek. The concrete dam located upstream of Animas Creek was identified by CDFW (Nelson, 2006a), as well as in the PWA in-stream surveys (Site #1, Map 5), and neither considered the dam to be a barrier to adults, but juvenile salmonid migration is likely impaired during summer low flow periods.

Table 5.1.  Surveyed fish barriers (FB), San Jose Creek Study, Monterey County, California.

Type of barrier Stream
Map identification
and location
(stream mi)
Figures
Concrete dam San Jose Creek Site #1, 1.90 Figures 5.1 and 5.2 Bedrock falls/cascade San Jose Creek FB #1, 2.85 Figures 5.3 through 5.6 Log jam San Jose Creek Site #47, 5.05 Figures 5.7 through 5.15; Figures 5.17 and 5.18
Log jam San Jose Creek FB #2, 7.5 Figures 5.19 through 5.23 Log jam San Jose Creek FB #3, 8.0 Figures 5.24 through 5.26 Concrete dama Seneca Creek Site #37, 1.74 Figures 5.28
Log jama Seneca Creek Site #41, 1.9 Figures 5.29 a Longitudinal profiles and/or cross sections were not completed at the two identified barriers along Seneca Creek.

However, the sites were evaluated during the instream assessment.

19 The three surveyed fish barriers (FB) are identified as FB #1, FB#2, and FB #3 and are located on Map 5 Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1066, Packet Page 1166 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 58 Based on the CDFW instream assessment along Seneca Creek (Nelson, 2006b), numerous features (log jams, culverted road crossings, and a flashboard dam) were identified as being potential temporary impediments to fish passage under certain flow regimes. However, the most significant potential fish passage barrier (FB) during most flow regimes for both life phases of steelhead was a LWD log jam located within habitat unit #414 at stream mile 1.9 (Site #41, Map 5).

The results of other smaller log jams encountered during the PWA in-stream sediment sources inventories are presented in Section 4.2.2. The in-stream survey occurred along 5.45 miles of 3rd, 4th, and 5th Strahler Order streams, or along 13% of 3rd order, 16% of 4th order, and 54% of all 5th order streams for a total of 8% of all stream orders in the SJC watershed (Table 4.4; Map 5).

None of these smaller log jams identified were considered to be more than minor barriers to juvenile migration at summer low flows (Nelson, 2006 a-b).

5.2.1 San Jose Creek Potential Barriers
Based on the CDFW instream assessment along SJC (Nelson 2006a), three sites were identified as potential impediments to fish passage. The downstream-most barrier is a permanent20 natural feature, a bedrock falls/cascade (Fish Barrier FB #1 and PWA Site #7, Map 5), located within CDFW habitat unit #305 at stream mile 2.85 that would impede fish passage for both juveniles and adults at low flows. The second barrier is a large natural temporary21 log jam located in habitat unit #957 at stream mile 7.5 (FB #2, Map 5), and the upstream-most barrier is also a large natural temporary log jam located in habitat unit #1050 at stream mile 8.0 (FB #3, Map 5). The middle log jam (FB #2) was identified by CDFW as the most significant fish barrier along the main stem of SJC (Nelson, 2006a).

Longitudinal profiles were surveyed at each of the 5 SJC features, and estimates of overall stability, persistence, volume of channel stored sediment and habitat improvement potential were evaluated. The 5 features will be discussed in an upstream direction starting with the concrete dam near the mouth of Animas Creek (Site #1, Map 5).

5.2.1.1 Concrete dam at station # 1.51 upstream of Animas Creek confluence A long-abandoned 3’ tall by 12’ wide concrete stream diversion dam spans 90% of SJC within a boulder-bedded, average 3% to 4% in gradient “run” reach of the creek (Figure 5.1). The dam age is unknown, but likely it was constructed at least four or more decades ago. Over the years since construction, SJC has eroded the alluvial left bank resulting in the creation of a steep gradient, boulder-bedded low flow channel around the dam (Figure 5.2). Winter high flows have resulted in the formation of a 0.5’ pool downstream of the dam face. The dam poses minimal potential to impede adult steelhead movement, however, juvenile upstream migration could be limited at a variety of flow regimes.

20 A Permanent fish barrier refers to a barrier that is not likely to “blow out” or “dislodge” during the larger flood events that the creek has/will experienced.
21 A Temporary fish barrier refers to a barrier that has the potential to “blow out” or “dislodge” as a result of any episodic natural event.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1067, Packet Page 1167 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 59 The flanked dam contains minimal amounts of channel stored sediment, estimated at <6 yd3 of primarily coarse sand sized particles. The dam itself is of home-made construction, appears to lack any rebar, and consists of <10 yd3 of concrete. The main, low use access road up SJC main stem is immediately adjacent the dam and channel, providing excellent access to the dam for heavy equipment. It is recommended the dam be removed to fully re-establish the natural flow regime and eliminate any potential fish passage concerns22. Refer to Section 7.4 Recommendations for Future Work for further discussion of recommended removal of this potential barrier. Impacts to SJC would be very minimal and short lived during and following removal of the relict manmade structure.

Figure 5.1. View from road across SJC to right bank at low concrete dam (Site #1, Map 5) and channel breach around dam on left bank, in foreground.

22 Mr. Hecht has been told of the pond behind the dam ‘blowing out’ during 1983 and 1995. We believe it also blew out during 1998. Considering this history, effects on aquatic habitat downstream of the dam are not likely to have a lasting adverse effect.

Boulder
Dam
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1068, Packet Page 1168 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 60 Figure 5.2. View downstream at concrete dam in center of photo with thalweg in channel to left of large natural boulder in SJC.

5.2.1.2 CDFW barrier #1: Bedrock falls at station # 2.85 The permanent bedrock falls and box canyon barrier #1 (FB#1 and Site #7, Map 5) identified by CDFW (Nelson, 2006a) and PWA during the instream survey confines SJC for a distance of 70’ and contains an elevation change of approximately 6’ over a distance of <20’ (Figure 5.3 and Figure 5.4). The 6’ elevation change consists of a upper 3’ tall, average 45o bedrock chute and a lower near-vertical 3’ bedrock falls that flows into a small 8’ x 10’ x 1’ deep bedrock pool (Figure

5.3). Downstream of the small pool is a large >3’ deep bedrock pool, which at moderately high winter flows will merge with the small pool to improve the potential for adult steelhead to negotiate the chute and falls (Figure 5.3). Near the head of the large pool, besides approaching the 6’ falls and chute, the box canyon shrinks to an averages 10-12’ wide channel. The constriction of SJC at the upstream end of the large pool will result in greatly increased velocities as fish approach the barrier (Figure 5.3 and Figure 5.5).

Upstream from the top of the bedrock chute, the channel of SLC is unconfined and twice as wide as through the box canyon. Stored sediment depths are minimal (<1’ -2’) and the active channel exhibits a broad sinuous pattern with low lying alternating point bars. At the downstream riffle crest, spawning habitat is moderately good and ocular estimates consist of small, well rounded gravels with some sand (Figure 5.4).

Dam
Thalweg
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1069, Packet Page 1169 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 61 Figure 5.3. Fish barrier #1, Longitudinal Profile and Cross Section at bedrock falls natural barrier on San Jose Creek.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1070, Packet Page 1170 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 62 Figure 5.4. View upstream at CDFW barrier #1 (Site #7, Map 5) taken from downstream riffle crest.

Figure 5.5. Close up of 6’ tall bedrock chute and falls at CDFW barrier #1 behind person (Site #7, Map 5). The bedrock box canyon narrows to +/‐ 11’ just above the large 3’ deep pool in the foreground.

Bedrock
falls
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1071, Packet Page 1171 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 63 Preliminary hydraulic modelling was performed to estimate the water surface profile and velocities at the bedrock falls barrier. Hydrology for the model was based on flood frequency analysis of the peak flows from the MPWMD (Table 3.1). Results of the coarse HEC-RAS hydraulic model are shown on Figure 5.6.

Figure 5.6. Estimate hydraulics from course HEC‐RAS model at bedrock falls barrier The coarse HEC-RAS hydraulic model and field evidence indicates the bedrock falls and chute is a total barrier for juvenile steelhead, by virtue of velocity, depth or hydraulic drop. More 100 120 140 160 180 200 220
90 92
94 96
98 100
102
104
106
108
BedrockFalls Plan: EG with LP3 flows 2/27/2014 Geom: EG Main Channel Distance (ft)
Elevation (ft)
Legend
WS Q100 (804 cfs)
WS Q50 (705 cfs)
WS Q25 (503 cfs)
WS Q10 (369 cfs)
WS Q5 (267 cfs)
WS Q2 (132 cfs)
WS Q1.25 (58cfs)
WS 50 cfs
WS 20 cfs
WS 5 cfs
WS 1 cfs
Ground
San Jose Creek Bedrock Falls
100 120 140 160 180 200 220
0 2
4 6
8 10
12 14
BedrockFalls Plan: EG with LP3 flows 2/27/2014 Geom: EG Main Channel Distance (ft)
Vel Chnl (ft/s)
Legend
Vel Chnl Q100 (804 cfs)
Vel Chnl Q50 (705 cfs)
Vel Chnl Q25 (503 cfs)
Vel Chnl Q10 (369 cfs)
Vel Chnl Q5 (267 cfs)
Vel Chnl Q2 (132 cfs)
Vel Chnl Q1.25 (58cfs)
Vel Chnl 50 cfs
Vel Chnl 20 cfs
Vel Chnl 5 cfs
Vel Chnl 1 cfs
San Jose Creek Bedrock Falls
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1072, Packet Page 1172 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 64 adept and athletic fish (juveniles and adults, depending on the flow) could possibly move through the barrier with burst speed. In general, the barrier seems to begin to back water somewhere around 50 to 60 cfs, estimated as the 1.25 year flow and above, but average channel velocity is high and possibly prohibitive for passage. In addition, there still appears to be a prominent drop in the water surface (greater than 0.5 feet maximum, a guideline for passage of juveniles in CDFG, 2009). More detailed ground survey and advanced hydraulic modeling, beyond the scope of this study, will more clearly define the hydraulic nature of this barrier and implications to passage of juvenile and adult steelhead Modifying natural bedrock falls/barriers to fish migration that have been present for thousands of years is not routinely embraced by state and federal regulatory agencies. However, if stakeholders desire to increase fish passage over a wider range of streamflows, the bedrock chute portion of the barrier could be most easily eliminated or modified in height by using drilling and explosive techniques. However, additional investigation would be necessary for design considerations and to determine the subsurface conditions and depth to bedrock within the channel upstream of the top of the bedrock barrier.

The barrier is very isolated within a steep, confined reach of stream, and with no vehicular access to the site other than by walking more than ½ mile up the channel of SJC. Constructing a roughened channel to reduce the height of the barrier would be cost-prohibitive, technically challenging, and result in eliminating one of the largest and deepest pools observed along the main stem of SJC. We recommend discussing the natural barrier with CDFW and NOAA Fisheries. Please refer to Section 7.4 Recommendations for Future Work for additional discussion.

5.2.1.3 PWA Site #47 log jam at station # 5.05
The largest temporary log jam observed by PWA during the channel surveys is located approximately 3,200’ upstream from the mouth of Seneca Creek (Site #47, Map 5). The double log jam appears to have formed after 2006 since the CDFW stream surveys (Nelson, 2006a) made no specific mention on the presence of the jam. The key lower log jam is composed of several 2’ to 3’ diameter logs and other rafted woody debris that span the channel of SJC just upstream of two large live redwoods located on the left and right bank of the channel (Figure

5.14). The second upper log jam is located 35’ farther upstream and is composed of a single 2’ to 3’ windfall tree and other rafted debris (Figure 5.13). Cumulatively, the jams elevate the streambed nearly 4’ vertically, and store a wedge of primarily sand sized material for a distance of 100’ upstream from the lower jam (Figure 5.7 through Figure 5.15, Figure 5.17 and Figure

5.18). Currently the jams store approximately 150 to 175 yd3 of sediment.

The channel aggradation has lowered the channel slope (Figure 5.17) triggered channel widening in the vicinity of the 2 log jams (Figure 5.18), and resulted in the formation of a highflow channel along the right unconfined, alluvial channel bank (Figure 5.13 and Figure 5.14).

The-high flow channel is flowing over former floodplain deposits containing abundant roots from the large adjacent redwood tree.

The log jam is likely a barrier to juvenile fish migration during most summer and winter low flow conditions, however, during normal or greater winter flows, adult steelhead should have minimal difficulty traversing either the jam or the high flow channel.

It should be noted that approximately 450’ upstream from the log jam at Site #47, a tremendous amount of recent SOD mortality is occurring along several hundred feet of SJC. The fallen trees have created an impenetrable woody debris accumulation that currently stores minimal amount Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1073, Packet Page 1173 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 65 of sediment (Figure 4.14, Figure 4.15, and Figure 5.16). However, the potential to form a new large log jam and barrier to fish passage is significant. MPRPD personnel should evaluate the location and evaluate whether some amount of debris removal would be advantageous. Please refer to Section 7.4 Recommendations for Future Work for additional discussion.

Figure 5.7. Site #47: View upstream from cross section #1 (Figure 5.18).  Start of survey in pool below spanning bay trees (Figure 5.17).  Note coarse pebble to cobble riffle crest.

Start Long Profile
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1074, Packet Page 1174 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 66 Figure 5.8. Site#47: View downstream from cross section #1 (Figure 5.18) at upstream end of channel stored sediments (Figure 5.17).  Note the well sorted sands burying the tail of the riffle composed of large gravels at the bottom of the photo.

Figure 5.9. Site#47: View upstream from cross section #2 (Figure 5.18) at aggraded channel reach.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1075, Packet Page 1175 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 67 Figure 5.10. Site#47: View downstream from cross section #2 (Figure 5.18) at upper logjam.

Figure 5.11.  Site#47: View upstream from cross section #3 (Figure 5.18).

Upper
Logjam
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1076, Packet Page 1176 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 68 Figure 5.12. Site#47: View downstream from cross section #3 (Figure 5.18) at upper and lower logjams.

Upper
Logjam
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1077, Packet Page 1177 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 69 Figure 5.13. Site#47: Photo mosaic taken from right end stake of cross section #4 (Figure 5.18) showing the upper and lower logjams, associated channel stored sediment, and the right bank high flow channel in foreground.

Upper
Logjam Lower Logjam
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1078, Packet Page 1178 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 70 Figure 5.14. Site#47: Photo mosaic taken from thalweg looking upstream at lower logjam (in center of photo) and developing high flow channel and sediment filled plunge pool on left below exposed redwood roots. Note presence of boulders on natural streambed downstream of the logjam.

High Flow
Channel
Lower Log
Jam
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1079, Packet Page 1179 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 71 Figure 5.15. Site#47: View downstream from near centerline of San Jose Creek illustrating typical SJC riffle characteristics, as well as a small <5 yd3 left bank erosion site at yellow arc, and the end point for the long profile at the arrow.

Figure 5.16. Extensive Sudden Oak Death mortality and fallen trees potentially forming a large log jam 450 ft upstream from PWA Site #47.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1080, Packet Page 1180 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 72 Figure 5.17. Longitudinal Profile of Logjam, Site #47 along San Jose Creek.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1081, Packet Page 1181 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 73 Figure 5.18. Cross Sections #1 through #5 of Logjam, Site #47 along San Jose Creek.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1082, Packet Page 1182 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 74

5.2.1.4 CDFW barrier #2 near CDFW station # 7.5
The second barrier identified by CDFW (Nelson, 2006a) consists of a large 8’+/- redwood that fell from the steep left bank hillslope across the channel of upper SJC (Map 5 and Figure 5.19 and Figure 5.20). The tree was located on the outside of a bend in SJC and likely due to bank erosion, the tree and rootwad fell and nearly 100% plugged the well-incised channel of SJC.

Based on numerous vertical redwood sprouts growing on the fallen tree trunk, it is estimated the tree fell approximately 25 to 30 years ago, at the minimum. Due to the near complete blockage of the active channel of SJC, it appears woody debris and sediment in transport rapidly backfilled the natural channel to near the height of the downed redwood trunk over the next decade or so. CDFW indicated barrier #2 was probably the most significant barrier to anadromous fish passage in the watershed (Nelson, 2006a).

Currently the temporary log jam barrier is very sound and stable, forms a 8’ near vertical step in the channel and is likely to remain intact for many decades, if not much longer. There is evidence of minor seepage from the base of the jam, but this is unlikely to trigger release of the channel stored sediments upstream of the tree truck and rootwad.

Channel aggradation associated with barrier #2 extends approximately 500’ upstream based on the presence of a near uniform, fine grained sandy streambed and the lack of visible coarser pebble, cobble and boulder bed materials (Figure 5.21 and Figure 5.22). Upstream of the zone of aggradation, the channel of SJC coarsens to a more typical pebble to boulder streambed and the channel gradient increases slightly.

Figure 5.19. View upstream from SJC at CDFW log jam barrier #2 in distance and at headcut on Log Jam
Diversion channel
on terrace
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1083, Packet Page 1183 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 75 right bank terrace where diverted SJC streamflow re‐enters the main stem of SJC.

Figure 5.20. View downstream taken from top of CDFW barrier #2 with 8’ vertical step in channel. Note old‐growth terrace on right bank where stream diversion flows re‐enter SJC.

Figure 5.21. View downstream from near station #180 in the long profile at aggraded sandy streambed and stored sediment upstream of barrier #2. About 50’ downstream of the large leaning redwood is the location where SJC is diverted onto the right bank terrace.

Diversion channel
on terrace Scale
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1084, Packet Page 1184 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 76 Figure 5.22. Longitudinal profile of Fish Barrier #2, large redwood logjam with side channel on San Jose Creek.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1085, Packet Page 1185 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 77 We estimate the volume of channel stored sediment, primarily sand-sized material, that has been deposited upstream of barrier #2 is approximately 1,400 yd3. It is estimated the original natural channel grade beneath the stored sediment is approximately 4.6% (Figure 5.22).

Approximately 210’ upstream from the log jam, it appears aggradation within the natural channel elevated the natural streambed to equal the floodplain/terrace surface elevation, and SJC streamflow has been diverted onto the right bank surface and out of the natural channel.

For at least a decade or so, it appears essentially all of SJC streamflow has been flowing through an average 4’ to 6’ wide, very poorly incised diversion channel along and across the right bank terrace surface that hosts old growth redwoods (Figure 5.23). The diverted SJC streamflow reenters the main stem of SJC about 30’ downstream from barrier #2 (Figure 5.19). Streamflow has created a quasi-stable approximately 8’ tall headcut off the right bank terrace. The headcut is a maze of dense old growth root systems that are severely limiting migration of the headcut (Figure 5.20).

We consider barrier #2 and the current terrace channel and headcut to be a near permanent, long term barrier to both adult and juvenile steelhead migration. The barrier is located in a very remote portion of upper SJC, there is no easy access for heavy equipment to reach the site, and the quality and extent of habitat upstream of the barrier is likely far less than the habitat located downstream of the barrier. Consequently, the costs to remove the barrier verses instream habitat gains and potential impacts to SJC suggest efforts to modify or remove the barrier are questionable. It is potentially possible to cut a large deep, average 10’ wide notch Figure 5.23. View downstream from near station #350 in long profile at diverted SJC stream across old growth terrace. The channel averages 4’ to 6’ wide, is very poorly incised and characterized by a sandy streambed.

Diversion channel
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1086, Packet Page 1186 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 78 through the trunk of the fallen redwood, but this would result in the release of most of the channel stored sediment, since there is no easy way to excavate the sediment. The large rootwad may also be problematic, since it would likely continue to block the channel and in the future, potentially capture additional organic debris in transport.

5.2.1.5 CDFW barrier #3 near CDFW station 8.0
The third and upstream-most barrier identified by CDFW (Nelson, 2006a) consists of a ½ dozen large old growth logs and trees with rootwads and lots of medium and small wood debris that has been rafted onto the core wood (Figure 5.24). It appears the barrier has been blocking the channel of SJC for 3 or more decades. Formation of the core log jam is difficult to explain in the old growth forest location (i.e. no evidence of tree throw or bank erosion). The log jam appears very solid and stable and forms a 7’ to 8’ steep in the channel (Figure 5.25). There is a minor amount of seepage at the base of the jam.

The log jam has stored sediment, primarily sand sized material, for a distance of 190’ upstream (Figure 5.27), and all stored sediment is contained within the active channel (Figure 5.26). It appears at some time in the past sediment accumulated in the channel to the current top of the log jam. Subsequently, field evidence indicates some form of piping/sink holes must have occurred/formed immediately upstream of the jam that has resulted in re-incision and transport of a portion of the stored sediment through the jam (Figure 5.25 and Figure 5.26). The incised notch varies in the upstream direction, but averages 1’ to 3’ deep and 5’ to 10’ wide.

Figure 5.24. View upstream at CDFW barrier #3 taken from top right bank. The complex log jam creates a 7’ to 8’ tall fish barrier and total blockage of upper SJC.

Log Jam
Scale
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1087, Packet Page 1187 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 79 Figure 5.25. View downstream at crest of log jam (at survey rod) with channel stored sediment covered by litter. Note inactive sinkhole near jam crest and recently incised channel upstream of the sinkhole.

Figure 5.26. View downstream taken from station #80 in long profile at recently re‐incised channel through previously stored sediment.

Log jam crest
Sink hole
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1088, Packet Page 1188 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 80 In the past, we estimate the maximum amount of stored sediment behind the jam was approximately 320 yd3. The more recent incision of the channel stored sediment wedge released approximately 80 to 85 yd3 of mostly sand sized material through the log jam. Currently, we estimate 235 to 240 yd3 is still stored behind the barrier. It is estimated that the buried natural channel gradient is approximately 7.5% through the stream reach influenced by the log jam (Figure 5.27).

Based on the evidence of past incision through previously stored sediments, the fate of the remaining stored sediment upstream of the jam is questionable. Unlike at CDFW barrier #2, the remaining stored sediment at barrier #3 has a higher potential to be released during future high flow events. The field evidence suggests the completeness of the debris blockage in the subsurface above the jam is less tight than at barrier #2.

Currently, barrier #3 is a total barrier to salmonid migration. While the potential for the log jam to fail or breach is extremely low, it exhibits a moderate to high potential to periodically release stored sediment from above the jam. In 2006, Nelson ended her habitat typing surveys at barrier #3 suggesting minimal anadromous habitat is present upstream from this point in the watershed. This is supported by the steeper projected natural channel gradient of 7.5% above the barrier. This coupled with the near permanent blockage of fish passage at barrier #2 suggest treating the feature would be more likely classified as a sediment management project rather than a fish passage project.

Restoration options at barrier #3 are more feasible than at CDFW barriers #1 and #2. While in an old growth setting, the adjacent natural hillslope are not steep, and heavy equipment access to the barrier and its’ stored sediment wedge could be fairly easily accomplished. Utilizing heavy equipment to remove the sediment within the lower 90’ to 100’ of channel above the jam, approximately 75% of the total volume of channel stored sediment upstream of the barrier could be excavated and safely spoiled upon the adjacent right bank terrace/fan surfaces. The log jam could also be fairly easily dis-assembled, and the wood could be re-introduced to the channel reach so as to provide for improved habitat and channel complexity.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1089, Packet Page 1189 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 81 Figure 5.27. Longitudinal Profile at Fish Barrier #3, large multi logjam on San Jose Creek.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1090, Packet Page 1190 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 82

5.2.2 Seneca Creek Potential Barriers
There are two potential barriers to fish passage identified by both Nelson (CDFW 2006b) and PWA during 2013. The downstream man-made partial barrier is located in the middle watershed adjacent the old homestead at stream mile 1.74 (Site #37, Map 5). The upper barrier is a temporary barrier at stream mile 1.9 composed of several large root wads and rafted smaller debris (Site #41, Map 5).

5.2.2.1 Barrier at 1.74 mile
The man-made barrier consists of a 4’ tall x 20’ wide concrete dam that spans the channel at a ford road crossing located about 200’ upstream of the old homestead house (Figure 5.28). There is a 6’ wide opening in the dam face for flashboard installation in order to impound stream flow annually.

A multi-stepped concrete apron creates a 2.5’ vertical step in the channel that extents approximately 13’ downstream from the spillway and results in a shallow and fast water cascade (Figure 5.28) eliminating jump pools and impairing passage. It appears it has been many decades since the dam was fully functional and the dam currently served as a total temporary barrier to upstream adult and juvenile steelhead migration. In its present condition, juvenile fish passage would be very difficult because of high velocities and shallow water depths over the apron during most flows.

Because the homestead is no longer a residence, it may be advantageous and a relatively straight-forward task to completely remove the man-made structure if it is not determined to be a site of cultural and/or historical significance. Vehicular access to the location is easily available and the stream channel is low gradient and poorly incised at the dam location. We estimate approximately 20 yd3 of primarily sand-sized channel stored sediment is located upstream of the dam. In removing the channel stored sediment, the approaches to the ford crossing may need to be slightly modified, lessened in steepness, in order to accommodate post dam removal vehicular access.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1091, Packet Page 1191 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 83 Figure 5.28. View upstream at concrete dam face and apron on Seneca Creek (Site #37, Map 5) located at stream mile 1.47 near the old homestead.

5.2.2.2 CDFW barrier at station 1.9 mile
Based on the 2006 CDFW instream assessment along Seneca Creek, numerous features (log jams, culverted road crossings, and a flashboard dam) were identified as being potential impediments to fish passage under certain flow regimes. However, the only identified feature classified as a barrier to both fish life stages during most flow regimes was a LWD log jam located within habitat unit #414 at stream mile 1.9 (Site # 41, Map 5). This log jam is comprised of 2 LWD stumps with SWD filling interstitial spaces and is 5’ to 7’ high with no jump pool located downstream of the jam (Figure 5.29).

During the instream assessment and field reconnaissance along Seneca Creek streamside roads, we observed many of the log jams that are potential impediments to fish during low flow regimes. In addition, we identified and confirmed that the log jam at stream mile 1.9 was a temporary fish barrier for all life stages at most flows. We did not survey this log jam as part of our fish barrier analysis. However, the log jam is located in a Strahler Order 3 stream with an active channel width that rarely exceeds 8’ in width. While still within a perennial stream reach, summer low flows are extremely low thereby limiting the long term benefit of the habitat located upstream of the jam.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1092, Packet Page 1192 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 84 Figure 5.29. View looking upstream at site #41 (Map 5) at station #1.9. Photo taken from Nelson, (2006b) see also Figure 4.11.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1093, Packet Page 1193 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 85

6. SAN JOSE CREEK LONGSHORE LAGOON

6.1 Introduction and background
We investigated conditions at the San Jose Creek lagoon using a set of study methods developed for other Monterey Bay lagoons supporting anadromous salmonids. Budget available for the lagoon study was equivalent to about 5 days of work. The investigation, summarized below, show that SJC has a lagoon atypical of the regional norm in that:
 The lagoon occupies the lower portion of backdune river channel. The creek flows north for a distance of about 1200 feet parallel to the beach, then enters Carmel Bay at a point where it is deflected across the dunefield into the bay by a bedrock outcrop. Few other coastal lagoons have this property of flowing parallel to the beach for a distance of about 80 channel widths or more.
 SJC has developed a channel through the backdune reach with its bed and banks composed of clean, well-sorted coarse sand and fine gravel. Infiltration into the bed and through the dunefield to the ocean occurs throughout this reach. We have observed several occasions in which flows of 3 to 5 cfs (estimated by a hydrologist) a short distance upstream of the Highway 1 bridge have completely infiltrated in the coarse sand and fine gravels before flow can reach the lagoon.  Once annual flows recede below 3 to 5 cfs, the lower portion of the lagoon is kept watered by tidewater flowing through the sands into the lagoon. The tidal inflows through the dunes cannot keep ponded the upper two-thirds of the channel up to the bridge, where the channel seems to have an elevation of about 10 to 11 feet above mean lower low water (0.14 feet above the NAVD88 datum). In many years, seasonal recession will have formed a flow barrier relatively early during the smolt downmigration period from early April through mid-June.  The ‘lagoon’ most resembles that which forms at the mouth of Pilarcitos Creek, near Half Moon Bay. This stream also conveys a coarse-sand sediment with a predominantly granitic origin. It enters the backdune channel reach with a bed elevation of about 15 feet MLLW, and infiltrates 1 to 2 cfs before entering the ocean. Pilarcitos Creek has been studied as part of an enhancement planning process, discussed below.

6.2 Work Conducted
Three monitoring activities were performed in the lagoon during this study (Figure 6.1).

1. A water level sensor was set up just downstream of the Highway 1 bridge.

2. Two time lapse cameras were set up to monitor the lagoon hydrography during the salmonid migration season

3. Ground survey was performed to monitor geomorphology of the lagoon and compare to Lidar topography.

6.2.1 Unique geomorphology
The hydrography and geomorphology of the lagoon differs from that for most central coast lagoons. As noted above, the lagoon has always formed in a backdune channel with a long reach between the bridge and the ocean. It is underlain by a thick wedge of coarse sand over Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1094, Packet Page 1194 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 86 bedrock. Our impression, and several observations comparing flows at the MPWMD gage and the Highway 1 bridge, suggest that infiltration occurs upstream of the bridge (see Appendix A).

6.2.2 Similar unique role in salmonid ecology
Relative to other lagoons for streams of similar size along the Santa Cruz and Monterey coasts (such as Aptos or Scott Creek, or the lagoons at the mouths of Watsonville Slough and the Little Sur River) beach bar opening are likely to be more infrequent, with greater daily fluctuations perhaps limiting rearing value, and not sufficient to sustain ponding to near the bridge, where essentially the downstream limit of continuous cover ends.

6.2.3 No role in amphibian or reptile ecology
It appears to us that the diurnal fluctuations limit values of the lagoon for amphibians and reptiles. The backdune channel is highly trafficked by visitors, which may also limit use.

6.2.4 Pilarcitos Creek Lagoon as site analog
Work done to enhance Pilarcitos Lagoon can inform the conceptual approach to the San Jose Lagoon. Pilarcitos Lagoon also has a supratidal point of entry into a long backdune channel which ponds intermittently. It has been studied intensively for potential enhancement (Parke and Hecht, 2009; Siegel and others, 2010). Three alternative concepts were assessed to improve steelhead upmigration and smolt downmigration, as part of a larger watershed-enhancement plan directed by the resource agencies:
a. Deflecting flow directly into the Pacific Ocean at the elbow where the backdune channel began, using a set of groins
b. Using step-pool morphology to minimize infiltration into the dune and stabilize a relatively permanent channel.
c. No action for the backdune channel, with enhanced willow planting to provide shade, cover, and resting habitat for the migrating salmonids.

Ultimately, the resource agencies chose to not pursue any of these alternatives, given the level of management that each might need, as well as the general intrusiveness of modifying the stream course. We suspect that much can be learned from the Pilarcitos experience. An effort to define the biological values of the lagoon would be a good place to direct further progress in working with the San Jose Lagoon.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1095, Packet Page 1195 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 87

6.3 Water Level Gaging and Ground Survey
In December 2012 a staff plate was installed downstream of the Highway 1 bridge (Figure 6.1).

On February 5, 2013 Balance installed a water level recorder at the same location, about 30 feet downstream of the Highway 1 bridge, within a thicket of cattails/bulrush. The flow pattern under the bridge appears to be a wide flat channel on a sand bed, which then enters thick aquatic vegetation and emerges on the beach to follow a long path northward through the dunes to the mouth at the ocean. The water level sensor was installed to monitor the lagoon/outlet levels. The gaging record is shown on Figure 3.8. Water-level data were adjusted vertically to approximately NAVD 88, based on ground survey performed May 2, 2013 and datum corrections to match Lidar data from the 2009 - 2011 CA Coastal Conservancy Coastal Lidar Project. NGS survey benchmarks were not recoverable near the site, so vertical adjustments are based on survey shots on the bridge deck, compared to the Lidar data.

As shown on Figure 3.8, the water levels fluctuate daily. During WY2013, pools dried up on June 4, 2013, and remained subsurface through January 2014. The low water level and dry creek at the Highway 1 bridge is primarily linked to the critically dry two year period of this study and stream gaging effort.

Ground surveys were also used at the lagoon to monitor changes in the channel cross section and profile through the study and during openings of the creek, as available. Five cross sections and a long profile of the creek were performed on May 2, 2014 (Figure 6.1). At the time of the ground survey, the creek was not flowing to the ocean and had not been since March, due to the critically dry year. The surveyed cross sections are shown on Figure 6.2, in comparison to Lidar data developed from the 2009 - 2011 CA Coastal Conservancy Coastal Lidar Project.

6.4 Photo documentation of lagoon with time lapse cameras Two remote time-lapse cameras were set up and recorded images at 5-minute intervals from February 16 to June 4, 2013, to document the anticipated opening and closing of the mouth of SJC during the salmonid migration/passage season. One camera was positioned to monitor the lagoon water levels, lagoon hydrography, and geomorphology (i.e. view to south parallel to beach and Highway 1), and the other was oriented to monitor the opening and closing of the mouth of SJC (i.e. view to west toward pacific ocean). A typical cycle of tidal inundation and cycling of lagoon water is shown on Figure 6.3.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1096, Packet Page 1196 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 88 Figure 6.1. San Jose Creek Lagoon survey, time lapse camera locations, and water level recorder location
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1097, Packet Page 1197 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 89 Figure 6.2. San Jose Creek Lagoon surveyed cross sections and profile from May 2, 2013 (blue lines), compared to 2009 – 2011 CA Coastal Conservancy Coastal Lidar Project (red lines).

Elevations in NAVD 88, approximately MLLW datum.
0 2
4 6
8 10
12 14
16 18
20 0 50 100 150 200 250 300
Cross Section 2
2 4
6 8
10 12
14 16
18 20
22 0 50 100 150 200 250 300
Cross Section 3
2 4
6 8
10 12
14 16
18 20
22 0 50 100 150 200 250 300
Cross Section 4
2 4
6 8
10 12
14 16
18 20
22 0 50 100 150 200 250 300
Cross Section 5
0 2
4 6
8 10
12 0 200 400 600 800 1000 1200 1400 1600
Channel profile
Ocean
HWY 1
Eucalyptus
root wad
0 2
4 6
8 10
12 14
16 18
20 0 50 100 150 200 250 300
Cross Section 1
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1098, Packet Page 1198 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 90 Figure 6.3. Typical photos from time lapse photography showing relation of lagoon inundation to tidal cycle.

6.5 Observations

6.5.1 Alignment and geometry of San Jose Creek lagoon
As shown on historic aerial photography and historic oblique photography from the Coastal Project (see Appendix A), the lagoon retains a long-shore orientation in water year 2013, running northward from the Highway 1 crossing along the back side of the beach for about 1,200 feet until a bedrock formation pushes deflects the flow in the lagoon seaward through a low spot in the barrier dune. The cross section geometry and profile did not change appreciably during this dry water year.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1099, Packet Page 1199 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 91 The cross section and profile of the lagoon seemed to remain somewhat unchanged during the course of this study, with the exception of wave over-run that altered dune structure near the mouth of the creek, at the northern end of Monastery Beach. In addition, about 300 feet from the mouth, a large 15’ diameter (estimated) rootwad from fallen eucalyptus trees is blocking and deflecting flow in the lagoon and forming a sediment plug within the lagoon. Beyond these geomorphic features, aeolian processes and public access are the next most influential factors affecting lagoon geomorphology.

Also note that the ground survey on May 2, 2013 is not that different from the LiDAR cross sections (Figure 6.2), with the prominent dune between the lagoon and the ocean. There are some minor topographic differences between the lagoon and highway, but this can possibly be explained by limited survey shots due to poison oak, or LiDAR inaccuracies from vegetation interference. A comparison of the two data sets set suggests and supports the concept that the lagoon is typically a narrow long-shore lagoon that is deflected north, bounded by a barrier dune and impacted by tidal seepage.

6.5.2 Opening and closing of lagoon
Opening and closing of the lagoon is a complex interaction of tidal dynamics, wave energy, fluvial activity, groundwater levels, hyperheic flow, and site lithology. An attenuated shallow opening of the mouth of San Jose Creek appears most frequently with a high tide of above 4.5 MLLW. A summary of observed openings and closings of the lagoon in the salmonid migration season is shown on Table 6.1. The process of lagoon opening appears to be related to tidal water permeating through the dune structure into the lagoon and waves that manage to traverse the dune structure to strengthen the opening. Inflow from SJC also has an influence, but the baseline sequence for opening seems to be a function of the tide and wave interaction, and mostly limited to opening and ponding at the mouth.

Table 6.1 Observed opening and closing of San Jose Creek lagoon at Monastery Beach during salmonid migration season for water year 2013, according to review of time-lapse photography Dates Open/closed observations
2/17 – 2/27 Partial day
opening
Changes with high tide, outflow shallow, 0.5’ or less
3/7 – 3/12 Partial day
opening
Changes with high tide, outflow shallow, likely
opening due to wave activity
3/22 – 3/23 Partial day
opening
Wave overrun imported sand and filled remnant
channel, flow does not extend further inland anymore
4/8 Partial day
opening
Wave overrun and short duration impoundment
4/9 Closed Closed for rest of summer and fall , time lapse camera removed 6/4/13
Opening of the lagoon also appears related to a bedrock structure about 100 feet from the mouth that deflects outbound flow ocean-ward. Periods of higher baseflow and peaking flows from storms from SJC appear to provide a setting for more frequent openings of the lagoon.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1100, Packet Page 1200 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 92 Based on comparison of tidal peaks and recorded images from the time-lapse cameras, it is estimated a high tide of about 4.5 feet MLLW typically raises the ocean level sufficiently and permeates through 50 to 70 feet of beach dunes to create a temporary lagoon. However, during the dry WY 2013, it is unlikely the small tidally created lagoon rarely received sufficient streamflow from SJC to provide for either upstream or downstream fish passage.

According to a sample of beach sand, the average diameter (D50) of the poorly-graded (somewhat uniform grain size) coarse sand is about 2 mm, which correlates to a permeability of 12 feet per hour (0.1 cm/s), per Freeze and Cherry (1979). This equates or about four to six hours of travel time to traverse the barrier dune from the waves to the narrow long-shore lagoon, with less travel time towards the mouth of th lagoon, where the dune width is less. The relatively short travel time through the coarse dune sand remotely links tidal cycles and the presence of water in the lagoon, also evidenced by time lapse photo sequences, as shown on Figure 6.3. Unlike in many coastal lagoons, flow in SJC has relatively little effect on the depth and extent of the lagoon. We do not have sufficient data to establish the range of flows which open the barrier beach; our impression is that even small freshets generate sufficient flow to open the lagoon in April, May and June, the months when steelhead smolt outmigrate, provided that flow at the bridge exceeds 3 to 5 cfs. On two separate occasions, we observed flows of 2.5 to 3 cfs upstream of the bridge infiltrating into the sands before reaching the lagoon.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1101, Packet Page 1201 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 93

7. KEY FINDINGS AND RECOMMENDATIONS

7.1 San Jose Creek Stream Gaging
Available data reveals WY2012 was dry, estimated as 66% of mean rainfall at San Clemente Dam (14.0 inches versus 21.3 inches) and water year 2013 was even drier at about 69% of mean precipitation (14.6 inches, estimated, versus 21.3 inches). These data are based on rainfall records dating to 1922. Water year 2014 is also looking to be critically dry to date. During the measurable storms during this study, limited bedload movement was observed with an initial pulse of fine sediment flushed through the system during initial storms. Ultimate disposition of fine sediment is indeterminate, but is possibly in-channel or flushed out to sea. As base flow recedes in the lower reach, it eventually sinks into the sandy stream bed, leaving the stream bed dry by the Highway 1 crossing.

7.1.1 Storm and Stream Gage Monitoring
Due to the dry nature of WY 2012/2013, we recommend that the gaging stations (Fig 6.1, Map 5) occupied and/or established as part of this study continue to be monitored. Gathering quantitative flow data in conjunction with sediment concentrations during significant storm event(s) will assist in painting an overall picture of episodic sediment sources in the watershed.

7.2 Sediment
As suggested by prior work in the early 1990s, sediment transport in SJC is relatively low relative to other Carmel-area channels, at about 3 to 30 tons per square mile during typical years. Lower values were measured in WY2013 and the first 4 months of WY2014. About half of the transport is sand.

Very little coarse gravel, and virtually none of the cobbles or boulders making up the stream bed are transported during typical years. Most of cobbles and boulders are in a position suggesting that they have not moved for many tens or perhaps 100s of years, with many being shaped by sandblasting, suggesting many, many years of immobility.

Bedload transport rates at the V-Notch gage suggest that the stream is now transporting higher loads at a given streamflow than were measured in 1991-1995 using similar equipment and methods. Because flows were so low during the recent sampling, this finding should be verified at higher flows before it is accepted. We believe that most of the additional sediment originates from a single active-incision tributary which enters SJC from the north side just upstream of Van Winkley Canyon, at Mile 5 on Rancho San Carlos Road. If so, the additional sediment could be mitigated by restoring this channel.

The work of the Balance/PWA team occurred during a period with virtually no episodic disturbance.  Our observations and measurements characterize a period of chronic erosion, quiescent relative to the periods of episodic sedimentation.  Overall‐ long‐term sediment yields may be expected to be half‐again‐as‐large, or twice as large, as those measured during this study.  Erosion may focus on the hillslopes during periods of episodicity, rather than chronic periods where the ridgetops (and roads) may be focus of sediment entrainment as identified in the USLE analysis above.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1102, Packet Page 1202 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 94 The identified major source of sediment found is identified as coming from upland sources, such as head cuts, roads, and hill slope erosion. There is limited and minor bank erosion, otherwise well secured stream banks with redwood forest was typical of the main stem and major tributaries. The supply of gravels for spawning material seemed limited, but present in smaller fractions.  The stream bed was typically bimodal, consisting of sand and cobbles. This study has determined that upslope erosional features are the most likely sources of sediment in the SJC watershed. One of the most important watershed management elements of long‐term restoration and maintenance of both water quality and fish habitat is the reduction of future impacts from upland erosion and sediment delivery.

7.2.1 Quantitative Road/Trail Assessment
Unlike many watershed improvement and restoration activities, erosion prevention through "storm-proofing" rural, ranch, and forest roads provides immediate benefits to the streams and aquatic habitat of a watershed (Weaver and Hagans, 1994, 1999; Weaver et al., 2006). It measurably diminishes the impact of road related erosion on the biological productivity of the watershed's streams, and allows future storm runoff to cleanse the streams of accumulated coarse and fine sediment, rather than allowing continued sediment delivery from managed areas.

We have seen the benefit of implementing erosion prevention and sediment reduction treatments along roads and trails through the comprehensive assessment utilizing CDFW recommended protocols, followed by the successful storm-proofing of approximately 8 mi of roads within the Williams Canyon subwatershed. By implementing these treatments, approximately 6,030 yd3 of sediment was saved from entering Williams Canyon and the main stem of SJC, where based on this study, the residence time may be quite long.

We recommend that future efforts be made to assess the remaining approximately 74 mi of road/trail within the watershed. The assessment(s) should identify and quantify all sources of future erosion, and most importantly, quantify the volume of future erosion being prevented from entering streams, as well as provide site-specific recommended treatments to reduce erosion and prevent future sediment delivery. In addition, a prioritized, treatment plan complete with a cost estimate and necessary labor and equipment needs should be a deliverable product.

7.2.2 Quantitative Upslope Erosion Assessment
Besides man-caused road related erosion and sediment delivery, based on preliminary review of historical and current aerial photography as well as reconnaissance field evaluations, nonroad related hill slope mass wasting and fluvial erosion are considered to be significant source of sediment in the SJC watershed. Therefore, we recommend conducting a comprehensive upslope erosion assessment of non-road related sediment sources.

The continued upslope migration of 1st order stream channels via headcutting and sub-surface piping processes, particularly on south facing hill slopes underlain by Miocene sedimentary bedrock within SJC, should be evaluated on a case by case basis with prioritized treatments developed in concert with land management goals.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1103, Packet Page 1203 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 95 A study identifying sources of erosion, quantifying past and future sediment delivery volumes and prescribing future treatment options can lead to developing a prioritized, treatment plan complete with a cost estimate and necessary labor and equipment needs to reduce future sediment delivery from upslope sources.

7.2.3 Maintain Installed Sediment Basins
To reiterate, this study has determined that upslope erosional features are the most likely sources of sediment in the SJC watershed. The Project Team installed four small sediment basins (SB #1-4, Map 5) in order to collect quantitative data from identified non-road-related upslope sediment sources. Due to the lack of rainfall during WY2013 and WY2014, sediment basins did not retain measurable quantities of episodic sediment from the upslope gullies. We recommend that these sediment basins continue to be maintained and monitored after significant rainfall events in order to quantify sediment delivery volumes from the upslope gullies due to episodic events. If these sediment basins are properly maintained, quantifiable data could assist in completing the upslope sediment source budget.

Prior to next winter, we recommend that the basins be evaluated for structural integrity and reinforced as deemed necessary. Basins should be evaluated after significant rainfall events and measurements taken and volumes recorded to document sediment derived from the upslope erosional features. Regular maintenance will also be required as the storage volume behind each dam is small (i.e. <2 yd3). The volume of sediment removed from the basins should be dispersed at a location that does not have the potential to deliver to any nearby watercourse.

7.2.4 Treatment of Identified Instream Sites
Although instream sources were not determined to be a significant input of sediment into the SJC watershed, based on the instream assessment we recommend that 15 of the 57 inventoried sites be treated to reduce sediment, improve instream habitat, and/or improve fish migration.

Of the 15 sites recommended for treatment, 8 are road crossings, 5 are log jams, and 2 are concrete instream dams (Table 7.1).

Table 7.1. Inventoried instream sites recommended for treatment, San Jose Creek Study, Monterey County, California.

Problem type
Instream sites
Inventoried
(#)
Recommended for
treatment (#)
Bank erosion 3  ‐‐
Landslide 1  ‐‐
Log jam 38 5
Other
Road crossing 12 8
Dam 2 2
Bedrock Cascade 1  ‐‐
Total 57 15
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1104, Packet Page 1204 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 96 Based on this study, we recommend 4 different types of instream treatments along the inventoried reaches of SJC and Seneca Creek. Recommended treatments primarily consist of removing instream obstructions and excavating/removing material from the stream channel (Table 7.2). For additional treatment information, treatment summaries are provided in Table

7.2, which refer to information in Appendix C.

For the 2 concrete non-functioning dam sites, both can easily be removed with minimal disturbance to the adjacent stream banks and the stream bed, following cultural clearance that they are not significant structures. Both have good vehicular access and the stream channels are low gradient and mildly incised at the dam locations. At Site #1 on the main stem of SJC (Map 5), not including moving a small track driven excavator and dump truck to and from the dam site, it is estimated approximately 10 hours of work will allow for demolishing the dam, and loading the concrete into a dump truck for removal from the SJC watershed, or to a proper spoil disposal location within the watershed. It is unlikely any of the stored sediments behind the dam could be retrieved primarily because it is deposited in and amongst the natural boulders contained in the streambed.

At Site #37 on Seneca Creek (Map 5), not including moving a small track driven excavator and dump truck to and from the dam site, it is estimated approximately 20 hours of work will allow for demolishing the dam, reshaping the bed and banks, and loading the concrete into a dump truck for removal from the SJC watershed, or to a proper spoil disposal location within the watershed. This time on site will allow for modifying the road approaches to the ford crossing located immediately upstream, as well as excavating the approximately 20 yd3 of primarily sand-sized channel stored sediment within the crossing and upstream of the dam. The approaches to the ford crossing need to be slightly modified, lessened in steepness, in order to accommodate post dam removal vehicular access. Ample locations are available on the nearby farmed terraces for disposal of the excavated bank materials and stored sediment.

At both dam sites, temporary coffer dams will need to be constructed both upstream and downstream of the instream construction sites (i.e. the dam removal site work area) in order to isolate the work area and maintain water quality through the use of pumps. In addition, Table 7.2. Recommended treatments for inventoried instream sites, San Jose Creek Study, Monterey County, California.

Treatment type No. Comments
Remove existing culvert and install
armored ford (wet) crossing 3 Remove existing culvert and all road fill and convertinto a ford crossing (Site #26, 29, and 30) Soil excavation 5
At 5 sites, excavate and remove a total of 225 yd3 of
sediment at 3 stream crossings ( Site #26, 29, and 30)
and concrete at 2 dams (Sites #1 and 37).

Remove, clear, or cut notch in
LWD/SWD log jam 5
At 5 sites, Remove, clear a portion, or cut a notch in
the existing log jam to allow for fish passage (Site
#33, 49, 50, 51, and 52)
Rolling dip 17 Install to improve road drainage and reduce delivery of road related sediments to streams.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1105, Packet Page 1205 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 97 electrofishing will be required to relocate fish and amphibians. All bare soil areas where surface erosion could deliver sediment to the stream should be seeded and mulched with appropriate materials.

Table 7.3 details the 15 sites recommended for treatment and lists treatment immediacy, complexity, and effectiveness as well as the total estimated volume of excavated material and estimated hours for heavy equipment. The equipment needs are reported as equipment times, in hours, to treat each individual sites. These estimates only include the time needed for the actual treatment work, and do not include additional construction activities such as mobilizing equipment, materials and/or field personnel, and traveling between sites. An estimated total cost to implement the recommended instream treatments for the project was not developed, but we can assist with that if requested.

7.2.5 Further Specific Instream and Upslope Assessments This study has determined that the primary source of instream sediment is likely derived from lower (1st and 2nd) order tributaries. A small, unnamed (2nd order) tributary opposite and just upstream of Van Winkley Canyon, draining south facing hill slopes, was identified as visibly undergoing active incising with active headcuts. In addition, higher sediment concentrations were observed from this tributary during synoptic storm monitoring. Therefore, should the adjacent land owners be willing, we recommend finding where sediment originates and determining alternatives and actions to stabilize the tributary.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1106, Packet Page 1206 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 98 Table 7.3. Estimated heavy equipment and labor requirements based on treatment immediacy and effectiveness for instream inventoried sites recommended for treatment, San Jose Creek Study, Monterey County, California.

Site # Treatment
immediacy Complexity Effective‐ness
Excavated
volumea
(yd3)
Excavator
(hr)
Dozer
(hr)
Dump
truck
(hr)
Labor
(hr)b
1 Moderate Low High 10 8  ‐‐  8 10
2 Low Low High‐
moderate ‐‐  ‐‐  6  ‐‐  ‐‐
15 Low Low High‐
moderate ‐‐  ‐‐  2  ‐‐  ‐‐
26 High‐
moderate
Moderate‐
low High 70 3 2  ‐‐  2
29 High‐
moderate Low High 75 3 2  ‐‐  ‐‐
30 High‐
moderate
Moderate‐
low High 70 2 2   2
33 Moderate Low High  ‐‐  ‐‐  ‐‐  ‐‐  2
37 High Low High 35 20   20 20
38 Low Low Moderate ‐‐  ‐‐  5  ‐‐  ‐‐
42 Moderate Low Moderate ‐‐  ‐‐  2  ‐‐  ‐‐
46 Moderate ‐
low Low Moderate ‐‐  ‐‐  2  ‐‐  ‐‐
49 High‐
moderate Moderate High  ‐‐  ‐‐  ‐‐  ‐‐  60
50 High‐
moderate Moderate High  ‐‐  ‐‐  ‐‐  ‐‐  40
51 Moderate Moderate‐
low
High‐
moderate ‐‐  ‐‐  ‐‐  ‐‐  6
52 Moderate Low High‐
moderate ‐‐  ‐‐  ‐‐  ‐‐  6
Total 260 36 23 28 148
Note: Equipment and labor times do not include hours necessary to move in and out of the project area and traveling between sites.
a Excavated volume includes material permanently removed and/or stored at a stable location out of the floodplain. b Labor time includes using chainsaws and other hand tools, seeding and mulching activities and pumping activities.

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7.3 Fish Barriers
Evaluation of previously identified fish barriers by CDFW was completed for the purposes of this study. The natural permanent barrier (FB #1, Map 5) presents complexities of hydraulics beyond the scope of this study. The barrier on SJC, located between Animas and Seneca Creeks, appears to be a significant bedrock drop at low flows (6 feet total for a chute and 3-foot vertical drop to pool) within a very constricted section of creek. In higher flows the backwater will raise the tailwater and ease the jump heights, but it still may reflect a significant barrier with a drop and a plunging water fall. Only strong adult swimmers would make it past this lower barrier, and we suspect few juvenile move upstream at any flows.

The two temporary natural fish barriers (FB #2 and FB #3, Map 5) are in the upper quarter of the watershed and based on field observations, it is suggested that these should be left in place.

Each are composed of old growth logs and root wads, are very stable and near permanent features in SJC with average vertical changes in bed elevation of 7-8’, and each are 100% fish barriers to all life phases of resident and anadromous salmonids.

7.3.1 Further Evaluation and Treatment of FB #1
A detailed hydraulic study is recommended for the bedrock falls barrier, identified as FB #1 (Map 5). Because this barrier is low in the watershed, a better understanding of existing hydraulic conditions and required swimming performance will inform access to the rest of the watershed for spawning and rearing habitat in order to develop recommendations to improve fish passage. As a result, if there is concern for passage and restoration beyond a natural barrier, the stakeholders might consider chipping the bedrock to facilitate a lower drop in step elevation or some other measures, but additional analysis will also be needed upstream of the falls to determine channel conditions and depth to underlying bedrock.

7.3.2 Treatment of FB #2 and FB #3
Until a decision is made to conduct further study and investigation of FB #1, it is not advised to pursue efforts and study to facilitate removal of the natural and stable log jams/temporary fish barriers at FB #2 and FB #3 ( Map 5). Stakeholders must consider the extent of instream habitat gains upstream of each barrier versus potential impacts of the release of channel stored sediments in relation to the level of effort to eliminate the features as barriers.

7.3.3 Monitoring of Sudden Oak Disease and Treatment of Potential Fish Barriers There is a section of SJC upstream of the confluence with Seneca Creek and PWA site #47 (Map 5) that may be delivering anomalously high wood loadings volumes to SJC from sudden oak death.  The potential for formation of future additional wood jams that could impede fish passage is high.  This area should be closely monitored for future additional instream impacts.

Based on decisions regarding treatment of fish barriers located downstream, existing and future log jams may be modified and/or removed to enhance fish migration.

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7.4 Long-Shore Lagoon
The hydrography and geomorphology of the SJC lagoon differs from that for most central coast lagoons. SJC has been identified as an important anadromous watershed. Sediment impairment is a primary restorative goal. However, field observations, surveys, and time lapse photography demonstrate lagoon opening is an important factor when discussing fish migration. Littoral processes and tidal dynamics play a large role in lagoon opening. The anomalously dry year in which our necessarily limited study was conducted should be balanced with an additional more thorough coastal dynamic and fluvial study will better answer how often the lagoon will open.

Based on this study, it appears the timing/opportunities for adult upstream migration, as well as outmigration for juvenile salmonids are very limited in many water years.

7.4.1 Further Lagoon Studies
We recommend conducting a study to understand the lagoon cycling and evolution. Such a study would combine an analysis of coastal dynamics, hydrology, and further geologic investigations of the Monastery Beach area. We suggest that a biological reconnaissance identify the potential uses of the lagoon, such that a more site-specific set of observation can be made.

Based on monitoring performed within the scope of this study, we doubt whether the physical attributes of the lagoon will support the same suite of habitat values characterizing other central coast coastal lagoons. We suggest that a biological reconnaissance identify the potential uses of the lagoon, such that a more site-specific set of observation can be made. The anomalously dry year in which our necessarily limited study was conducted should be balanced with an additional more thorough coastal dynamic and fluvial study that will better answer how often the lagoon will open. Littoral processes and tidal dynamics appear to play a large role in lagoon opening and the duration of time the lagoon would remain open. A recommended study to understand the lagoon cycling and evolution would combine a study of coastal dynamics, hydrologic analysis, and further geologic investigations of the Monastery Beach area.

7.4.2 Recapture Lagoon Photo Points and Surveys
As part of either further study or independently, we recommend re-occupying the photo point locations, longitudinal profile lines, and cross sections (Figure 6.1, Map 5) established for the purposes of this study. Photographic evidence combined with quantitative data could reveal important conclusions in regards to available and potential fisheries habitat.

7.5 Limitations
This report was prepared in general accordance with the accepted standard of hydrologic practice existing in Northern California at the time the investigation was performed. No other warranties, expressed or implied, are made. As is customary, we note that readers should recognize that interpretation and evaluation of subsurface conditions and physical factors affecting habitat is a difficult and inexact art. Judgment leading to conclusions and recommendations are general made with an incomplete knowledge of the conditions present.

More extensive or extended studies, including additional hydrologic investigations through additional water years, can reduce the inherent uncertainties associated with such studies. If the client wishes to further reduce the uncertainty beyond the level associated with this study, Balance should be notified for additional consultation.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1109, Packet Page 1209 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 101 We have used standard environmental information -- such as rainfall, soil mapping, and geology mapping -- in our analyses and approaches without verification or modification, in conformance with local custom. Information specific to the San Jose Creek Watershed is current through January 2014. New information or changes in regulatory guidance could influence the plans or recommendations, perhaps fundamentally. As updated information becomes available, or as pertinent water-quality information is issued for the region, the interpretations and recommendations contained in this report may warrant change. To aid in revisions, we ask that readers or reviewers advise us of new plans, conditions, or such data of which they are aware.

Concepts, findings and interpretations contained in this report are intended for the exclusive use of the MPRPD and the MPWMD. Their use elsewhere could lead to environmental or structural damage, and/or to noncompliance with water-quality policies, regulations or permits. Data developed or used in this report were collected and interpreted solely for the purposes described in the report. They should not be used for other purposes without great care, updating, review of sampling and analytical methods use, and consultation with Balance staff familiar with the site. In particular, Balance Hydrologics, Inc. should be consulted prior to applying the contents of this report to geotechnical or facility design, instream restoration, or for other purposes not specifically cited in this report.

Finally, we ask once again that readers who have additional pertinent information, who observed changed conditions, or who may note material errors should contact us with their findings at the earliest possible date, so that timely changes may be made.

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8. REFERENCES
(Alphabetized)
Calfire, 2013, California Forest Practice Rules Title 14, California Code of Regulations Chapters 4, 4.5 and

10. Prepared for: California Licensed Timber Operators and California Registered Professional Foresters Compiled by: The California Department of Forestry and Fire Protection Resource Management, Forest Practice Program. Available from: www.fire.ca.gov California Department of Fish and Game, 1962. Memorandum, San Jose Creek, Entire Headwaters, Personal observations and knowledge of Warden Lester Golden, 7p.

California Geological Survey, 1999, Factors affecting landslides in forested terrains: Department of Conservation, Division of Mines and Geology Note 50, 5 p. Available from:
http://www.conservation.ca.gov/cgs/information/publications/cgs_notes/note_50/Documents/n ote50.pdf
California Geological Survey, 2010, Palo Corona Regional Park, Back-Country Parcel, Draft Final Wild Lands Road Assessment: Report prepared for MPRPD, 30p. with maps Cook, T.D. 1978. Soil survey of Monterey County, California. U.S. Department of Agriculture Soil Conservation Service and Forest Service in cooperation with the University of California Agricultural Experiment Station. Washington, D.C
Cruden, D.M. 1991. A Simple Definition of a Landslide. Bulletin of the International Association of Engineering Geology, No. 43, pp.27-29.

Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes, in Turner A.K., and Shuster, R.L., eds., Landslides: investigation and mitigation: Washington, D.C., Transportation Research Board Special Report 247, p. 36-75.

Flosi, G., Downie, S., Hopelain, J., Bird, M., Coey, R., and Collins, B., 2010, California Salmonid Stream Habitat Restoration Manual, 4th edition. California Department of Fish and Game, Sacramento, California: Report, Volumes 1 and 2.

Furniss, M., Flanagan, S., and McFadin, B., 2000, Hydrologically-connected roads: An indicator of the influence of roads on chronic sedimentation, surface water hydrology and exposure to toxic chemicals: Fort Collins, CO, USDA Forest Service, Rocky Mountain Research Station, Stream Systems Technology Center, Stream Notes, July, 2000, p. 5-7.

Furniss, M.J., Roelofs, T.D., and Yee, C.S., 1991, Road construction and maintenance, in Meehan, W. R., ed., Influences of forest and rangeland management on salmonid fishes and their habitats. American Fisheries Society Special Publication 19, p. 297-323.

Freeze, A., and Cherry, J., 1979, Groundwater, Prentice-Hall, Inc., Englewood Cliffs, N.J., 604p.

Hagans, D.K., Weaver, W.E., and Madej, M.A., 1986, Long-term on-site and off-site effects of logging and erosion in the Redwood Creek Basin, Northern California, in Papers presented at the American Geophysical Union meeting on cumulative effects (December 1985): New York, NY, National Council for Air and Stream Improvement Technical Bulletin, no. 490, p. 38-66. Available from:
http://www.krisweb.com/biblio/gen_ncasi_hagans_etal_1986_roadsed.pdf Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1111, Packet Page 1211 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 103 Harr, R.D., and Nichols, R.A. 1993, Stabilizing forest roads to help restore fish habitats: A northwest Washington example: Fisheries, v. 18, no. 4, p. 18-22.

Hecht, B., 2000, Drought, fire, and geology – key watershed influences in the northern Santa Lucia Mountains, California: Peninsula Geological Society, Spring Field Trip, Big Sur Coast, 2000 Guidebook, Salinia Nacimiento Amalgamated Terrane, 7 p.

Hecht, B., 1981, Sequential changes in bed habitat conditions in the upper Carmel River following the Marble-Cone fire of August 1977, in Warner, R.E. and Hendrix, K.M., eds., California Riparian Systems: Ecology, Conservation and Productive Management, University of California Press, pp. 134- 141.

Hecht, B., 1993, South of the Spotted Owl - restoration strategies for episodic channels and riparian corridors in Central California: proceedings of the Society of Wetlands Scientists, Western Wetlands Conference, Davis, California, March 25-27, 1993, 7 p., 7 figs.

Hecht, B. and Napolitano, M., 1995, Baseline characterization of sediment transport and sedimentation at the Santa Lucia Preserve, Monterey County: Interim Report prepared by Balance Hydrologics, Inc. for Santa Lucia Preserve, 58p.

Hecht, B., Woyshner, M., Brown, S., Donaldson, E., Richmond, S., and Owens, J., 2013: Deeply weathered granitics: Resilient aquifer storage for a changing climate: State of the Estuary Conference, Oakland, CA, Oct. 28-30, 2013. Poster and abstract.

Higgins, P.T., Dobush, S., and Fuller, D, 1992, Factors in Northern California threatening stocks with extinction: Arcata, CA, American Fisheries Society, Humboldt Chapter, 25 pp.

NMFS (National Marine Fisheries Service), 2000, Salmonid guidelines for forestry practices in California:

Long Beach, CA, National Marine Fisheries Service, Southwest Regional Office, 16 p. Available from:
http://swr.nmfs.noaa.gov/psd/sgfpc.htm
NMFS (National Marine Fisheries Service), 2001, Guidelines for salmonid passage at stream crossings:

Long Beach, CA, National Marine Fisheries Service, Southwest Regional Office, 14 p. Available from:
http://swr.nmfs.noaa.gov/hcd/NMFSSCG.PDF
Nelson, J., 2006a, Stream Inventory Report, San Jose Creek, California Department of Fish and Game:

Report, 42p.

Nelson, J., 2006b, Stream Inventory Report, Seneca Creek, California Department of Fish and Game:

Report, 24p.

Pacific Watershed Associates, 2007, Williams Creek Watershed Erosion Prevention Planning Project Mitteldorf Preserve, Monterey County, California PWA Report No. 07074101, 43 pp [prepared for the California Department of Fish and Wildlife].

Pacific Watershed Associates, 2010, Mitteldorf Preserve Sediment Reduction Project, Monterey County, California PWA Report No. 11085701, 78 pp [prepared for the California Department of Fish and Wildlife].

Paddock, E., Stoner, K., and Smith, D., 2012, Installation and initial analysis of two stream gages on San Jose Creek, Monterey County, CA. Central Coast Watershed Studies (CSUMB): Report, 25p.

Rosenberg, Lewis I., 2001, Digital geologic map of Monterey County, California.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1112, Packet Page 1212 Balance Hydrologics, Inc.

San Jose Creek Wshed Assessment Final Report 03-31-14.docx 104 Strahler, A. N., 1952. “Dynamic basis of geomorphology". Geological Society of America Bulletin 63: 923–938.

Titus, R. G., Erman, D. C., and Snider, W. M., 2011, History and status of steelhead in California coastal drainages south of San Francisco Bay. Report for California Department of Fish ‘And Game, in preparation, 302p.

University of California Santa Cruz, 2013. Access to aerial photography reserves and scanning of historic aerial photos by Balance staff.

USDA Forest Service, 2000, Water/road interaction field guide: San Dimas, CA, San Dimas Technology and Development Center, 68 p. Available from: http://www.fs.fed.us/eng/pubs/pdf/00771803.pdf USDA National Agriculture Imagery Program (NAIP), 2010, Digital orthoimagery of Monterey County, California: ortho_1-1_1n_s_ca053_2010_1, USDA-FSA Aerial Photography Field Office, Salt Lake City, Utah (downloaded on 8/29/2011 from http://datagateway.nrcs.usda.gov/) USDA National Agriculture Imagery Program (NAIP), 2012, Digital orthoimagery of Monterey County, California: ortho_1-1_1n_s_ca053_2012_1, USDA-FSA Aerial Photography Field Office, Salt Lake City, Utah (downloaded on 8/08/2012 from http://datagateway.nrcs.usda.gov/) USGS, 1995, Mount Carmel, California [map]: Washington, D.C., U.S. Geological Survey, 7.5 Minute Map 36121D7, scale 1:24,000.

USDA NRCS Websoil Survey online server, 2014. Accessed February 24, 2014 to list soils and soil properties of areas within San Jose Creek Watershed.  Website:
http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
USGS Streamstats online server, 2014. Accessed February 23, 2014 to tabulate watershed characteristics and estimate peak flows at mouth of San Jose Creek and at bedrock falls per Nelson (2006). Website:
http://water.usgs.gov/osw/streamstats/california.html
Wagner, David L., Cal Geology Nov 1991/vol.44/no 11. “Decomposed Granite” p243-249.

Weaver, W.E., and Hagans, D.K., 1994, Handbook for forest and ranch roads: a guide for planning, designing, constructing, reconstructing, maintaining and closing wildland roads: Ukiah, CA, Mendocino County Resource Conservation District, 198 p. Available from:
http://www.mcrcd.org/publications/
Weaver, W.E, and Hagans, D.K., 1999, Storm-proofing forest roads, in Sessions, J., and Chung, W., eds., Proceedings of the International Mountain Logging and 10th Pacific Northwest Skyline Symposium, Corvallis, Oregon, April 1999: Oregon State University, Forest Engineering Department, p 230-245.

Available from: http://www.iufro.org/science/divisions/division-3/30000/30600/publications/ Weaver, W.E., Hagans, D.K., Weppner, E., 2006, Part X: Upslope erosion inventory and sediment control guidance, in Flosi, G., Downie, S., et al., eds., California salmonid stream habitat restoration manual, 3d. ed.: Sacramento, CA, California Department of Fish and Game, 207p.

Google Earth, May 12, 1994, September 6, 1998, June 6, 2002, October 1, 2002, July 30, 2003, July 30, 2004, December 30, 2005, June 19, 2007, August 14, 2008, June 5, 2009, July 13, 2010, December 2, 2011, and May 5, 2012. Accessed between December 2012 and January 2014.

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Monterey Peninsula, Carmel Bay, and South Monterey Bay
Integrated Regional Water Management Plan Update
APPENDIX 2-G

CANYON DEL REY MASTER DRAINAGE PLAN
UPDATE

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Monterey Peninsula, Carmel Bay, and South Monterey Bay
Integrated Regional Water Management Plan Update
This Page Intentionally Left Blank

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1115, Packet Page 1215 Canyon del Rey
Master Drainage Plan
July 2014
Prepared for:

Monterey Peninsula Water Management District
Monterey County Water Resources Agency
City of Seaside, California
Funded by the California Department of Water Resources With Local Matching Funding Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1116, Packet Page 1216

CANYON DEL REY MASTER DRAINAGE PLAN
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July 14, 2014
Canyon Del Rey Master Drainage Plan
Prepared for:

Monterey Peninsula Water Management District
5 Harris Court, Building G
Monterey, CA 93942
(831) 658-5600
800 Bancroft Way, Suite 101 ~ Berkeley, California 94710-2800 ~ (510) 704-1000 ~ [email protected] 224 Walnut Avenue, Suite E ~ Santa Cruz, California 95060 ~ (831) 457-9900 9699 Blue Larkspur Lane, Suite 105 ~ Monterey, California 93940 ~ (831) 649-5225 100 Campus Center ~ Seaside, California 93955-8001 ~ (831) 582-3000 © 2014 Balance Project Assignment: 212050
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TABLE OF CONTENTS
EXECUTIVE SUMMARY 1
1 INTRODUCTION 6

1.1 Background 6

1.2 Location description 7

1.3 Study objectives 7

1.4 Acknowledgments 8
2 WATERSHED AND FACILITIES DESCRIPTIONS 9

2.1 Hydrologic setting 9

2.2 Watershed description 10

2.3 Development within the watershed 11

2.4 Facilities overview 12

2.5 Descriptions of existing facilities 17

2.6 Lake surveys and mapping 29
3 HYDROLOGIC CONDITIONS AND PRECIPITATION ESTIMATES 36

3.1 Available precipitation gages and data 36

3.2 Precipitation data quality and utility 37

3.3 Selected methodology for developing rainfall depth estimates and storm hyetographs 39

3.4 Mean annual precipitation estimates 40

3.5 Depth-Duration-Frequency estimation from NOAA Atlas 14 results 43 4 HYDROLOGIC ANALYSES AND MODELING 45

4.1 Available data 45

4.2 Description of alternative methodologies 46

4.3 Modeling methodology applied for this work 48

4.4 HMS model development 49

4.5 Conditions modeled 56

4.6 HEC-HMS model calibration 57

4.7 Runoff modeling results 60
5 STORM DRAINAGE CRITERIA USED IN THIS STUDY 66

5.1 General 66

5.2 Hydrologic analysis approach 68

5.3 Hydraulic analysis approach 69

5.4 Hydraulic design methodology and criteria 69
6 HYDRAULIC ANALYSES 71
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6.1 Hydraulic performance calculation methods 71

6.2 Facility capacity estimates 71

6.3 Effects of facilities on existing and future flows 74 7 EROSION AND SEDIMENTATION 77

7.1 Purposes of the geomorphic study 77

7.2 Historical and archival analysis 88

7.3 Evidence from the field 107

7.4 Management implications of the sediment study 113
8 RECOMMENDED FACILITY IMPROVEMENTS 117

8.1 General 117

8.2 Additional rain and stream gages 117
9 FACILITY IMPROVEMENT COST ESTIMATES 119

9.1 Cost estimation methodology 119

9.2 Facility improvement costs 120
10 RECOMMENDATIONS 123

10.1 Utilization of this report and electronic submittals 123

10.2 Lake management and preservation recommendations 123

10.3 Facility improvement priorities and execution 124

10.4 Impact of facility improvements on flood flows 124

10.5 Flow control measures for future development 125

10.6 Recommendations regarding sediment management 125

10.7 Use of this Master Drainage Plan in ongoing planning and design of storm 126

10.8 Future revisions and updates to this plan 127
11 REFERENCES 128
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TABLE OF TABLES
Table 2-1: Detention basin facilities in Canyon del Rey watershed. 14 Table 2-2: Roadway crossing culverts in Canyon del Rey watershed. 15 Table 2-3: Percent area and volume of Lakes for corresponding depth zones. 32 Table 3-1: Correlations of annual precipitation values between gage records. 38 Table 3-2: Precipitation-Duration-Frequency results for Canyon Del Rey watershed. Precipitation depths in inches. 40
Table 3-3: Comparison of Atlas 14 and 1977 Master Plan DDF estimates. Precipitation depths in inches. 40
Table 3-4: Comparison of Mean Annual Precipitation Estimates. 41 Table 3-5: 10 year and 100 year, 24 hour duration rainfall depth estimates. 44 Table 4-1: Runoff related HEC-HMS input parameters for Canyon del Rey sub-watersheds. 50 Table 4-2: Stream flow routing related HEC-HMS input - Canyon del Rey basin. 51 Table 4-3: Detention basins within the Canyon del Rey watershed. 53 Table 4-4: Detention basin parameters used in the HEC-HMS model. 54 Table 4-5: HEC-HMS model calibration results. 58
Table 4-6: Canyon del Rey sub-watershed runoff before routing. 61 Table 4-7: Stream flow statistics for 10 year and 100 year simulated events. 62 Table 4-8: Storage basin operating statistics for the 10 year and 100 year events. 64 Table 4-9: Storage basin performance statistics for the 10 and 100 year events. 65 Table 6-1: Culvert design parameters used in hydraulic analyses. 72 Table 6-2: Comparison of culvert capacities with 10 and 100 year flows. 73 Table 6-3: Recommended upgrades to existing culverts. 74 Table 7-1: Well Log for abandoned well east of GJMD and east of Frog Pond. 80 Table 7-2: Summary and description of active major compartments in Canyon del Rey watershed. 92
Table 7-3: Summary of incised reaches and characteristics based on LiDAR analysis and field observationsa. 101
Table 7-4: Summary of compartments, estimated sediment loads, and capacity, Canyon del Rey. 102
Table 8-1: Recommended culvert upgrades. 118
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Table 9-1: Preliminary construction cost estimates. 120

TABLE OF FIGURES
Figure 2-1: Rainfall and annual runoff recurrence curves for streams in the eastern Monterey Bay area. 11
Figure 2-2: Bathymetry of Roberts Lake (top) and Laguna Grande (bottom) shown in 1 foot depth zones as elevation in feet NAVD88. 31
Figure 2-3: Elevation-volume curves for Laguna Grande and Lake Roberts. 33 Figure 2-4: Acoustic backscatter echo return intensity classified into natural breaks. 34 Figure 2-5: Unclassified sidescan sonar backscatter intensity mosaic - hydrologic conditions and precipitation estimates 35
Figure 3-1: Rainfall gages with useful records. 37
Figure 3-2: Mean Annual Precipitation statistics overlain on the 1977 isohyet map. 42 Figure 3-3: Mean Annual Precipitation Isohyets for Canyon del Rey. 43 Figure 4-1: Annual peak flows for Arroyo del Rey at Del Rey Oaks gage. 45 Figure 4-2: Arroyo del Rey at Del Rey Oaks annual peak flows plotted against return period. 46 Figure 4-3: Example 100 year, 24 hour rainfall hyetograph, sub-watershed 18. 56 Figure 4-4: Stream flow at Frog Pond Gage for the March 2006 Calibration Event. 59 Figure 4-5: Stream flow at Frog Pond Gage for the April 2006 Calibration Event. 59 Figure 4-6: Sample HEC-HMS output hydrographs for Reach 29. 63 Figure 4-7: Simulation peak discharge results for Canyon del Rey. 63 Figure 5-1: Peak flow rates along main stem for alternate conditions. 67 Figure 5-2: Runoff volumes along main stem for alternative conditions. 68 Figure 6-1: Effect of proposed culvert upgrades on future peak flows. 75 Figure 6-2: Effect of proposed culvert upgrades on future flow volumes. 76 Figure 7-1: Profile and geomorphic interpretation of lower reach of Canyon del Rey, Monterey County, California. 81
Figure 7-2: Profile and geomorphic interpretation of upper reaches of Canyon del Rey Monterey County, California. 82
Figure 7-3: Historic Photo of Laguna del Rey showing original lake and Hotel del Monte, circa 1880 per Cain (2005). 83
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Figure 7-4: Historic progression of delta in Laguna del Rey. 85 Figure 7-5: Conceptual profile of aggradation process upstream of a dam per Brown and Jackson (1973). 86
Figure 7-6: Conceptual geomorphic cross sections of incision process per Schumm (1984). 87 Figure 7-7: Time line of major sedimentological events in Canyon Del Rey Watershed. 89 Figure 7-8: Canyon Del Rey watershed sediment sources and sinks (lower watershed). 93 Figure 7-9: Canyon Del Rey watershed sediment sources and sinks (middle watershed). 95 Figure 7-10: Canyon Del Rey watershed sediment sources and sinks (upper watershed). 97 Figure 7-11: Compartments mapped on aerial photograph, with subwatershed boundaries, Canyon Del Rey, Monterey County 99
Figure 7-12: Flow at MPWMD gage at Del Rey Park for water year 2013. Flow data courtesy of Greg James of the MPWMD. 104
Figure 7-13: Locations of sediment sample sites in lower Canyon del Rey. 105 Figure 7-14: Sediment size distribution of Canyon Del Rey bed-material samples and surrounding dunes downstream from Tarpy's corner. 111
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OVERSIZED FIGURES
Figure 1: Canyon del Rey sub-watersheds and Map Index.

Figure 2: Watershed slope map.

Figure 3: HEC-HMS model structure for the Canyon del Rey basin.

APPENDICES
Appendix A. Sub-watershed Maps
Appendix B. Facility Inspection Records
Appendix C. Selected Facility Photographs
Appendix D. Depth-Duration-Frequency Estimates from NOAA Atlas 14 Appendix E. Preliminary Cost Estimate Details for Proposed Facility Upgrades Appendix F. Included Electronic Files: Data DVDs 1 and 2 Appendix G. Geology and Soil Maps
Appendix H. Historical Maps and Aerial Photography
Appendix I. Sedimentological and Mineralogical Analysis of Lower Canyon Del Rey: Channel and Dune Descriptors
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EXECUTIVE SUMMARY
This report presents an update to the Master Drainage Plan for Canyon del Rey originally prepared for the Monterey County Water Resources Agency (MCWRA) in 1977. This updated plan accounts for the changes in hydrologic and hydraulic conditions in the watershed, as well as the addition of new and updated flood management facilities. It also provides a new investigation and evaluation of sediment related processes in the watershed, including analyses of sediment transport, erosion, and deposition within the stream channel system. Project activities included:
• field investigation of existing storm water management facilities, • surveying of selected culverts and storm water basin outlet structures, • field investigations of stream channel morphology and sediment migration,
• bathymetric surveys of Roberts Lake and Laguna Grande, • collection of available rainfall and stream flow gauge data within and in proximity to the watershed
• updating of rainfall data and depth-duration-frequency estimates and mean annual rainfall estimates ,
• completion of a comprehensive hydrologic model of the entire watershed including both designed and de facto stormwater storage areas,
• hydraulic analysis (using FHA HY-8) of conveyance capacity of culverts and crossings to determine their adequacy for handling design storm events,
• preliminary cost estimating for recommended upgrades to selected facilities,
• analysis of sediment mobilization, transport and deposition processes in the watershed, and
• consideration of alternative future watershed and flood management activities.

A set of recommendations for ongoing management of the watershed is provided for your consideration.

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The following summarizes the principal results, conclusions, and recommendations.

1. Work in this study included the survey and subsequent hydrologic and hydraulic analyses of 51 culverts, 32 water and sediment basins, and the two lakes in the overall 14.3 square mile watershed.

2. Bathymetric surveys of Roberts Lake and Laguna Grande revealed that very little deposition of sediments has occurred since the last time that the lakes were dredged. This indicates that relatively small amounts of sediment have been transported into the lakes from the creek. Fine sediments have accumulated to a greater degree in Laguna Grande than in Lake Roberts but both sedimentation rates have been markedly low.

3. Management practices before, and particularly since, the 1977 plan have resulted in the construction of numerous stormwater basins within the watershed. Furthermore, flow restrictions at the many highway crossings and associated culverts create significant additional de facto detention storage areas which dramatically alter stream flows and sediment transport throughout the system.

4. The evaluation of available precipitation gage records concludes that local gage data are not sufficiently robust to be used to update the 1977 mean annual rainfall isohyet map. This data are also not sufficient to enable development of improved depth/duration/frequency relationships for the watershed.

5. However, the detailed analyses underlying the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 data are appropriate and were used to generate depth-duration-frequency estimates for hydrologic modeling purposes.

6. Analogously, data from the Parameter-elevation Relationship on Independent Slopes model (PRISM) is technically robust and was used to produce an updated mean annual precipitation isohyetal map for the watershed.

7. A comprehensive hydrologic model using the U.S. Army Corps of Engineers’ HEC-HMS platform was created for the watershed, which was divided into 37 sub-watersheds for modeling purposes. Model operation was calibrated using data from the former USGS gage site now operated by the Monterey Peninsula Water Management District, with very close correlation.

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8. Predicted peak discharge for the watershed is summarized at numerous points for both 10-year and 100-year design storms. Predicted peak discharge at Fremont Boulevard in Seaside is 209 cfs for the 10-year storm and 753 cfs for the 100-year storm. These values are somewhat lower than predictions of future flows in the 1977 plan, which were 214 cfs for the 10-year storm and 870 cfs for the 100-year storm. For comparison, the FEMA 2009 Flood Insurance Study predicts discharges at Fremont Boulevard equal to 250 cfs for the 10-year storm and 675 cfs for the 100-year event. In all three cases, the ratios of 100 year peak flows to 10 year peak flows (approximately 3 to 1 up to 4 to 1) differ noticeably from the regional norm of about 2 to 1, reflecting the unique attributes of the sandy soils.

9. Flow and sediment transport in Canyon del Rey are attenuated in 8 to 10 natural mainstream compartments which pond water and settle sediment. Constrictions between the compartments are largely natural and persistent.

10. Sediment yields in Canyon del Rey are normally quite low in most years, yet can be very high during extreme events. While many watersheds function this way, the differences in Canyon del Rey are much more extreme than the norm. This indicates that different processes may be in effect during the 10-year flood than during the larger 100-year event, leading to unusually large differences in sediment transport during 100- year events than during smaller storms. Episodic events – such as wildfires, large magnitude storms, and landslides and their after-effects – probably account for much of the sediment delivered to the Canyon del Rey valley floor. Such events are sufficiently rare and/or poorly understood that they are not generally incorporated in drainage or watershed plans.

11. Despite the widely-held view that windblown sand is a substantial contributor to sedimentation of Roberts Lake and Laguna Grande, particle sizes, the overwhelmingly angular grains, mineralogy, and diagnostic statistical metrics establish that the source of nearly all sediment in Laguna Grande and probably in Roberts Lake is from stream deposition.

12. Bathymetric surveys of Roberts Lake and Laguna Grande revealed that very little deposition of sediments has occurred since the last time that the lakes were dredged. This indicates that relatively small amounts of sediment have been transported into the lakes from the creek. Fine sediments have accumulated to a greater degree in Laguna Grande than in Lake Roberts but both sedimentation rates have been markedly low.

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13. Dredging of Laguna Grande and Roberts Lake occurred in 1870 in conjunction with a real-estate promotion, and then again in 1983.

Assuming that post-1983 deposition patterns persist, no need for dredging during coming decades is anticipated. The lakes are not likely to be a regular source of dredged material for beach nourishment.

14. Analysis of the behavior of the constructed and de facto natural detention basins (which are part of the defined “compartments” in this report), many of which are located on the main stem, indicates that the basins are an important contributor to flood flow management under existing conditions. Seven of the basins reduced incoming peak storm flows by 50% or more, and supplemental hydrologic runs indicate that peak flow entering Laguna Grande is reduced by approximately 60% due to the combined effects of upstream storage.

15. Two approaches to sediment management in the watershed are identified and developed: 1) continuation of the existing and largely natural compartmental accumulation of produced sediments and 2) modification of channel and storage elements to emphasize passing sediment downstream to Laguna Grande. It appears that continuation of the existing compartmental storage mechanisms will have multiple benefits for the watershed in terms of both sediment management and flood flow attenuation. However, the multiple implications of such a policy call for a broader planning perspective. Likely effects on habitat, existing policies and regulations, flood mapping, public safety, and maintenance costs all warrant greater consideration before a suitable approach can be adopted.

16. Evaluation of culvert capacities and recommendations regarding culvert improvements are predicated on a continuation of the existing compartmental storage mechanisms in the stream system.

17. Upgrades to 16 culverts are recommended, based on their inability to safely pass the 100-year peak discharge. The recommended upgrades are designed to reduce the depth of overtopping of the roadway associated with the culvert to 0.5 feet or less.

18. The impacts of the recommended upgrades on flows in the creek were estimated by modifying the HEC-HMS model to represent the changed culvert hydraulics. The model predicts that peak flows would increase in some locations by no more than 20% and decrease in some locations by no more than 7%.

19. The impacts of the recommended culvert upgrades on sediment transport, channel stability, and delivery of sediment to the lakes were not analyzed explicitly in this study. Given the relatively small changes in peak flows resulting from the upgrades, substantial changes in sediment Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1128, Packet Page 1228

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movements and/or channel stability seem unlikely. However, ongoing observation of stream behavior and sediment accumulation in the compartments is warranted.

20. The costs of the recommended upgrades were estimated using a preliminary design and a unit pricing methodology. Unit prices were obtained from the CalTrans Contract Cost Database and bid results for similar projects. Total cost for all 16 upgrades is estimated at $6.5 million, with the individual costs varying from $158,000 to $1,645,000.

21. Extensive amounts of data were collected during this work and are provided in a set of electronic files. It is recommended that these data be placed in an active data server and made available to District personnel and other users.

22. Changes in runoff due to changes in the watershed are likely to change storm flows and sediment transport. Decisions regarding land development, wildfire management, road improvements, and upgrades to the flood conveyance facilities should be made with full understanding of potential impacts on peak flows, creek channel stability, and sediment transport into the lakes. The HEC-HMS model developed for this project can be used in the future to estimate hydrologic effects of changes in the watershed.

23. Regular upgrades to this plan, on an approximately 10 year schedule, are recommended to provide that the plan remains reasonably up to date and useful in planning for the watershed.

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1 INTRODUCTION

1.1 Background
This master drainage plan update was authorized by the Monterey Peninsula Water Management District and funded in part through a grant from the California Department of Water Resources. The purposes and scope of this update acknowledge the changes in hydrologic and hydraulic conditions in the watershed, as well as the addition of new and updated flood management facilities within the watershed, that have occurred since the prior master drainage plan (Koretsky King, et al, 1977) was completed. In addition, changes in land use have also occurred as a result of building within the watershed.

There is also evidence that erosion and sediment transport have changed stream channel conditions, which should be evaluated from a flood management perspective.

The Master Drainage Plan for Canyon del Rey (Koretsky King, et al, 1977) was produced in 1977 for the Monterey County Flood Control and Water Conservation District. The purpose of the previous report was to provide a basis for orderly land development and design of associated flood control facilities.

Within this general purpose were specific objectives, including: establishing flood control facility design criteria, assessing the adequacy of existing flood control facilities, determining the drainage patterns and hydraulic characteristics, defining the need for an estimated cost of needed flood control facilities, and recommending legal instruments needed to implement the plan.

The 1977 plan included a hydrologic analysis of existing data, predictions of future flows, a hydraulic analysis of existing facilities, and made recommendations for future improvements. One recommendation in that plan was “A periodic updating of the Master Plan to reflect actual conditions, as development occurs, is strongly recommended as the basis on which to provide for future conditions.” No update of the plan has occurred since completion of the study.

Improvements recommended in the 1977 drainage study were proposed to reduce flooding damage to structures as a result of the expected 100-year return period flood. Additional analyses and recommendations were made in the 1977 study to pass the expected 10-year storm flows at road and highway drainage facilities. To fund needed improvements, the 1977 study recommended both property tax assessments and fees for new developments.

However, with the passage of Proposition 13 in June 1978, property taxes were capped and only the development fee was initiated. In the interim, some improvements and erosion prevention measures on individual parcels developed since that time have been completed.

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1.2 Location description
The Canyon Del Rey watershed is located in Monterey County on the Central Coast of California, just east and north of the City of Monterey (see Figure 1-1).

Canyon Del Rey Creek (also called Arroyo del Rey) is an intermittent stream that drains to the Pacific Ocean from an area of 14.3 square miles (approximately 9,137 acres) along Highways 68 and 218, beginning near the Laguna Seca raceway at the eastern end of the watershed and flowing west into Monterey Bay (see Oversized Figure 1). 1 The watershed includes portions of Seaside, Del Rey Oaks, Monterey, and unincorporated areas in Monterey County.

1.3 Study objectives
This report and the efforts contributing to these results and recommendations are focused on updating the 1977 plan to account for: changes to the watershed over the intervening 37 years; additions to flood control facilities; additional hydrologic, hydraulic and geologic information that is available now; improvements in analytical, computational, and modeling methods; and changing needs for development and flood management. In particular, this report provides expanded analyses of sediment production, transport and fate within the watershed and relates those analyses to facility improvement and management. Specific study objectives include:

1. Updating the hydrologic model contained in the 1977 Drainage Study.

The 1977 study included rainfall data from the mid-1930’s to the mid1970s. An additional 35 years of rainfall data are now available for use in updating isohyetal maps and depth-duration-frequency curves. In addition, assumptions about land use, the effects of development, and the effect of various drainage facilities on surface flows were evaluated and updated.

2. Surveying and documenting the design and condition of the existing flood conveyance and management facilities.

3. Evaluating the hydraulic capacity of facilities to pass existing flows. Using information gathered in the field and an update of flow estimates, the hydraulic characteristics of each primary and selected secondary drainage facility will be evaluated.

4. Updating expected erosion and sedimentation rates. The 1977 drainage study postulated that the creek was in a “juvenile” stage of channel development and that, despite transportation of silt from the

1 Many of the topographic drainage areas used throughout this report are approximate, given that boundaries are often drawn at the crest of dunes, which may or may not reflect how water actually drains.

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headwaters, the amount of sediment carried by the creek was limited.

The historic, existing, and future rate of sediment transport to the lakes needed review.

5. Producing a plan report which updates the 1977 plan and provides the new results and findings.

1.4 Acknowledgments
This project was jointly funded by the Monterey Peninsula Water Management District and the Monterey County Planning Department under the direction of Larry Hampson (Chief Engineer) and Tom Moss (Floodplain Manager), respectively. This project also benefited from, and likely contributed to, a concurrent planning study for Frog Pond Wetland Preserve, sponsored by the Monterey Peninsula Regional Parks District, under the supervision of Tim Jensen, conservation planner.

Many individuals and entities contributed to this effort. Whitson Engineers conducted all surveys, and helped in many ways with documenting the history of the watershed. The Whitson staff, under the direction of Rich Weber and Tom Hannon, also assisted in preparing for and conducting the stormwater measurements.

CSUMB, through Prof. Rikk Kvitek, conducted the bathymetric mapping and bed characterization of Laguna Grande and Roberts Lake. This work and the findings to which it led were central to identifying the role of compartments, and in guiding subsequent sedimentologic, geomorphic, and beach-sand recruitment analyses.

The geomorphic team benefited from discussions with Prof. Doug Smith (CSUMB), who also provided the key well log identifying the character of the valley-floor sediments upstream of Frog Pond. Elizabeth Geisler, CSUMB graduate student in hydrology generously shared her thoughts and observations. Jonathan Lear, MPWMD hydrogeologist, contributed helpful observations regarding the variable depth of dune sands and groundwater fluctuations throughout the watershed.

Hydrologic records, including rainfall and streamflow gauge data, were provided by multiple parties, including U. S. Geological Survey, Monterey Peninsula Water Management District, Laguna Seca Golf Course, Naval Post Graduate School, and the National Weather Service.

Finally, we wish to thank Greg James, chief hydrographer for MPWMD, who provided both up-to-date and historical information for daily and 15-minute flows at the District’s gage in Work Park (formerly site of the USGS gage discussed throughout the 1977 report).

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2 WATERSHED AND FACILITIES DESCRIPTIONS

2.1 Hydrologic setting
High flows during the extremely wet years of 1995 and 1998 enable observations of drainage system behavior and confirmed some of the predictions made in the 1977 study, especially concerning head-cutting in Canyon Del Rey Creek (the creek or CdR creek) and sedimentation of culvert and other road drainage facilities. Upland areas in the watershed continued to experience erosion and observations indicated sediment transport in the channel system.

These conditions raised questions regarding potential sedimentation and resultant reduction of flood storage (volumetric) capacity in Laguna Grande and Roberts Lakes.

It should be noted that Monterey County implemented regulations to prevent or reduce sediment and runoff from new development beginning in the 1960’s.

In addition, since completion of the 1977 drainage plan, the City of Monterey has implemented all recommendations from that plan for new development within the city limits of Monterey.

Along the Central California Coast, which contains a significant number of watersheds underlain with sandy soils, it has been noted recently that the hydrologic effects of development may be much greater in deep sandy soils than in loamy, clay and/or shallow soils (Hecht and Woyshner, 1984). Only limited data were available regarding lake sedimentation.

There have been reports that the stream channel suffers from bank instability, head-cutting, and erosion problems during high flows in multiple locations due to a variety of factors such as increased stream flows during rain events, changes in sediment supply in the watershed, and creek side development.

Evidence of these effects is very limited.

The watershed is heterogeneous and its creek system complex. Land use, soils, slopes, and land cover vary widely (see Sections 2.2 and 4.5), with particularly large differences between the sub-watersheds north of Hwy 68 and those south of the highway. Urbanization has occurred in some sub-watersheds and has been completely absent in others (see Section 2.3 and the map plates in Appendix A), increasing the complexity of watershed response during storms.

The morphology of creek channels and depositional areas is highly variable, with erosion dominating in some reaches while deposition dominates in other reaches (see Chapter 7). The results from this study provide considerably more detail regarding these conditions and propose a set of hydrologic and sedimentation behaviors that conform to observations in the watershed (see Chapters 4 and 7).

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2.2 Watershed description
To better analyze the watershed, with an overall area of 14.3 square miles, we divided it into 37 sub-watersheds. Of these, 16 flow into CdR creek from the south and 19 from the north. The shape and extent of these sub-watersheds were developed to enable and simplify simulation of hydrologic processes, leading to variation in area from 8 acres to 1.88 square miles. While the subwatershed boundaries generally follow drainage divides, boundaries also are set to represent changes in land use or soils. The delineations of these subwatersheds are shown in Oversized Figure 1 and on the map plates in Appendix A. A map showing areas within the watershed with certain ranges of ground slopes is provided as Oversized Figure 2. Slopes vary from essentially flat to well in excess of 25%. High slope areas extend throughout the southern and western portions of the watershed.

Significant portions of the watershed to the south of Highway 68 are also sparsely developed and rise to 1,300 feet in elevation with a mix of coastal scrub, pine, and oak woodlands. Much of the runoff to the creek is from this area, which is also subject to relatively higher rates of erosion. Areas to the north of Highway 68 are generally low rolling grassy hills covered with sandy soils that generate little storm runoff to the creek, except during the wettest periods.

The west end of the basin, in the Cities of Monterey, Seaside, and Del Rey Oaks there is a high degree of urbanization with a mix of single- and multi-family units and commercial development.

The peak 100-year flow in the creek (Koretsky King, et al, 1977) ranged from 800 cfs (existing) to 1,000 cfs (future conditions). Many of the primary facilities surveyed in 1977 could pass less than one-half of the 100-year peak. The most recent update of the FEMA Flood Insurance Rate Map (April 2, 2009) shows zones of shallow flooding in the 100-year event that would affect residences and businesses adjacent to the creek between Roberts Lake and the intersection of Highway 68/Highway 218 2. Portions of both highways would be inundated during a 100-year flood.

Geologic evidence and soil moisture budget analyses (Yates et al, 2003) both indicate the presence of a shallow alluvial aquifer system that sustains wetlands and phreatophytic vegetation and also supports base flow in Canyon Del Rey in wet years. Hydro-geologic conditions along Canyon Del Rey suggest that infiltration into the aquifer system from the channel or valley bottom is dominant and runoff in the creek normally represents a small fraction of the annual rainfall.

2 Locally known as Tarpy’s Corners, a name we frequently use in this report.

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2.3 Development within the watershed
Urbanization of Canyon del Rey has a two sided history. In Seaside, North Monterey and Del Rey Oaks, development occurred primarily during the three decades following WWII, perhaps coincident with the growth of the functions of Fort Ord. The period of most rapid expansion was roughly 1949 to 1959. The ditches and drainage ways fully developed during these years. Relatively little urbanization has progressed since that time. Laguna Grande and Roberts Lake were dredged in 1983, following this period of growth (see Appendix H).

Conversely, in the upper half of the watershed only limited development occurred prior to 1980; significant expansion followed, including residential areas, infrastructure, business parks, and intensive open-space recreational uses such as golf courses, a speedway, and equestrian boarding and facilities.

Figure 2-1: Rainfall and annual runoff recurrence curves for streams in the eastern Monterey Bay area.

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It is clear that the volume of recharge reaching the bedrock aquifers beneath the valley has been diminishing over the past 25 years (e.g., Yates and others, 2003). Additionally, streams have incised into valleys floors which did not support streams, including parts of the main stem in the upper half of the valley.

Channel incision allows storm runoff to move more quickly through the watershed. More runoff (and sediment) leaves the watershed, with less time for infiltration. Figure 2-1 shows that the sandy watershed previously yielded only a small fraction of incident rainfall, commonly less than 10% of the runoff from watersheds with more loamy and clay soils, The opportunities to both reduce storm runoff and increase recharge are an important part of developing an integrated drainage plan.

Many of the improvements recommended in the 1977 study have been implemented. General Plans for development in the cities and unincorporated areas have changed, water quality standards for storm water runoff have become more stringent, and tools to understand and predict water and sediment flows have improved. (Monterey County implemented regulations to prevent or reduce sediment and runoff from new development beginning in the 1960’s. In addition, since completion of the 1977 drainage plan, the City of Monterey has implemented all recommendations from that plan for new development within the city limits of Monterey. It is unknown which improvements have been implemented within the city of Del Rey Oaks.

2.4 Facilities overview
Existing storm water conveyance and management facilities within the study area which were evaluated and included in the modeling and analysis consist of: 51 culverts (primarily under roadway crossings), 32 water and sediment detention basins, and the Roberts and Laguna Grande impoundments. These facilities vary greatly in size, design, and physical condition. Table 2-1 provides a list of the detention basins. Table 2-2 provides a list of the road crossing culverts. Details of these facilities and their condition are provided in Section

2.5. The nomenclature used in identifying specific facilities is defined in the facility ID as follows:
• the first two digit number is the sub-watershed identifier, • followed by a letter indicating whether the facility is a basin (B) or a culvert (C),
• followed by a two digit number that is the facility sequence number within the sub-watershed, and
• occasionally followed by a letter indicating that the object is a component of a facility, with all components sharing the same number.

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Abbreviations used in defining the materials of construction for culverts are defined at the end of Table 2-2.

The 1977 study (Koretsky King, et al, 1977) differentiated between primary and secondary facilities, with primary facilities defined as structures (culverts) providing at least 12 square feet of usable flow area. Secondary facilities were those culverts with lesser usable flow area. The 1977 plan recommended improvements for primary structures. Evaluation of structures was based on ability to carry the 10 year peak storm flow for secondary facilities and the 100 year peak storm flow for primary facilities.

This plan update does not differentiate between primary and secondary facilities. It does exclude certain very small structures which are located on private property. All facilities are evaluated according to the same criteria (described in Section 5.4).

Canyon del Rey creek (main stem) has its source at the crest of Hwy 68, runs mostly westerly along Hwy 68 until the junction of Hwy 68 with Hwy 218, at which point it follows Hwy 218 north and west to Laguna Grande, Roberts Lake, and finally Monterey Bay. Both highways cross the creek at many locations, creating a series of flow restrictions and associated impoundments. These restrictions and impoundments, along with both naturally occurring ephemeral lakes and marshes and basins constructed for storm water detention, dramatically alter the natural runoff from the watershed and the peak storm flows in the creek.

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Facility ID Location
LS_B_01 Lake at Laguna Seca Raceway.
04_B_01 South of Hwy 68 at S.P.C.A. facility.
04_B_02 South of Hwy 68 at S.P.C.A. facility.
05_B_01 South of Hwy 68, approx. 1'650 feet west of S.P.C.A. entrance road. 06_B_01 North of Hwy 68, approx. 500 feet west of S.P.C.A. entrance road. 07_B_01 North of Hwy 68, approx. 1,000 feet east of Boots Road. 08_B_01 South of Hwy 68. Approx. 250 feet southeast of Boots Road. 08_B_02 South of Hwy 68. Approx. 250 feet southeast of Boots Road. Not modeled. 09_B_01 Approx. 200 feet south of where Boots Road and Whip Road meet near Hwy 68. 10_B_01 Pasadera golf course pond, approx. 200 feet south of Las Laderas Drive. 10_B_02 Pasadera golf course pond, approx. 230 feet west of Las Brisas Drive. 10_B_03 Pasadera golf course pond, approx. 200 feet southeast of Pasadera Country Club. 11_B_01 Pasadera golf course pond, approx. 180 feet west of Mirasol Ct. Not modeled. 11_B_02
10_B_04
11_B_03 1'400 feet west of Pasadera Drive and 180 feet north of Hwy 68. Part of the Laguna Seca Golf Ranch. 12_B_01 1'650 feet west of Pasadera Drive and 270 feet north of Hwy 68. Part of the Laguna Seca Golf Ranch. 12_B_02 2'050 feet west of Pasadera Drive and 270 feet north of Hwy 68. Part of the Laguna Seca Golf Ranch. 14_B_01 Canyon del Rey reach west of Pasadera Road and South of Hwy 68. 19_B_01 Directly west of where Wilson Road and York Road meet. 21_B_01 North of Hwy 68. Approx. 1'300 feet east of Ragsdale Drive. 22_B_01 North of Hwy 68. Approx. 1'000 feet east of Ragsdale Drive. 24_B_01 West side of Hwy 218. Approx. 1'400 feet south of the Hwy 68 and Hwy 218 interchange. 24_B_02 West side of Hwy 218. Approx. 1'400 feet south of the Hwy 68 and Hwy 218 interchange. 25_B_01 Approx. 650 feet northwest of the Ragsdale Drive and Lower Ragsdale drive T intersection. South of the Harris Ct business development.
25_B_02 Directly south of the Hwy 68 and Hwy 218 interchange. South of the Monterra Subdivision. 26_B_01 South of Hwy 68 and west of 218 at interchange. North of the Monterra subdivision entrance. 27b_B_01 Directly east of Hwy 218 at Pheasant Ridge Road. 29_B_01 Frog Pond Wetland Preserve.
29_B_02 Northeast of the Monterey Airport. North of N road. 29b_B_01 Park behind Safeway, west of hwy 218 and south of Wilson Way. 30_B_01 Laguna Del Rey and Roberts Lake combined. North and south of Del Monte Blvd.

Pasadera golf course pond, at the intersection of Pasadera Drive and Via Del Milagro. Modeled as one (10_B_04).

Table 2-1: Detention basin facilities in Canyon del Rey watershed.

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Facility ID Location Description
LS_C_01 Lake at Laguna Seca Raceway 15" circular CPE
01_C_01 Crossing under Hwy 68, approx. 1'240 feet east of Laureles Grade 18" circular CMP 01_C_02 North of Hwy 68 across from Laureles Grade. Parallel to Hwy 68. 18" circular CMP 01_C_03 Crossing under Laureles Grade. 24" circular CMP 02_C_01 Crossing under Hwy 68, just west of Laureles Grade. double 28" x 20" oval CMP 02_C_02 Crossing just east of the S.P.C.A. entrance and parallel to Hwy 68. South o 18" circular CMP 02_C_03 Crossing under S.P.C.A. driveway entrance. 18" circular CPE 03_C_01 Crossing under the east gate to Laguna Seca Raceway. 48" x 30" oval CMP 03_C_02 Crossing under the main entrance to the Laguna Seca Raceway. 40" circular HDPE-S 04_C_01 Crossing under Hwy 68 just west of S.P.C.A. 24" circularCMP 04_C_02 Crossing under Hwy 68, approx. 770 feet west of S.P.C.A. 24" circular HDPE-S 05_C_01 Crossing under Hwy 68, approx. 1'750 feet west of Laguna Seca Raceway 24" circular CMP 06_C_01 Crossing under gated access road, approx. 2'130 feet west of Laguna Seca 48" and 30" circular CMP 07_C_01 Crossing under Hwy 68, approx. 1'160 feet east of Pasadera entrance. 24" circular HDPE-S 08_C_02 Crossing under Hwy 68, just west of Boots Road. 52" circular RCP 09_C_01A Crossing under Whip Road and Boots Road. 40" circular HDPE-S 10_C_01 Crossing diagonally under Hwy 68, approx. 950 feet east of Pasadera entr 36" circular CMP 10_C_02 Crossing under Boots Road, south of Hwy 68. 60" circular RCP 10_C_03 Crossing under Pasadera entrance, north of Hwy 68. Flows from Pasadera 36" circular RCP/CMP 12_C_01 Crossing under Hwy 68 at Laguna Seca Golf Ranch. 48" circular CMP 14_C_01 Crossing under Hwy 68 at Laguna Seca Golf Ranch. West of 12_C_01. 48" circular CMP 15_C_01 Crossing parallel to Hwy 68, approx. 2'800 feet east of York Road. North o 48" circular CMP 16_C_01 Crossing under Hwy 68, approx. 3'600 feet east of York Road. 24" circular CMP 17_C_01 Crossing under York Road. 14.4' x 8' concrete box with earth floor
18_C_01 Crossing under Hwy 68, just west of York Road. 6' x 4' RCB 21_C_01 Crossing under Hwy 68, approx. 1'800 feet west of York Road. triple 28" x 24" synthetic fiberglass pipes
25_C_01 Crossing under Monterra entrance (Hwy 218). double 48" and triple 18" circular RCP
25_C_02 Crossing under Hwy 68, just west of Monterra entrance. double 48" circular RCP Table 2-2: Roadway crossing culverts in Canyon del Rey watershed.

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Facility ID Location Description
25_C_03 Crossing under southeast entrance of the Stone Creek Center. Parallel to 14' x 7.7' RCB 26_C_01 Crossing under Hwy 68. Starting approx. 650 feet northwest of Monterra 36" circular HPDE-S 27_C_01 Crossing under northeast entrance of the Stone Creek Center. Parallel to 14' x 8' RCB 27_C_02 Crossing under entrance to storage lot, just north of the Stone Creek Cen 14' x 6.7' RCB 27_C_03 Crossing under Del Rey Gardens Drive, west of Hwy 218. Parallel to Hwy 2 87.6" circular CMP 27_C_04 Crossing under Hwy 218, approx. 520 feet southeast of General Jim Moor 6' x 8' RCB 28_C_01 Crossing under General Jim Moore Blvd. at Hwy 218. 10.1' x 8' RCB 28_C_02 Crossing under General Jim Moore Blvd approx. 500 feet northeast of Hw 3' x 3' RCB 29_C_01 Crossing under Hwy 218 at the Frog Pond, approx. 50 feet northwest of V 6' x 8' RCB 29_C_02 942 Angelus Way. not modeled
29_C_03 938/934 Angelus Way driveway. wooden bridge with concrete walls
29_C_04 930/926 Angelus Way. wooden bridge with
concrete walls
29_C_05 Across Angelus Way from Altura Pl. 48" circular CMP 29_C_05A Across Angelus Way from Altura Pl. 12' x 8' concrete bridge 29_C_06 Angelus Way, just west ofAvalon Pl. concrete bridge 29_C_07 Crossing under Rosita Road at Angelus and Rosita intersection. 6' x 8.25' RCB 29_C_08 Crossing under Fremont Blvd, starting at the park behind Safeway. 8' x 8' RCB 30_C_01 Crossing at the southeast end of Laguna Grande Park. East of Laguna Gran 6' x 6' RCB 30_C_02 Bridge crossing at Laguna Grande Park southeast of Branner Ave. 100' x 7'(in middle) Wooden arched bridge
30_C_03 Laguna Del Rey Lake crossing under Del Monte Avenue. double 16' x 7' RCBs 30_C_03B Laguna Del Rey Lake crossing under Del Monte Avenue. double 21.36' x 7' RCBs 30_C_04 Roberts Lake crossing under Roberts Avenue. double 8' x 6' RCBs 30_C_05 Crossing under Hwy 1 off-ramp at Hwy 218. Outlet at Seaside beach east Quad 6' x 6' RCBs Materials:

CMP: CORRUGATED METAL PIPE
CPE: CORRUGATED POLYETHYLENE PIPE - CORRUGATED EXTERIOR/INTERIOR HDPE-S: HIGH DENSITY POLYETHYLENE PIPE - TYPE S - CORRUGATED EXTERIOR/SMOOTH INTERIOR PVC: POLYVINYL CHLORIDE PIPE
RBC: REINFORCED BOX CULVERT
RCP: REINFORCED CONCRETE PIPE
OTHER: OTHER MATERIAL; SEE FIELD NOTES
Table 2-2: Roadway crossing culverts in Canyon del Rey watershed (continued).

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1140, Packet Page 1240

CANYON DEL REY MASTER DRAINAGE PLAN
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2.5 Descriptions of existing facilities
SUB-WATERSHED LS:
• Map Panel: C-7
• Basins: LS_B_01
• Culverts: LS_C_01
• Drainage area (acres): 188
• Hydrologic Characteristics: This watershed encompasses the Laguna Seca Lake (LS_B_01) and its contributory drainage area.
• Hydraulic Facilities: A single 15 inch pipe, LS_C_01, runs 2,640 feet to a creek which is tributary to the main stem, dropping in elevation from 743 feet to 508 feet. An outlet gate in a 3 foot wide flume controls flow from the lake into the pipe. The gate controls lake water level between 738.85 and 746.85 feet.

SUB-WATERSHED 01:
• Map Panel: C-7 and C-8
• Basins: None
• Culverts: 01_C_01, 01_C_02, 01_C_03
• Drainage area (acres): 128
• Hydrologic Characteristics: Sub-watershed 01 is located at the eastern upstream limit of the study area at the watershed divide. While the watercourse is poorly defined in places, three culverts carry flow under roads.
• Hydraulic Facilities: Culvert 01_C_01 is a 18 inch RCP carrying local drainage to the north side of Hwy 68. Culvert 01_C_02 is a pair of 18 inch CMP pipes in series which carry flow along the north side of Highway 68.

Culvert 01_C_03 is a structure consisting of 18 inch and 24 inch RCP, a junction manhole, and a 24 inch CMP running under Laureles Grade Road. This structure drains a detention pond on the southwest side of the intersection of Hwy 68 with Laureles Grade Road.

SUB-WATERSHED 02:
• Map Panel: C-7 and C-8
• Basins: None
• Culverts: 02_C_01, 02_C_02, 02_C_03
• Drainage area (acres): 484
• Hydrologic Characteristics: Sub-watershed 02 is located on the south side of Hwy 68 and is drained by a swale running in part along Laureles Grade Road. Runoff is concentrated where the swale approaches the highway.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1141, Packet Page 1241

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• Hydraulic Facilities: Culvert 02_C_01 is a pair of 28 inch by 20 inch elliptical RCP, which carry flow from this sub-watershed north across Hwy 68.

Culvert 02_C_02 is a pair of 15 inch CMP running along the north side of Hwy 68, carrying flow under a side road. Culvert 02_C_03 is an 18 inch CPP running under a driveway.

SUB-WATERSHED 03:
• Map Panel: C-7
• Basins: None
• Culverts: 03_C_01, 03_C_02
• Drainage area (acres): 250
• Hydrologic Characteristics: Sub-watershed 03 is located between subwatersheds LS and 01, north of Hwy 68, and drains steep slopes between Laguna Seca lake and the highway. Much of the runoff is routed to the highway via a drainage swale running due south.
• Hydraulic Facilities: Culvert 03_C_01 is a 48 x 30 inch CMP running under the Laguna Seca access road and gate, on the north side of Hwy 68. 03_C_02 is a 40 inch HDPE pipe running under the Laguna Seca main entrance road on the north side of Hwy 68.

SUB-WATERSHED 04:
• Map Panel: C-7 and C-8
• Basins: 04_B_01, 04_B_02
• Culverts: 04_C_01, 04_C_02
• Drainage area (acres): 53
• Hydrologic Characteristics: Sub-watershed 04 is a small shed located immediately south of Hwy 68 between sheds 02 and 05. A small creek drains the area.
• Hydraulic Facilities: Basins 04_B_01 and 04_B_02 are located on the creek in series just south of Hwy 68. Basin 04_B_01 flows over a concrete weir into basin 04_B_02, which drains to a swale south of Hwy 68 via a 12 inch PVC pipe. Culvert 04_C_01 (24 inch CPP) carries the flow from the basins north under Hwy 68, while culvert 04_C_02 (24 inch CPP) carries local flow under the highway.

SUB-WATERSHED 05:
• Map Panel: C-7 and C-8
• Basins: 05_B_01
• Culverts: 05_C_01
• Drainage area (acres): 290
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1142, Packet Page 1242

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• Hydrologic Characteristics: Sub-watershed 05 is located on the south side of Hwy 68 and the Laguna Seca ranger station. It drains mostly steeps slopes that extend nearly to the southern drainage divide via narrow valleys.
• Hydraulic Facilities: Basin 05_B_01 captures virtually all of the runoff from the sub-watershed. Culvert 05_C_01, a 24 inch CMP, drains water from the basin and carries it north under Hwy 68.

SUB-WATERSHED 06:
• Map Panel: C-7
• Basins: 06_B_01
• Culverts: 06_C_01
• Drainage area (acres): 229
• Hydrologic Characteristics: Sub-watershed 06 is located between Hwy 68 and the Laguna Seca Raceway basin and drains mostly steeps slopes that extend to the northern drainage divide via a narrow valley. The channel runs parallel to the Laguna Seca main entrance road and carries flow from Laguna Seca sub-shed (LS). A number of minor basins and small culverts along the west side of the road (not modeled or field surveyed) carry water downslope.
• Hydraulic Facilities: Basin 06_B_01 has a substantial storage area, is controlled by culvert 06_C_01 and extends 2000 feet eastward along the north side of Hwy 68. Culvert 06_C_01, consisting of 30 inch and 48 inch CMP, run on the north side of Hwy 68 and carry water under the ranger station access road. A wetland area has developed upstream from the culvert, while the culvert outlets are suspended 8-12 feet above the downstream channel.

SUB-WATERSHED 07:
• Map Panel: C-7 and C-8
• Basins: 07_B_01
• Culverts: 07_C_01
• Drainage area (acres): 86
• Hydrologic Characteristics: Sub-watershed 07 is a small shed which spans Hwy 68 between sheds 05-06 and sheds 08-11. A small creek drains the southern area. Detention basin 07_B_01 is a long, narrow storage area controlled by culvert 10_C_01.
• Hydraulic Facilities: Culvert 07_C_01, a 24 inch CPP, carries water from the south side under Hwy 68 to the main stem. This culvert has grated concrete box inlet.

SUB-WATERSHED 08:
• Map Panel: C-7 and C-8
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1143, Packet Page 1243

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• Basins: 08_B_01, 08_B_02
• Culverts: 08_C_01, 08_C_02
• Drainage area (acres): 388
• Hydrologic Characteristics: Sub-watershed 08 is located between Hwy 68 and the south drainage divide. A single channel drains much of the shed, which are mostly steep slopes.
• Hydraulic Facilities: The two basins, 08_B_01 and 08_B_02, which are in series, control nearly all of the runoff. Basin 08_B_01 is very shallow and has a riser outlet with a grate on top. The downstream location of the outlet was not found in the field survey. Basin 08_B_02 is located immediately downslope from basin 08_B_01 and is a shallow, small depression without obvious outlet or inlet structures. Culvert 08_C_01 is a short 24 inch CMP which drains basin 08_B_01. Culvert 08_C_02, a 52 inch RCP, carries all of the main stem flow north under Hwy 68. Both the inlet and outlet are on concrete headwalls; the outlet is at creek bed while the inlet is partially silted.

SUB-WATERSHED 09:
• Map Panel: C-6 and C-8
• Basins: 09_B_01
• Culverts: 09_C_01
• Drainage area (acres): 189
• Hydrologic Characteristics: Sub-watershed 09 extends from south of Hwy 68 to the southern drainage divide. A narrow ravine along Boots Road drains the area, which has moderate to very steep slopes. • Hydraulic Facilities: Basin 09_B_01 is a large, constructed detention basin with an engineered notched weir outlet structure. Culvert 09_C_01 is a 40 inch HDPE pipe which captures water from the basin outlet structure via a wide swale, carries the flow under Whip Road, and then under Boots Road, with manhole access between the crossings. Flow exits the culvert via an energy dissipation outlet structure into sub-watershed 08.

SUB-WATERSHED 10:
• Map Panel: C-7
• Basins: 10_B_01, 10_B_02, 10_B_03, 10_B_04
• Culverts: 10_C_01, 10_C_02, 10_C_03
• Drainage area (acres): 308
• Hydrologic Characteristics: Sub-watershed 10 extends from the northern drainage divide to Sub-watershed 07. Slopes vary from steep to relatively flat and the lower slope portions of the shed are occupied by the Pasadera Golf Course. Multiple basins and drainage pipes form the golf Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1144, Packet Page 1244

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course storm water retention and drainage system. Of these, four basins and three culverts were investigated and modeled in this study. • Hydraulic Facilities: Basins 10_B_01, 10_B_02, 10_B_03, 10_B_04 interconnect via pipes and surface swales such that storm water moves down slope in series through the basins. The storage in the basins was included in the hydrologic modeling, but the pipes and swales were not investigated in detail or analyzed. Culvert 10_C_01 is a 36 inch CMP carrying flow from basin 07_B_01 south under Hwy 68. Culvert 10_C_02 is a 60 inch RCP which carries the main stem under Boots Road at the Hwy 68 junction. Culvert 10_C_03 is a 36 inch CMP which drains basin 10_B_04 and carries the flow west under the Pasadera entrance at Hwy 68.

SUB-WATERSHED 11:
• Map Panel: C-5 and C-7
• Basins: 11_B_01, 11_B_02, 11_B_03
• Culverts: None
• Drainage area (acres): 344
• Hydrologic Characteristics: Sub-watershed 11 is west of shed 10, includes the north and west portions of Pasadera Golf Course and portions of Laguna Seca Golf Ranch, and contains several detention ponds associated with the golf courses. Three basins were included in the hydrologic modeling, but no culverts or other pipes were included in the hydraulic analyses or field investigation.
• Hydraulic Facilities: Basin 11_B_01 is a large pond in the south-central portion of the shed and receives water from a small portion of the shed. It is drained by two risers leading to 18 inch and 30 inch CMPs. Basin 11_B_02 was combined with 10_B_04 for modeling purposes. It has a single submerged pipe which carries water to basin 10_B_04. Basin 10_B_04 outlet is a grated concrete structure with a 36 inch CMP, leading to a junction with culvert 10_C_03. Basin 11_B_03 (sheet C-5) sends water to basin 12_B_01 via an overflow across a golf cart path; the installed outlet pipe is buried.

SUB-WATERSHED 12:
• Map Panel: C-5 and C-7
• Basins: 12_B_01, 12_B_02
• Culverts: 12_C_01
• Drainage area (acres): 90
• Hydrologic Characteristics: Sub-watershed 12 drains a portion of the Laguna Seca Golf Ranch. Slopes vary from moderate in the north to relatively low in the vicinity of the basins.
• Hydraulic Facilities: The two basins operate in series with two 18 inch CMPs connecting them under a cart path. Basin 12_B_02 drains to a swale via Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1145, Packet Page 1245

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24 inch and 12 inch CMPs. Culvert 12_C_01 carries the main stem flow southward under Hwy 68 in a 48 inch CMP.

SUB-WATERSHED 13:
• Map Panel: C-5
• Basins: None
• Culverts: None
• Drainage area (acres): 99
• Hydrologic Characteristics: Sub-watershed 13 is a narrow drainage north of Hwy 68 which extends to local hilltops. There is no well-defined water course; runoff continues overland to the main stem.
• Hydraulic Facilities: None
SUB-WATERSHED 14:
• Map Panel: C-5 and C-6
• Basins: None
• Culverts: 14_C_01
• Drainage area (acres): 148
• Hydrologic Characteristics: Sub-watershed 14 extends south from Hwy 68 up a very steep rise to a local ridge separating this shed from shed 18. A single incised channel carries the bulk of the runoff to the main stem. • Hydraulic Facilities: Culvert 14_C_01, a 48 inch CMP, carries the main stem and runoff from this shed north under Hwy 68. The channel is narrow with steep sides. The culvert entrance is at a concrete headwall, while the outlet is above a concrete apron designed to resist scour. The apron is undercut and failing.

SUB-WATERSHED 15:
• Map Panel: C-5
• Basins: None
• Culverts: 15_C_01
• Drainage area (acres): 156
• Hydrologic Characteristics: Sub-watershed 15 extends north from Hwy 68 and includes much of Laguna Seca Golf Ranch. It consists of a broad westward sloping apron along Hwy 68 with steep slopes to the north. • Hydraulic Facilities: Culvert 15_C_01, a 48 inch CMP, carries the main stem parallel to Hwy 68 and under a drive way. The culvert entrance is a vertical shaft with a trash rack (not currently installed). The outlet is through a head wall with invert at channel invert.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1146, Packet Page 1246

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SUB-WATERSHED 16 A & B:
• Map Panel: C-5 and C-6
• Basins: None
• Culverts: 16_C_01
• Drainage area (acres): a: 65, b: 104
• Hydrologic Characteristics: Sub-watershed 16 a & b is a narrow shed along the south side of Hwy 68 which extends to a ridge dividing it from shed 18. Nearly the entire area is very steep, with a single defined drainage running north to the highway.
• Hydraulic Facilities: Culvert 16_C_01, a 24 inch CMP, carries local runoff northward under Hwy 68. The culvert entrance is flared, while the outlet opens above the main stem channel.

SUB-WATERSHED 17:
• Map Panel: C-5
• Basins: None
• Culverts: 17_C_01
• Drainage area (acres): 149
• Hydrologic Characteristics: Sub-watershed 17 extends north from Hwy 68.

Slopes are moderate, with a single drainage channel extended ½ way up into the watershed.
• Hydraulic Facilities: Culvert 17_C_01 is 14.5 foot by 8 foot elliptical culvert with a dirt floor. This culvert carries the main stem parallel to Hwy 68 and under York Road. Both entrance and outlet are flared with concrete headwalls.

SUB-WATERSHED 18:
• Map Panel: C-5 and C-6
• Basins: None
• Culverts: 18_C_01
• Drainage area (acres): 964
• Hydrologic Characteristics: Sub-watershed 18 is a large shed extending south from Hwy 68 to the watershed divide. A single dendritic creek drains the area, which converges at the highway immediately west of York Road. Slopes are highly variable, with multiple steep rises to hilltops.

Numerous dirt roads cross the area.
• Hydraulic Facilities: Culvert 18_C_01, a 6 foot by 4 foot box culvert, carries runoff from the shed north under Hwy 68 to the main stem. There are headwalls on both ends of the culvert.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1147, Packet Page 1247

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SUB-WATERSHED 19:
• Map Panel: C-5
• Basins: 19_B_01
• Culverts: None
• Drainage area (acres): 99
• Hydrologic Characteristics: Sub-watershed 19 is a small shed on the north side of Hwy 68 with York Road running through its center. Slopes are moderate with no well-defined channel. A local storm drain network installed in the Ryan Ranch development drains into basin 19_B_01 • Hydraulic Facilities: Basin 19_B_01 is a small basin at the southeast end of the shed immediately north of the main stem channel. Inflow is primarily from a 24 inch RCP and outflow is via a 24 inch CMP with a 24 inch riser covered by a trash rack. High flows are passed by a 3 foot wide trapezoidal grass overflow spillway and grass swale.

SUB-WATERSHED 20:
• Map Panel: C-3, C-5 and C-6
• Basins: None
• Culverts: None
• Drainage area (acres): 320
• Hydrologic Characteristics: Sub-watershed 20 is located between Hwy 68 and the southern drainage divide. Slopes are generally steep with one dendritic channel draining much of the area.

SUB-WATERSHED 21:
• Map Panel: C-5
• Basins: 21_B_01
• Culverts: 21_C_01
• Drainage area (acres): 123
• Hydrologic Characteristics: Sub-watershed 21 drains the relatively low slope Ryan Ranch industrial area between Hwy 68 and shed 28 via a local storm drain network. The network elements drain to a swale which flows to basin 21_B_01.
• Hydraulic Facilities: Basin 21_B_01 receives flow from a small upstream basin (not field investigated or modeled) via a concrete weir and spillway.

Both basins are located in the downslope, southwest corner of the shed.

The upstream basin receives the runoff from the storm drains. The outflow from basin 21_B_01 is controlled by a concrete outlet structure containing a square notch weir. Culvert 21_C_01 consists of three 28 inch by 24 inch elliptical fiberglass pipes and carries the main stem south across Hwy 68.

Both inlet and outlet are in concrete headwalls.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1148, Packet Page 1248

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SUB-WATERSHED 22:
• Map Panel: C-5
• Basins: 22_B_01
• Culverts: None
• Drainage area (acres): 9.6
• Hydrologic Characteristics: Sub-watershed 22 is a very small shed encompassing a single steep drainage immediately north of Hwy 68. • Hydraulic Facilities: Basin 22_B_01 receives flow from the drainage swale; outflow to local drainage swale is controlled by a weir in a narrow concrete flume.

SUB-WATERSHED 23:
• Map Panel: C-6 and C-3
• Basins: None
• Culverts: None
• Drainage area (acres): 454
• Hydrologic Characteristics: Sub-watershed 23 is a large shed south of Hwy 68 which extends to the watershed divide and is drained by an extensive dendritic channel network. Slopes are general steep with incised valleys.

There are no natural basins or ponds.

SUB-WATERSHED 24:
• Map Panel: C-3 and C-2
• Basins: 24_B_01, 24_B_02
• Culverts: None
• Drainage area (acres): 160
• Hydrologic Characteristics: Sub-watershed 24 is located between sheds 23 and 26 and drains a moderately steep, narrow area with a single swale. Two basins have been constructed in series at the north end of the shed, with lower basin discharging into the main stem.
• Hydraulic Facilities: Basin 24_B_01 receives flow from the swale; outflow to basin 24_B_02 is controlled by the combination of a 36 inch CMP vertical riser plus culvert and an 8.7 foot wide spillway. The riser opening is at the same elevation as the spillway crest. Basin 24_B_02 drains via three 15 inch CMP, which run under a driveway.

SUB-WATERSHED 25, 25B, 25C:
• Map Panel: C-2 and C-5
• Basins: 25_B_01, 25_B_02
Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1149, Packet Page 1249

CANYON DEL REY MASTER DRAINAGE PLAN
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• Culverts: 25_C_01, 25_C_02, 25_C_03
• Drainage area (acres): 161
• Hydrologic Characteristics: These sub-watersheds, combined, drain relatively low sloped areas between the main stem and shed 28, primarily on the northeast side of Hwy 68. Most of the runoff occurs overland, with two short defined channels in the vicinity of the highway. • Hydraulic Facilities: Basin 25_B_01 is located in Ryan Ranch business park and intercepts local runoff. Outflow from this basin is via both a 24/36 inch vertical, perforated riser connected to an 18 inch CMP culvert and a separate 18 inch CMP culvert. Basin 25_B_02, basin on the main stem, was not surveyed, but is controlled by culvert 25_C_01. Culvert 25_C_01 consists of three 18 inch RCP and two 48 inch RCP, which carry the main stem westward under Monterra Ranch Road. Culvert 25_C_02 consists of two 48 inch RCP, which carry the main stem northward under Hwy 68.

Culvert 25_C_03 is a 14 foot wide by 7.7 foot high concrete box culvert which carries the main stem under a driveway on the west side of Hwy 218.

SUB-WATERSHED 26:
• Map Panel: C-2 and C-3
• Basins: 26_B_01
• Culverts: 26_C_01
• Drainage area (acres): 324
• Hydrologic Characteristics: Sub-watershed 26 is long, narrow shed with an incised channel and moderately steep to very steep slopes, located between Hwy 68 and the southern drainage divide. Portions of this shed border the Monterey airport. The channel has one basin located near the highway.
• Hydraulic Facilities: Basin 26_B_01 controls much of the runoff from this shed and is located in the northern, downstream part of the shed.

Outflow is controlled by a high flow spillway and a lower flow outlet consisting of three 24 inch CMP risers and pipes through the containment berm. Culvert 26_C_01 is a 36 inch CPP which carries runoff from this shed northward under Hwy 68 and enters the storm drain system via a 3 foot by 4 foot concrete box with an 18 inch inflow and 36 inch outflow pipe.

SUB-WATERSHED 27 AND 27B:
• Map Panel: C-2
• Basins: 27b_B_01
• Culverts: 27_C_01, 27_C_02, 27_C_03, 27_C_03
• Drainage area (acres): 129
• Hydrologic Characteristics: Sub-watersheds 27 and 27b contain the Stone Creek Center and other commercial properties. Shed 27 extends from the Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1150, Packet Page 1250

CANYON DEL REY MASTER DRAINAGE PLAN
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airport boundary nearly to the Fort Ord reservation boundary and includes the main stem and Hwy 218. Shed 27b extends along the east side of Hwy 218 to a regional park. Slopes are highly variable, from very step at the airport boundary to very flat in the valley floor. Local storm drains route most of the flow in shed 27.
• Hydraulic Facilities: Culverts 27_C_01 and 27_C_02 are 14 foot by 8 foot high concrete boxes which carry the main stem under Stone Creek Center entrances from Hwy 218. Culvert 27_C_03 is a 7.3 foot diameter CMP which carries the main stem under the Del Rey Gardens entrance on the west side of Hwy 218. Culvert 27_C_04 is a 6 foot wide by 8 foot high concrete box culvert which carries the main stem eastward under Hwy 218. Both inlet and outlet have retaining wing walls.

SUB-WATERSHED 28:
• Map Panel: C-1, C-2, C-4, C-5
• Basins: None
• Culverts: 28_C_01, 28_C_02
• Drainage area (acres): 1201
• Hydrologic Characteristics: Sub-watershed 28 is a large, elongated shed which is entirely within the Fort Ord reservation and drains northwestward via both local swales and a well-defined channel near the south boundary of the shed. Slopes vary from relatively high in the eastern portions to low in the far west portion.
• Hydraulic Facilities: Culvert 28_C_01 is a 9.6 foot by 8 foot high box culvert which carries the main stem under General Jim Moore Road on the north side of its intersection with Hwy 218. Culvert 28_C_02 is a 3 foot by 3 foot concrete box which carries the runoff from this shed westward under General Jim Moore Road and into basin 29_B_01.

SUB-WATERSHED FP:
• Map Panel: C-1 and C-2
• Basins: 29_B_01
• Culverts: 29_C_01
• Drainage area (acres): 67
• Hydrologic Characteristics: Sub-watershed FP is a small area immediately surrounding the Frog Pond basin which includes portions of the regional park. Slopes are relatively low; much of the shed, other than the park, is developed. The main stem winds through a woodland before entering the basin.
• Hydraulic Facilities: Basin 29_B_01 is the frog pond, which detains flows on the main stem. It is a natural depression, with water level controlled by a spillway which is part of culvert 29_C_01. This culvert is a 6 foot wide by 8 foot high box culvert which carries the basin outflow south under Hwy 218.

Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1151, Packet Page 1251

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SUB-WATERSHEDS 29 AND 29B:
• Map Panel: C-2
• Basins: 29_B_02
• Culverts: 29_C_02, 29_C_03, 29_C_04, 29_C_05, 29_C_06, 29_C_07, 29_C_08 • Drainage area (acres): 29: 138; 29b: 246
• Hydrologic Characteristics: Sub-watershed 29 encompasses a relatively flat, high area and includes the majority of the municipal airport.

Drainage, including portions of the airport drained by culverts, flows west and north to a single swale with a basin controlling flow to shed 29b and the main stem. Sub-watershed 29b is a largely developed area, with exception of park lands along the channel. The channel runs alongside the park and through developments; it is a narrow, deep notch with limited capacity. Local runoff enters the channel via storm drains.

Multiple bridges and culverts carry the flow under driveways and streets. • Hydraulic Facilities: Basin 29_B_02 captures much of the drainage from the eastern portions of the airport. Outlet control consists of a 12 foot wide trapezoidal spillway, a 30 inch RCP under the berm, and a 48 inch RCP and riser with a 36 inch grate on the entrance. Culvert 29_C_03 is a private wood deck over the channel. Culvert 29_C_04 is a driveway bridge. Culvert 29_C_05 is a driveway bridge with a 48 inch CMP under a concrete apron. Culvert 29_C_06 is a driveway bridge. Culvert 29_C_07 is a 6 foot wide by 6.8 feet high concrete box carrying flow under Rosita Ave. Culvert 29_C_08 is a 8 foot wide by 8 foot high concrete box with wing walls carrying flow under Freemont Blvd. The inlet has a trash rack.

SUB-WATERSHEDS 30:
• Map Panel: C-1 and C-2
• Basins: Laguna Grande, Roberts Lake
• Culverts: 30_C_01, 30_C_02, 30_C_03, 30_C_03, 30_C_04, 30_C_05 • Drainage area (acres): 1011
• Hydrologic Characteristics: Sub-watershed 30 includes a portion of the regional airport and much of Seaside. Almost the entire shed is urbanized and drained by storm drain networks. Slopes are mild to flat. Roberts Lake and Laguna Grande and associated park land are located immediately upstream of Hwy 1 and the beach. These lakes control outflow to the ocean.
• Hydraulic Facilities: Culvert 30_C_01, a 6 foot by 6 foot concrete box, is located immediately north of Fremont Blvd. and carries flow under an earthen berm. Culvert 30_C_02 is a park foot bridge set on pilings over the channel at its entrance into Laguna Grande. Culvert 30_C_03 consists of a 16 foot wide by 7 foot high concrete channel connecting Laguna Grande to Roberts Lake. Bridges for Del Monte Blvd. and a foot path span the channel. Culvert 30_C_04 consists of two 8 foot wide by 6 foot high Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1152, Packet Page 1252

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concrete boxes and two 24 inch gate valved low flow pipes. An inlet weir sets the normal lake elevation. This culvert runs under Roberts Avenue.

Culvert 30_C_05 consists of four 6 foot wide by 6 foot high concrete boxes.

The outlets are partially filled with sand. This culvert carries flow under the Hwy 1 interchange structure and highway overpass and discharges adjacent to the Monterey Beach Hotel.

2.6 Lake surveys and mapping
The condition of the Laguna Grande - Roberts Lake complex and the rate of accumulation of sediment in the lakes were evaluated via a field investigation and a bathymetric survey of bottom elevations.

HYDROGRAPHIC MAPPING METHODS
The CSUMB Seafloor Mapping Lab performed comprehensive high resolution hydrographic survey of both Roberts Lake and Laguna Grande on April 7-10, 2013. Swath bathymetry and acoustic backscatter data were collected using an SEA SwathPlus interferometric sidescan sonar system coupled with an Applanix POS MV inertially aided GPS positioning and attitude measurement system mounted on a small skiff. All measurements were reference to the vertical and horizontal control benchmark specified by the sponsor (figure 1).

This benchmark was occupied with a Trimble NetR5 geodetic grade GPS receiver during the survey and the data were used to post-process the POS MV position data to better than 0.05 ft precision. The channel area located just south of the roadway separating the two bodies of water was surveyed manually using a leadline and GPS because the very low foot bridge at the south end of that channel prevented access by the sonar survey vessel.

All bathymetry data were processed to IHO Special Order standards in CARIS hydrographic software, and used to generate final bathymetry DEMs and soundings at 2ft horizontal spacing and 0.01 ft vertical precision. Sounds were gridded at 2ft cell size with 5x5 interpolations to eliminate any minor data gaps.

Acoustic backscatter data were processed into mosaics using SAE SwathPlus software and the results classified by echo return intensity in ArcGIS to reveal sediment texture patterns on the basin floors.

All results were project and referenced as follows:
• Coordinate system: State Plane NAD83 CA 4 US Feet
• Datum: NAVD88(Geoid12a) feet
• Reference bench mark position: N: 2115125.842 E: 5723380.903 EL:19.155 (NGS ’83, CA Zone IV, NAVD ’88, based on Geoid 12A)

FINAL PRODUCTS
Products provided in digital submittals include:

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• digital elevation models (DEM) in ArcView Grid formats • shaded relief images in greyscale and colored by depth in GeoTiff format • xyz point data at 2ft spacing as text files
• classified acoustic backscatter mosaics in GeoTiff format • Survey vessel tracks in shapefile format
• Channel xyz data in text and shapefile format
• ArcGIS project with all associated product data layers

MAPPING RESULTS
The bathymetric mapping results (Figure 2-2 and Table 2-3) show all of Roberts Lake and > 85% of Laguna Grande to be shallower than 7 ft. deep. The bottoms of the basins are both quite flat with the only relief found along the banks and associated with the small islands. Curves relating elevations (NAVD) to lake volumes are provided in Figure 2-3.

The acoustic backscatter (sidescan sonar) echo return intensity results suggest a greater coverage by finer sediments in the deeper areas of Laguna Grande than in Roberts Lake (purple class in Figure 2-2). Finer sediments have weaker echo returns. Of particular note are the surface tracks still visible on the floor of Roberts Lake that show in the backscatter image because they appear to have been filled in with finer (purple) sediments. These track marks are visible in both the classified mosaic (Figure 2-4) and the unclassified sidescan sonar mosaic (Figure 2-5), and were likely created either during basin construction or subsequent dredging. The fact that the tracks are still visible suggests that there has been relatively little in the way of sediment accumulation since they were formed.

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Cooler colors are deeper and warmer colors shallower. Horizontal and vertical control benchmark is shown on southwest shore of Laguna Grande.

Figure 2-2: Bathymetry of Roberts Lake (top) and Laguna Grande (bottom) shown in 1 foot depth zones as elevation in feet NAVD88.

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Depth Zone
(ft)
Laguna Grande Roberts Lake
Area (acres) Percent Area Percent Volume Area (acres) Percent Area Percent Volume 1 0.02 0.2% 0.0% 0.18 1.6% 0.3%
2 0.08 0.7% 0.2% 0.27 2.4% 0.9%
3 0.25 2.1% 1.0% 0.57 5.1% 2.7%
4 0.87 7.4% 4.7% 0.95 8.6% 6.1%
5 1.68 14.3% 11.5% 1.76 15.9% 14.1%
6 2.77 23.5% 22.6% 3.89 35.1% 37.2%
7 4.86 41.3% 46.4% 3.48 31.4% 38.8%
8 1.22 10.4% 13.3% 0.00 0.0% 0.0%
9 0.01 0.1% 0.2% 0.00 0.0% 0.0%
Total 11.77 100.0% 0.0% 11.10 100.0% 100.0%
Table 2-3: Percent area and volume of Lakes for corresponding depth zones.

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0 10
20 30
40 50
60 70
80 0 1 2 3 4 5 6 7 8 9 10
Lake Volume (acre-ft)
Elevation (Feet NAVD)
Elevation Volume Curves for Lakes
Laguna Grande
Roberts Lake
Figure 2-3: Elevation-volume curves for Laguna Grande and Lake Roberts.

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Results suggest a higher coverage of fine sediments (purple class) in the deeper areas of Laguna Grande than in Roberts Lake. Note the remnants of tracks now filled with finer sediments on the bottom of Roberts Lake still visible from either the original basin construction or past dredging.

Figure 2-4: Acoustic backscatter echo return intensity classified into natural breaks.

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Stronger returns are shown as lighter and weaker returns as darker grey. The relic track marks in the deeper parts of Roberts Lake are also clearly visible in these “raw” data.

Figure 2-5: Unclassified sidescan sonar backscatter intensity mosaic - hydrologic conditions and precipitation estimates
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3 HYDROLOGIC CONDITIONS AND PRECIPITATION ESTIMATES

3.1 Available precipitation gages and data
The following rainfall records were found for the vicinity of Canyon Del Rey.  Salinas Airport (Hrly) - 7/1/48-9/1/1951, 4/1/2001-11/20/2011 - incomplete
 Del Monte, Monterey (15 min) - 5/2/1971 - 6/29/1995 generally complete, some missing data
 Naval Post Graduate School (Daily) - 1970 – 2010 generally complete, some missing data
 Monterey County ALERT Data Stations (Cumulative Rainfall) - Point Pinos: 1/3/2007 - 2/11/2013 readings every approximately 12 hrs.  Mt Toro: 7/27/2006 - 2/11/2013 readings every approximately 12 hrs - Blanco Circle: 7/27/2006 - 2/11/2013 readings every approximately 12 hrs
 CIMIS Data - Carmel, #210: 10/24/2008 – current, hourly; Pacific Grove, #193: 10/26/2011 – current, hourly
 Fort Ord CDEC Station ( Hourly Rainfall) - WY 2002 -2011, Full record, good quality
 KMRY - Monterey Regional Airport, NWS (Daily Rainfall) - 1/1/1970 - 12/31/2010, Full record, good quality
 MPWMD - 187 Eldorado (Daily Rainfall) - 10/25/1991 - 9/23/2000 Hand recorded, good quality
 MPWMD - 5 Harris Ct., Ryan Ranch, Monterey (Daily Rainfall) - 10/10/2000 - 9/11/2012, Hand recorded, good quality
 Laguna Seca Golf Course (Daily Rainfall) - obtained limited data set:

WY2012 - current
 Weather Underground data sources - daily data, short term records The locations of rain gages with useful records are shown in Figure 3-1, which is a reproduction of the 1977 master plan isohyet map with gage locations added.

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3.2 Precipitation data quality and utility
Long term rainfall records are required to generate usable depth/duration/frequency relationships. With the exception of the NOAA del Monte and the NOAA Monterey gages, none of the gage records have a long enough duration to yield usable statistics. In addition, most of the records provide only daily values, rather than the hourly and 15 minute records needed for frequency analyses. Correlations between records (gage cross-correlation) can sometimes be used to fill and extend missing data in a gage records using the full data set for another gage record. Such correlations can also be used, when records are sufficiently long and detailed, to create hourly or 15 minute data for a gage record which is limited to daily or hourly data.

Figure 3-1: Rainfall gages with useful records.

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In order to quickly assess the potential for record filling using correlations, the annual precipitation values for all of the useful gages over the overlapping record periods were correlated, producing correlation coefficients for each gage pair. Table 3-1 provides the results of that exercise.

For most gage pairs, the number of values in the calculations varied from 3-12.

The lightly shaded results are for gage pairs using 23-39 values. Higher numbers of values produce correlation estimates which are more reliable, so that a number close to 1 for a gage pair with over 23 values indicates a well correlated gage pair. The strengths of the correlations, with the exception of the NOAA del Monte – NOAA Monterey pair, are poor to fair. Together, these two gages provide a record spanning 1949-2012, which is sufficient to generate long term statistics. However, poor correlations with the other useful gage records make extending and filling the other gage records unrealistic.

Conclusions regarding the utility of the available rainfall gage data for generating depth/duration/frequency estimates and other statistics are summarized below.
• Record lengths vary from 2 to 40 years; gage correlations only fair • Measurement frequencies are mostly daily; exceptions: Salinas AP is hourly; Monterey NWS is 15 min; ALERT twice daily
• Gage locations are mostly near sea level; exceptions: Laguna Seca Golf gage is at 370 ft, Ft Ord at 490 ft
• Quality of the data is generally good, but not tested • Extension of short gage records using longest records is possible, but the quality of results questionable; correlations are based on < 12 yrs Table 3-1: Correlations of annual precipitation values between gage records.

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• Poor spatial distribution of gages and inconsistency between gages limits use of gages for isohyet development
• Adjustment of precipitation estimates for elevation and topographic orientation problematic due to lack of data
• Production of adequate precipitation/ frequency estimates from gage data seems unlikely
The conclusion is that wholesale modification of the 1977 isohyet map cannot be supported by the available data. This is particularly true because the gage data produces substantially different MAP estimates over much of the watershed, so that isohyet map modifications would be controversial and difficult to support with the data. Further, the precipitation records are not sufficient to enable development of depth/duration/frequency relationships.

3.3 Selected methodology for developing rainfall depth estimates and storm hyetographs
NOAA Atlas 14 is the data server implemented precipitation-frequency atlas for the United States; Volume 6 of the atlas provides the data for California. The details of the atlas, including technical bases for its development, are provided in a very thorough report (NOAA, 2012). NOAA Atlas 14 contains precipitation frequency estimates for a range of durations and frequencies and other information on temporal distribution of rainfall in California. Estimates are provided with 90% confidence intervals and at a 30 arc-second (approximate ½ mile) spatial resolution. Event durations range from 5 minutes to 60 days.

Recurrence intervals range from 1 year to 1,000 years.

Locations for estimates are selected graphically on a California map, by specifying latitude and longitude, or by selecting specific stations where the detailed analyses were developed.

Complete documentation of the NOAA Atlas 14 server is available at http://www.nws.noaa.gov/oh/hdsc/PF_documents/Atlas14_Volume1.pdf.

Precipitation-frequency estimates are obtained from the Atlas using a graphical user interface available at the following
URL: http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=ca.

Examples of Atlas 14 results for the Canyon Del Rey watershed are provided in Table 3-2, which shows precipitation depths for specific storm durations and frequencies at 10 points which represent a variety of elevations and orientations to incoming storm events.

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These results are internally consistent and reasonable when compared with PRISM results and the San Francisco Bay Depth-Duration-Frequency tables which were used in the 1977 Master Plan. Table 3-3 provides a brief comparison of results for locations at low, moderate and higher elevations.

The Atlas 14 results are generally higher than the Plan results for less frequent events and very similar for more frequent events. The variation of precipitation depths with mean annual precipitation is similar. The differences appear to be within the estimation error range.

3.4 Mean annual precipitation estimates
Table 3-4, below, summarizes the mean annual precipitation statistics for the useful gaging stations and compares them with the average annual precipitation estimates taken from the 1977 Master Drainage Plan.

Grid Results from NOAA Atlas 14
Location Lattitude Longitude Elevation
100 yr
24 hr
100 yr
6 hr
100 yr
60 day
10 yr
24 hr
10 yr
6 hr
10 yr
60 day
Laguna Del Rey 36.6037 -121.8556 13 4.78 2.76 21.2 2.95 1.73 14.5 Naval Post Graduate School 36.5972 -121.8776 35 4.7 2.74 21.1 2.89 1.72 14.2 Hwy 68/ 218 junction 36.5812 -121.8279 123 5.13 2.92 22.9 3.16 1.84 15.8 Hwy 218/ Moore Blvd Jct 36.5911 -121.8322 146 5 2.86 22.3 3.08 1.8 15.3 Laguna Seca Golf Ranch 36.572 -121.7876 382 5.54 3.08 24.5 3.43 1.96 17.2 Fort Ord CDEC gage 36.627 -121.786 480 5.12 2.87 22.7 3.17 1.81 15.9 S Boundary Rd, N of Laguna Seca Ranch 36.5795 -121.7885 600 5.48 3.05 24.5 3.39 1.94 17.2 Tehama Golf, S of 68/218 Jct 36.5587 -121.8329 750 5.34 3 24.1 3.29 1.9 16.7 Boots and Saddle Rd Jct, S of Laguna SR 36.5528 -121.7825 1056 5.76 3.15 25.4 3.58 2.02 17.9 Laureles Grade, top of grade 36.5446 -121.7534 1243 5.83 3.16 25.7 3.65 2.04 18.1 Table 3-2: Precipitation-Duration-Frequency results for Canyon Del Rey watershed. Precipitation depths in inches.

Comparison of NOAA Atlas 14 DDF Results with 1977 Master Plan - SF Bay DDF Table Longitude Lattitude Elevation
Mean Ann
Precip
(PRISM)
Precip
from Plan
Table
Atlas 14
100yr 12
hr
977
Master
100yr 12
hr
Atlas 14
10yr 12 hr
1977
Master
10yr 12 hr
Atlas 14
100yr 1 hr
1977
Master
100yr 1 hr
-121.75 36.546 668 20.18 20 4.17 3.45 2.63 2.6 1.33 1
-121.8131 36.5818 279 17.85 18 3.75 3.23 2.34 2.42 1.4 0.96 -121.8776 36.5972 36 15.95 16 3.35 3.01 2.08 2.24 1.43 0.91 Table 3-3: Comparison of Atlas 14 and 1977 Master Plan DDF estimates. Precipitation depths in inches.

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Long term average annual precipitation estimates were computed for three of the gages (MPWMD at Harris Court, MPWMD at El Dorado Way, and Laguna Seca Golf Course) by taking the ratio of mean annual precipitation (MAP) for each of these gages to the MAP for the NOAA Monterey gage for the same period of record and then multiplying that ratio times the long term average MAP for the NOAA Monterey gage.

Figure 3-2 provides the MAP at these gages on the 1977 Isohyetal map, allowing a quick comparison between the 1977 MAP estimates and those obtained from the gage data. Several observations can be made from Figure 3-2 and Table 3-4:
• The MAP for each gage is similar to the values shown on the isohyetal map in the 1977 Master Drainage Plan at only a few points: MPWMD Harris Court, NOAA del Monte, and Laguna Seca Golf Course. These gages are located along the centerline of the watershed.
• Gages located away from the watershed centerline have large variations from the isohyetal map. The reasons for this variation are not obvious; possibilities include: much longer records are now available for some gages – resulting in shifts in gage statistics, the 1977 map utilized gages whose records are no longer available, and more gage locations are available now.
• The table shows large differences in MAP for gages in close proximity, making interpretation of the results problematic.
• The spatial distribution of the gage sites is inadequate for defining new isohyet contours and, therefore, for generating a revised isohyet map.

CDEC Fort
Ord
MPWMD
Harris
MPWMD El
Dorado
NOAA del
Monte
NOAA
Monterey
Naval Post
Grad School
Laguna Seca
Golf Course
Record Duration
WY 2002-
2011
WY 2001-
2012
WY 1992-
2000
WY 1949-
1994
WY 1996-
2011
WY 1971-
2010
WY 2012-
2013
Avg Ann Precip (inch) 11.53 15.04 23.2 12.44 14.48 20.29 14.91 Long Term Avg Ann Precip 17.8 20.4 15.63
Location in watershed NE of bndry Central W of bndry N edge W Central NW Edge E Central Avg Ann Precip from Map 13-14 15 14-15 12.5 12.5 13 14.8 Ratio of gage to isohyet 85.4% 100.3% 160.0% 99.5% 115.8% 156.1% 100.7% Table 3-4: Comparison of Mean Annual Precipitation Estimates.

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The PRISM (Parameter-elevation Regressions on Independent Slope Model) climate mapping system produces estimates of long term climate parameter statistics and time series for any location in the United States. The technical basis for the PRISM methodology is published in Daly, et al, 2008. The PRISM data can be found at http://prism.oregonstate.edu/normals/, while descriptions of the PRISM data sets can be found
at: http://prism.oregonstate.edu/documents . Mean annual precipitation (MAP) is one of the available parameters, making PRISM a possible source for long term annual precipitation estimates. The PRISM data server was queried for points within the Canyon Del Rey watershed. The resulting MAP data is shown as isohyetal lines in Figure 3-3. The PRISM results, when compared with the gage data, appear to under-predict the effects of orientation and elevation and generally reduce the spatial variability of rainfall within the watershed. The correlation between gage annual means and PRISM results is also relatively low. However, the PRISM system uses all of the long term precipitation gage records in the vicinity of the study area and, as a result, is the most complete estimate of mean annual precipitation.

Figure 3-2: Mean Annual Precipitation statistics overlain on the 1977 isohyet map.

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3.5 Depth-Duration-Frequency estimation from NOAA Atlas 14 results Rainfall depth estimates were generated from NOAA Atlas 14 output for each sub-watershed within the Canyon del Rey watershed. The centroid of each sub-watershed was used for the estimation location, which was input into the NOAA Atlas application. The depth values were extracted for return periods of 10 and 100 years and durations of 15 minutes through 24 hours. Table 3-5 provides a summary of generated depths for each of the sub-watersheds for 10 year and 100 year return periods and 24 hour durations.

Figure 3-3: Mean Annual Precipitation Isohyets for Canyon del Rey.

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TOTAL 24 HOUR RAINFALL DEPTHS (inches)
Sub-watershed 10-year 100-year Sub-watershed 10-year
100-
year
Sub_LS 3.42 5.50 Sub_17 3.38 5.47
Sub_01 3.49 5.60 Sub_18 3.50 5.65
Sub_02 3.56 5.72 Sub_19 3.32 5.39
Sub_03 3.47 5.58 Sub_20 3.32 5.39
Sub_04 3.51 5.65 Sub_21 3.27 5.30
Sub_05 3.57 5.74 Sub_22 3.21 5.22
Sub_06 3.48 5.61 Sub_23 3.24 5.27
Sub_07 3.50 5.68 Sub_24 3.17 5.16
Sub_08 3.56 5.74 Sub_25 3.21 5.22
Sub_09 3.51 5.66 Sub_25b 3.21 5.21
Sub_10 3.44 5.54 Sub_25c 3.16 5.13
Sub_11 3.47 5.60 Sub_26 3.15 5.12
Sub_12 3.51 5.66 Sub_27 3.16 5.13
Sub_13 3.43 5.54 Sub_27b 3.08 5.00
Sub_14 3.47 5.61 Sub_28 3.27 5.31
Sub_15 3.43 5.54 Sub_FP 3.08 5.00
Sub_16a 3.47 5.61 Sub_29 3.04 4.95
Sub_16b 3.43 5.54 Sub_29b 3.04 4.95
Sub_30 2.97 4.82
Table 3-5: 10 year and 100 year, 24 hour duration rainfall depth estimates.

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4 HYDROLOGIC ANALYSES AND MODELING

4.1 Available data
Available stream flow gage data is limited to peak flow records for 1967-1978 and 2003-present and 15 minute stream flow data for 2003-2013. All records are for the Arroyo del Rey gage at Del Rey Oaks. Figure 4-1 shows the peak flow at this gage for the years of record. No large storm events are included in the years of record (in particular, the 1995 and 1998 events are missing), resulting in relatively low peak flows.

Based on the available gage data, a return period analysis can be made, producing the relationship in Oversized Figure 3. While this relationship may be representative of frequent events (recurrence intervals of 5 years and less), it is not descriptive of less frequent events.

Figure 4-1: Annual peak flows for Arroyo del Rey at Del Rey Oaks gage.

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4.2 Description of alternative methodologies
The 1977 drainage plan used the following methods to develop runoff estimates:
• Define 28 sub-watersheds based on drainage divides, slope, land use • Create a mean annual precipitation (MAP) isohyetal map from rainfall gage data
• Develop MAP for sub-basins from USGS MAP data (from Rantz) • Estimate total storm precipitation from regional Depth Duration Frequency tables
• 100 year return period event used for primary structures • 10 year return period event used for secondary structures • Create design storm runoff for each sub-watershed using synthetic unit hydrograph method
• Obtain rainfall excess (volume of runoff) using: loss rates from USGS tables and times of concentration adjusted for urbanization
• Route runoff through the basin using Muskingum method, including effects of temporary storage above structures
Figure 4-2: Arroyo del Rey at Del Rey Oaks annual peak flows plotted against return period.

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For this study, both precipitation gage and stream flow gage data were collected and analyzed for application to the hydrologic analyses. Neither of these data sets was deemed adequate for use in developing design storm flows, due to short periods of record and inadequate areal coverage of the study area. As was true for the 1977 study, synthetic hydrologic methods are necessary.

Alternative hydrologic computation methods include:
• Frequency Analysis of Recorded Peak Flows for Study Stream • Adequate long, consistent time series record not available • Correlation with Frequency of Recorded Peak Flows in Nearby Watershed • A 40 year stream gage record on El Toro Creek is available but no suitable gage within Canyon del Rey is available for correlation • Regional Rainfall Frequency Analysis
• PRISM and NOAA Atlas on line datasets available and accepted • Rational Method
• Accepted today only for very small watersheds
• Synthetic Unit Hydrograph
• Method of choice for many settings; can be readily adjusted for variety of watershed conditions; provides complete hydrograph • Implementation via HEC-HMS, a widely used and accepted simulation package
• Continuous Simulation Modeling Using Detailed Moisture Accounting • Most robust method, representing detailed variations
• Costs not within in current project scope
Considering the above methods, the most appropriate approach involves:
development of design storm precipitation hyetographs, computation of runoff and stream flow from the rainfall for each sub-watershed, and routing of the runoff through the channel network. For this approach, selection of a rainfall loss method is required. Alternative methods include:
• Initial and Constant
• Appropriate only for watersheds lacking soil details
• Housing and Urban Development Methodology
• Similar to SCS method, but oriented specifically to central coastal region of California
• SCS Curve Number
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• Curve numbers assigned to different soil-cover complexes; pervious losses built into curve numbers
• Works well for highly pervious soils
• Soil Moisture Accounting (continuous, long term simulation) • Most complex; costs not included in project scope
Rainfall temporal distributions must also be developed and storm return periods selected. Methods for developing rainfall distributions include:
• Individual rainfall distributions for each storm duration and frequency, based on historical storms
• Available storm rainfall data is inadequate to enable this method • Balanced rainfall distribution with one distribution for all events • Includes 3, 12, 24 hour peaks in one hyetograph
• Conservative; usually adequate for flood planning

4.3 Modeling methodology applied for this work
The following methodology was selected from the methods previously described. This methodology incorporates generally accepted best practices for flood analysis, management, and design which can be executed with available information regarding the local rainfall and watershed conditions. • Storm rainfall depths obtained from the NOAA Atlas 14 regional rainfall frequency analysis application
• Comprehensive and well documented; makes maximum use of available rainfall data
• Synthetic Unit Hydrograph for hyetograph development
• Provides full storm hydrograph; allows for routing and variable losses • Balanced storm hyetograph for 24 hour rainfall event
• Includes shorter duration events; commonly accepted for flood control planning
• 10 year and 100 year return period events
• Represent appropriate range of risk; typically used in flood management planning and design
• SCS Curve Number method for rainfall loss estimation
• Widely accepted for use with permeable soils
• Muskingum routing of flows through channel system
• Properly represents runoff timing, storage, and travel lag times in natural channels
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• Use HEC-HMS simulation platform to develop stream flow hydrographs in locations within the study area
• Widely accepted as appropriate for runoff simulation and routing • Calibrate HEC-HMS against Del Rey Oaks and Frog Pond gage data • Rainfall and stream flow gage data available for several events • Improves and demonstrates representativeness of simulation results

4.4 HMS model development
The HEC-HMS simulation platform used in this study is thoroughly described in the program user manual available at
http://www.hec.usace.army.mil/software/hec-hms/documentation/HECHMS_Users_Manual_3.5.pdf and the technical reference manual available at
http://www.hec.usace.army.mil/software/hec-hms/documentation/HECHMS_Technical%20Reference%20Manual_(CPD-74B).pdf.

In this platform, a stream channel network and associated watershed is described as a series of sub-watersheds with connecting flow paths in a dendritic construction. Runoff is calculated from precipitation in each subwatershed using parameters including slope, sub-watershed geometry, percent impervious area, soil type, ground cover, and antecedent moisture. A runoff hydrograph is produced at the downstream end of each sub-watershed for a specific rainfall history (hyetograph). The hydrograph is then routed from the downstream end of the sub-watershed to a channel junction, combined at the junction with flows from other sub-watersheds and junctions, and then routed to a further downstream junction. Flow routing is calculated in the MuskingamCunge method using parameters including channel roughness, length, slope, and cross section.

Oversized Figure 3 depicts the HEC-HMS model used to simulate runoff and stream flow in the Canyon del Rey basin. The model is built using runoff elements (sub-basins), stream channel routing elements (reaches), stream confluence elements (junctions), and water storage or detention elements (reservoirs). All of these elements are shown in the figure. Water sources, sinks and diversions can also be simulated, but are not used in this study. The connectivity between sub-basins, reaches, junctions, and reservoirs are shown in the figure.

Runoff related parameters, including slope, ground cover, soil type, percent impervious, and surface roughness were developed from aerial photographs, topographic mapping, and geologic mapping. Specific input parameters for the HEC-HMS simulation platform were assigned, based on the above attributes, to each sub-watershed. The parameters used for the watersheds are provided in Table 4-1.

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SubWatershed IDArea
(sq miles)
Time Lag
(min)
Curve
Number
Percent
Impervious
Sub_LS 0.293 2 45.6 55
Sub_01 0.2 21.4 61.3 8
Sub_02 0.757 37.2 56.4 6
Sub_03 0.391 29 64 2
Sub_04 0.083 11 61.9 10
Sub_05 0.454 28.8 54 2
Sub_06 0.358 26.8 63.9 6.1
Sub_07 0.134 7.3 64.1 3
Sub_08 0.606 29.9 53.8 10
Sub_09 0.295 20 51.7 5
Sub_10 0.481 25.2 58.7 8
Sub_11 0.538 26.9 52.5 20
Sub_12 0.14 9.9 60.1 5
Sub_13 0.154 19 54.2 8
Sub_14 0.231 12.6 50.6 3
Sub_15 0.244 21 45.4 15
Sub_16a 0.101 11.5 48.8 0.5
Sub_16b 0.162 22.5 51 1
Sub_17 0.233 12.8 47.4 25
Sub_18 1.506 41.6 50.1 1.5
Sub_19 0.127 11.8 52.1 15
Sub_20 0.5 23.7 50.7 1
Sub_21 0.192 13.3 57 35
Sub_22 0.015 4 61.5 25
Sub_23 0.71 25 53 3.5
Sub_24 0.25 25.6 58.9 3.1
Sub_25 0.131 17.3 60.5 54
Sub_25b 0.018 5.4 56.2 75
Sub_25c 0.103 14.3 53.3 10
Sub_26 0.507 38.7 51.7 5
Sub_27 0.146 8.5 52.3 55
Sub_27b 0.056 12.2 42.6 20
Sub_28 1.876 60.5 28.8 2
Sub_FP 0.105 8 24.6 65
Sub_29 0.365 10.7 21.8 30
Sub_29b 0.385 12.4 28.4 35
Sub_30 1.506 17.4 22.6 70
Table 4-1: Runoff related HEC-HMS input parameters for Canyon del Rey sub-watersheds.

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Stream flow routing parameters, including channel length and geometry, hydraulic roughness, and channel slope, were developed from field reconnaissance, surveying of key cross sections, topographic mapping, and data from local agencies. Specific input parameters for the HEC-HMS simulation platform were assigned, based on the above attributes, to each routing reach. The parameters used for the reaches are provided in Table 4-2.

Reach Length (ft) Slope (ft/ft) Manning's n Shape
R_02_03 545 0.022 0.05 Trapezoid
R_06_07 807 0.0223 0.075 Trapezoid
R_07_08 1120 0.011 0.075 Trapezoid
R_11_12 1117 0.0143 0.075 Trapezoid
R_12_14 2122 0.029 0.075 Trapezoid
R_16a_15 2665 0.0056 0.075 Trapezoid
R_15_17 2086 0.012 0.075 Trapezoid
R_17_19 2064 0.0116 0.075 Trapezoid
R_19_20 1846 0.013 0.085 Trapezoid
R_23_25 3334 0.0093 0.15 Trapezoid
R_26_27 1650 0.00485 0.075 Trapezoid
R_27_27b 945 0.0106 0.075 Trapezoid
R_29 2900 0.0152 0.075 Trapezoid
R_29b 1060 0.0622 0.075 Trapezoid
R_29_30 268 0.0373 0.075 Trapezoid
A large number of detention basins currently exist in the watershed. Table 4-3 provides a list of the basins, with their location described. Some of the basins developed naturally and certain of those basins were further defined by culverts acting as outlet structures. Other basins were built specifically as storm water detention basins with engineered outlet structures.

Basins were characterized in the model by a stage-storage table and outlet parameters. Two of the basins were not included in the HEC-HMS model. Table 4-4 provides information about how these basins were represented in the HECHMS model. The maximum storage capacities shown in the table were calculated using survey derived elevations and topographic mapping. Since the detailed geometries of the basins were not surveyed, these volumes are necessarily approximate. As water flows into each basin, the basin begins to fill.

As water level in the basin rises, water begins to flow out of the basin, with Table 4-2: Stream flow routing related HEC-HMS input - Canyon del Rey basin.

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outflow increasing as the water level in the basin increases. When a basin overflows, then the outflow relationship changes to represent overbank or overtopping flow.

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Facility ID Location
LS_B_01 Lake at Laguna Seca Raceway.
04_B_01 South of Hwy 68 at S.P.C.A. facility.
04_B_02 South of Hwy 68 at S.P.C.A. facility.
05_B_01 South of Hwy 68, approx. 1'650 feet west of S.P.C.A. entrance road. 06_B_01 North of Hwy 68, approx. 500 feet west of S.P.C.A. entrance road. 07_B_01 North of Hwy 68, approx. 1,000 feet east of Boots Road. 08_B_01 South of Hwy 68. Approx. 250 feet southeast of Boots Road. 08_B_02 South of Hwy 68. Approx. 250 feet southeast of Boots Road. Not modeled. 09_B_01 Approx. 200 feet south of where Boots Road and Whip Road meet near Hwy 68. 10_B_01 Pasadera golf course pond, approx. 200 feet south of Las Laderas Drive. 10_B_02 Pasadera golf course pond, approx. 230 feet west of Las Brisas Drive. 10_B_03 Pasadera golf course pond, approx. 200 feet southeast of Pasadera Country Club. 11_B_01 Pasadera golf course pond, approx. 180 feet west of Mirasol Ct. Not modeled. 11_B_02
10_B_04
11_B_03 1'400 feet west of Pasadera Drive and 180 feet north of Hwy 68. Part of the Laguna Seca Golf Ranch. 12_B_01 1'650 feet west of Pasadera Drive and 270 feet north of Hwy 68. Part of the Laguna Seca Golf Ranch. 12_B_02 2'050 feet west of Pasadera Drive and 270 feet north of Hwy 68. Part of the Laguna Seca Golf Ranch. 14_B_01 Canyon del Rey reach west of Pasadera Road and South of Hwy 68. 19_B_01 Directly west of where Wilson Road and York Road meet. 21_B_01 North of Hwy 68. Approx. 1'300 feet east of Ragsdale Drive. 22_B_01 North of Hwy 68. Approx. 1'000 feet east of Ragsdale Drive. 24_B_01 West side of Hwy 218. Approx. 1'400 feet south of the Hwy 68 and Hwy 218 interchange. 24_B_02 West side of Hwy 218. Approx. 1'400 feet south of the Hwy 68 and Hwy 218 interchange. 25_B_01 Approx. 650 feet northwest of the Ragsdale Drive and Lower Ragsdale drive T intersection. South of the Harris Ct business development.
25_B_02 Directly south of the Hwy 68 and Hwy 218 interchange. South of the Monterra Subdivision. 26_B_01 South of Hwy 68 and west of 218 at interchange. North of the Monterra subdivision entrance. 27b_B_01 Directly east of Hwy 218 at Pheasant Ridge Road. 29_B_01 Frog Pond Wetland Preserve.
29_B_02 Northeast of the Monterey Airport. North of N road. 29b_B_01 Park behind Safeway, west of hwy 218 and south of Wilson Way. 30_B_01 Laguna Del Rey and Roberts Lake combined. North and south of Del Monte Blvd.

Pasadera golf course pond, at the intersection of Pasadera Drive and Via Del Milagro. Modeled as one (10_B_04).

Table 4-3: Detention basins within the Canyon del Rey watershed.

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The total precipitation depths for the 10 and 100 year events, described in Chapter 3 and Table 3-5, provided the basis for developing balanced 24 hour storm hyetographs. These hyetographs provided the temporal rainfall pattern Basin ID
Subwatershed
location
Peak Storage
Elevation
Before
Overflow (ft)
Max Storage
Capacity
(acre-feet) Modeling Method Outlet Type
LS_B_01 Sub_LS 748 12.8 Outflow Structures Culvert
04_B_01 Sub_4 413 0.2 Outflow Structures Spillway, horizontal grate, & orifice
04_B_02 Sub_5 400 0.1 Outflow Structures Culvert
05_B_01 Sub_5 388 14.7 Outflow Structures Culvert
06_B_01 Sub_6 388 28.7 Outflow Structures 2 Culverts
07_B_01 Sub_7 356 7.9 Outflow Structures Culvert
08_B_01 Sub_8 348 2.3 Outflow Curve Horizontal grate
09_B_01 Sub_9 375 1.7 Outflow Structures Weir & orifices 10_B_01 Sub_10 464 2.4 Outflow Curve Horizontal grate
10_B_02 Sub_10 449 0.7 Outflow Curve Horizontal grate
10_B_03 Sub_10 409 1.3 Outflow Curve Horizontal grate and weir 10_B_04 Sub_11 358 3.4 Outflow Curve Horizontal grate
11_B_03 Sub_12 318 0.4 Outflow Structures Spillway
12_B_01 Sub_12 318 1.6 Outflow Structures 2 Culverts
12_B_02 Sub_12 314 1.2 Outflow Structures 2 Culverts
14_B_01 Sub_14 304 64.6 Outflow Structures Culvert
19_B_01 Sub_19 194 0.2 Outflow Structures Spillway
21_B_01 Sub_21 142 1.3 Outflow Structures Weir & 2 orifices 22_B_01 Sub_22 140 0.1 Outflow Structures Weir
24_B_01 Sub_24 142 4.4 Outflow Curve Spillway and riser culvert 24_B_02 Sub_24 128 1.2 Outflow Structures 3 Culverts
25_B_01 Sub_25b 226 0.7 Outflow Structures 2 Culverts
25_B_02 Sub_25 122 7.6 Outflow Structures 5 culverts
26_B_01 Sub_26 210 4.0 Outflow Curve 3 Riser culverts & spillway 27b_B_01 Sub_27b 90 70.1 Outflow Structures Culvert
29_B_01 Sub_FP 82 32.5 Outflow Structures Box culvert
29_B_02 Sub_29 108 3.5 Outflow Structures 2 Culverts & Spillway 29b_B_01 Sub_29b 34 43.7 Outflow Structures Culvert
30_B_01 Sub_30 14 177.6 Outflow Structures 2 Box culverts Table 4-4: Detention basin parameters used in the HEC-HMS model.

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used in the model for each sub-basin. Consequently, runoff and routing simulations used a 24 hour period. Given the relatively small size of the watershed (14.3 square miles) and the relatively short maximum stream length (6.9 miles), a 24 hour period was sufficient to adequately represent variations in both runoff and stream flow during the selected design events.

Balanced 24 hour storm hyetographs were generated using standard methods.

An example of a typical application of this method can be found at: http://onlinemanuals.txdot.gov/txdotmanuals/hyd/hyd_apxf.pdf.

Specifically, rainfall depths are obtained for durations ranging from 15 minutes to 24 hours from the NOAA Atlas 14 application. The differences between rainfall depths are then plotted, with the first (and largest) difference plotted at 12 hours and then succeeding differences plotted at alternating later and earlier times until all values are used and the 24 hour hyetograph is completely specified. In this way a hyetograph which represents the 24 hour storm as well as the lesser duration storms is defined. While this hyetograph does not represent any particular storm event, it is representative of the rainfall depths which can be expected for storms of 24 hour and lesser duration. Figure 4-3 provides an example hyetograph for a 100 year, 24 hour rainfall event in one of the sub-watersheds in Canyon del Rey. Rainfall depths shown are for 15 minute periods.

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4.5 Conditions modeled
The highly pervious nature of watershed soils, particularly on the north side of the valley enables the watershed to absorb and retain large amounts of rainfall before substantial runoff is initiated. However, a long duration rainfall event or series of rainfall events will saturate the soils and cause much higher runoff to result from additional rainfall events. The 24 hour hyetograph that is used in this study is insufficiently long to create fully saturated soil conditions and thus can under-predict runoff. This behavior was confirmed in simulations which were initiated with minimal antecedent rainfall. Comparison of this behavior with available stream gage data indicated that more substantial antecedent rainfall (and high soil saturation) was needed to achieve results similar to gage data. Therefore, the SCS moisture accounting calculations were set to use Antecedent Moisture Condition I instead of condition II.

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.00 5.00 10.00 15.00 20.00 25.00
Rainfall Depth (inches)
Hyetograph Time (hours)
100 Year 24 Hour Hyetograph for Sub-watershed 18
Figure 4-3: Example 100 year, 24 hour rainfall hyetograph, sub-watershed 18.

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Impervious versus pervious conditions in each sub-watershed were assessed by calculating paved area for each sub-watershed using GIS data for land cover.

The results were than adjusted based on inspection of aerial photography for the watershed. It was found that model results were quite sensitive to percent impervious specification, due to the highly pervious nature of the soils. Trial and error adjustment of percent impervious was used in the model calibration process.

The constructed model was used to simulate the following conditions:
• 10 year, 24 hour storm with existing land use and flood management infrastructure
• 100 year, 24 hour storm with existing land use and flood management infrastructure
• 10 year, 24 hour storm with existing land use and proposed flood management infrastructure improvements
• 100 year, 24 hour storm with existing land use and flood management infrastructure improvements
Runoff and routing simulations using HEC-HMS were used to provide flow data for hydraulic calculations and assess performance of detention facilities and ponding areas. These data were produced for both existing conditions and potential future conditions. After hydraulic assessments were made and potential upgrades to the flood management infrastructure identified, the effects of such changes on runoff and routing of flows were calculated using the model.

4.6 HEC-HMS model calibration
The HEC-HMS model was calibrated by comparing model results with measured stream flow at the CSUMB “Frog Pond” gage. This stream flow gage record included adequate data for two short storm events: on April 3-4, 2006 and March 3-4, 2005. Corresponding data was available from the Fort Ord rain gage.

Antecedent moisture conditions (AMC) were adjusted to decrease rainfall loss rates in the watershed; AMC II conditions were used initially and then changed to AMC I to allow for the effect of multiple and longer duration rainfall events on soil moisture. Percent impervious calculations were also reviewed using aerial photographs and field investigation, resulting in increases in percent impervious for many sub-watersheds. These changes increased sub-watershed runoff and stream discharge results in the model. The calibration results are Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1181, Packet Page 1281

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summarized in Table 4-5. Figure 4-5 and Figure 4-6 show the correspondence between measured flow and HEC-HMS model results for the two events.

Both peak flow rates and total discharge volume correspond well with the gage results for the two rainfall events. The shapes of the HEC-HMS hydrographs correspond quite well with the gage record graphs. The model does appear to be somewhat more responsive to rainfall variations, as indicated by the somewhat larger response to early rainfall and the faster decline in flow after rainfall rates diminish. This level of calibration was deemed satisfactory for the current study purposes.

Peak Flows (cfs) Total Volume (ac-ft)
Known flow gage 80 191
HEC_HMS flow 96 214
Peak Flows (cfs) Total Volume (ac-ft)
Known flow gage 83 79
HEC-HMS flow 78 80
Calibration Results- April 2006 Storm
Calibration Results - March 2005 Storm
Table 4-5: HEC-HMS model calibration results.

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Figure 4-4: Stream flow at Frog Pond Gage for the March 2006 Calibration Event.

Figure 4-5: Stream flow at Frog Pond Gage for the April 2006 Calibration Event.

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4.7 Runoff modeling results
The HEC-HMS model of the watershed was used to produce runoff rates and volumes for the sub-watersheds, flow results for stream channels, operational data for the storage basins, and performance data for the culverts. Runoff statistics for each of the sub-watersheds is provided in Table 4-6. Peak flow and total flow volume in the stream channels (routing reaches), at confluences of channels (junctions), and from storage basins (basins) are provided in Table 4-7.

Figure 4-6 provides HEC-HMS output hydrographs for reach 29, which is immediately downstream of the “frog pond” basin.

A graph of peak flow at each stream channel location for the 10 year and 100 year events is provided in Figure 4-7. The locations along the channel from most upstream to most downstream (outlet) are identified with the station (facility) ID number. While stream flow generally increases from upstream to downstream, significant variations in stream flow can be seen and are caused by tributary flows from sub-watersheds and storage in basins within the channel sections.

Operating statistics for the storage basins – peak discharge, total outflow, peak storage, and peak water surface elevation – are shown for the 10 year and 100 year events in Table 4-8. The details of the basins are described in Chapter 6.

The impact of culverts at stream crossings on routing of flow is included in these results. Details of culvert geometry are provided in Chapter 6. The performance and adequacy of each culvert will be discussed in Chapter 7.

Statistics for the performance of the basins are shown in Table 4-9, which presents information on attenuation of the peak discharge by the basin and the portion of the basin nominal capacity that is utilized during a storm event.

Peak attenuation varies from negligible to 94%, depending on the outlet characteristics of the basin, storage volume available, volume of the storm hydrograph entering the basin, and the shape of the flow hydrograph. Storage volume utilized during the storm events varies from 2% to 878% of the nominal capacity of the basins. Nominal capacity is defined as the volume of water in the basin when the water level reaches the crown of the outlet culvert. Utilized capacity exceeds nominal capacity when the outlet culvert is surcharged.

The wide range in utilization of basin volumes and the effectiveness of the basins in attenuating storm runoff peaks emphasizes the reality that few of the basins were designed. Most basins resulted from road crossings and culverts designed and built to enable traffic and protect roadways and people. Nonethe-less, seven of the basins reduced storm peak flows by 50% or more and nearly all of the basins were filled to more than nominal capacity during the storm events. Consequently, these basins are an important contributor to flood flow management within the Canyon del Rey watershed.

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Subwatershed
ID
10-year
Peak Runoff
(cfs)
10-year Runoff
Volume (ac-ft)
100-year Peak
Runoff (cfs)
100-year Runoff
Volume (ac-ft)
Sub_LS 143 29.9 240 51.7
Sub_01 22 8.6 76 21.9
Sub_02 42 23.4 178 67.5
Sub_03 40 15.2 139 41.8
Sub_04 13 4.0 39 9.8
Sub_05 14 9.4 93 32.8
Sub_06 45 16.1 141 41.4
Sub_07 23 5.7 80 15.4
Sub_08 42 20.5 157 54.8
Sub_09 11 6.3 63 20.5
Sub_10 42 17.5 154 46.8
Sub_11 65 25.5 173 57.0
Sub_12 16 5.1 62 14.2
Sub_13 10 4.4 44 12.6
Sub_14 5 3.7 47 13.9
Sub_15 24 7.5 49 17.8
Sub_16a 1 0.9 15 4.8
Sub_16b 3 2.0 25 8.8
Sub_17 42 11.5 86 23.8
Sub_18 22 18.8 174 81.1
Sub_19 14 4.5 43 11.1
Sub_20 6 5.2 70 24.7
Sub_21 53 13.9 119 27.3
Sub_22 4 0.9 11 2.0
Sub_23 18 11.6 136 42.3
Sub_24 13 5.8 71 18.8
Sub_25 55 13.4 106 24.3
Sub_25b 13 2.4 23 4.0
Sub_25c 8 2.7 30 7.4
Sub_26 13 7.9 72 27.5
Sub_27 73 14.1 132 25.0
Sub_27b 8 1.9 15 3.8
Sub_28 16 6.4 25 10.6
Sub_FP 64 11.2 107 18.2
Sub_29 86 17.7 146 28.8
Sub_29b 101 21.8 170 35.5
Sub_30 772 166.2 1313 269.7
Table 4-6: Canyon del Rey sub-watershed runoff before routing.

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HEC-HMS
Station ID
Existing
Conditions 10-
year Peak Flow
Existing
Conditions 10-
year Runoff
Volume (ac-ft)
Existing
Conditions 100-
year Peak Flow
Existing
Conditions 100-
year Runoff
Volume (ac-ft)
J_01_02 59.9 31.9 241.4 89.4
R_02_03 59.8 31.9 238.5 89.3
J_03_04 110.1 50.8 412.5 140.5
06_B_01 54.3 41.3 377.7 170.6
R_06_07 54.1 41.1 375.2 170.3
07_B_01 56.4 46.1 389.2 175.5
R_07_08 56.3 45.8 371.7 174.8
J_08_09 77.3 72.6 494.1 249.7
J_10 139.3 114.9 684.8 351.2
R_11_12 138.4 114.3 669.5 350.1
11_B_03 138.1 113.1 660.9 347.4
12_B_01 145.4 116.2 662.2 358.1
12_B_02 144.7 114.2 660 354.3
R_12_14 143.1 113.4 648.9 352.8
14_B_01 143.4 120.6 480.4 325.1
J_16a 144.3 121.5 484.9 330.0
R_16a_15 144.1 119.4 471.1 325.6
J_15 149.1 126.9 482.6 343.4
R_15_17 149 125.7 477.4 340.7
J_17 156.4 137.1 489.1 364.4
J_19a 180.5 158.0 583.2 454.3
R_17_19 180.4 156.0 572.3 450.1
J_19 183.8 160.5 578.2 461.1
R_19_20 183.6 158.1 576.1 456.5
J_19_20 205 175.9 619.4 506.2
J_20_23 218.3 188.5 670.7 550.4
R_23_25 217.9 175.7 662.1 526.0
25_B_02 227.3 193.8 702.5 553.1
J_25_25c_26 236.5 204.1 760.4 586.6
R_26_27 236 202.0 745.7 582.0
J_27 240.7 216.1 768.9 607.0
R_27_27b 240.5 215.1 764.6 604.9
27B_B_01 232.3 208.9 756.4 583.3
29_B_01 197 189.0 723.9 552.0
R_29 197 186.0 712.6 546.3
J_29_fp_29b 203.2 207.8 731.1 581.7
J_29b 208.8 224.8 752.8 609.6
R_29_30 208.8 223.6 752.7 609.3
29b_B_01 208.7 222.3 701 599.5
30_B_01 246.8 273.8 752.4 664.5
Outfall 246.8 273.8 752.4 664.5
Table 4-7: Stream flow statistics for 10 year and 100 year simulated events.

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Figure 4-6: Sample HEC-HMS output hydrographs for Reach 29.

Figure 4-7: Simulation peak discharge results for Canyon del Rey.

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Basin ID
10-year
Peak
Discharge
(cfs)
10-year
Total
Outflow
(ac-ft)
10-year
Peak
Storage
(ac-ft)
10-year
Peak
Elevation
(ft)
100-year
Peak
Discharge
(cfs)
100-year
Total
Outflow
(ac-ft)
100-year
Peak
Storage
(ac-ft)
100-year
Peak
Elevation
(ft)
LS_B_01 10 7.8 22.2 749 14 14.4 37.38 750.5
04_B_01 13 3.7 0.38 414 39 9.5 0.45 415
04_B_02 12 3.7 0.06 399 35 9.4 0.79 400
05_B_01 2 1.5 12.80 388 24 12.8 24.97 390
06_B_01 54 41.3 37.43 389 378 170.6 48.43 390
07_B_01 56 46.1 0.48 347 389 175.5 12.05 358
08_B_01 41 20.5 2.48 348 158 54.6 2.99 349
09_B_01 9 6.3 0.21 370 61 20.3 1.03 373
10_B_01 38 17.3 0.76 463 119 46.2 3.66 465
10_B_02 33 17.1 0.65 449 131 45.9 2.70 452
10_B_03 32 17.5 1.24 409 107 45.8 2.77 410
10_B_04 64 42.3 2.92 358 199 101.5 11.66 362
11_B_03 138 113.1 0.31 318 661 347.4 0.66 319
12_B_01 145 116.2 1.52 318 662 358.1 3.29 320
12_B_02 145 114.2 1.37 314 660 354.3 2.49 316
14_B_01 143 120.6 1.48 276 480 325.1 64.28 304
19_B_01 14 4.5 0.32 195 41 11.1 0.41 195
21_B_01 20 12.6 3.85 144 44 25.0 7.26 145
22_B_01 3 0.9 0.25 141 9 1.9 0.33 142
24_B_01 7 5.6 0.85 137 28 14.5 5.22 143
24_B_02 7 5.1 1.67 129 20 13.4 3.01 130
25_B_01 7 2.4 0.77 226 12 4.0 1.02 227
25_B_02 227 193.8 5.11 121 703 553.1 15.05 124
26_B_01 11 7.6 0.53 205 93 26.1 1.69 207
27b_B_01 232 208.9 12.76 89 756 583.3 27.53 92
29_B_01 197 189.0 43.80 83 724 551.9 65.18 85
29_B_02 14 6.1 2.06 107 23 10.2 2.99 108
29b_B_01 205 211.5 1.58 22 694 581.9 15.78 29
30_B_01 258 282.4 154.27 13 758 679.7 253.13 16
Table 4-8: Storage basin operating statistics for the 10 year and 100 year events.

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Basin ID
Nominal
Storage
Capacity (acft)
% Attenuation
through Basin
% of Nominal
Storage Capacity
Utilized
% Attenuation
through Basin
% of Nominal
Storage
Capacity
Utilized
LS_B_01 12.8 93% 174% 94% 293%
04_B_01 0.2 2% 213% 2% 249%
04_B_02 0.1 7% 69% 10% 878%
05_B_01 14.7 85% 87% 75% 170%
06_B_01 28.7 66% 130% 33% 169%
07_B_01 7.9 2% 6% 2% 152%
08_B_01 2.3 2% 109% -1% 131%
09_B_01 1.7 16% 12% 3% 60%
10_B_01 2.4 9% 32% 23% 156%
10_B_02 0.7 13% 97% -10% 403%
10_B_03 1.3 2% 96% 19% 213%
10_B_04 3.4 17% 85% 7% 339%
11_B_03 0.4 0% 85% 1% 179%
12_B_01 1.6 1% 96% 2% 207%
12_B_02 1.2 0% 112% 0% 204%
14_B_01 64.6 7% 2% 29% 99%
19_B_01 0.2 1% 139% 6% 179%
21_B_01 1.3 63% 306% 63% 576%
22_B_01 0.1 24% 250% 20% 327%
24_B_01 4.4 46% 19% 61% 118%
24_B_02 1.2 6% 137% 26% 247%
25_B_01 0.7 47% 109% 49% 144%
25_B_02 7.6 3% 67% 1% 197%
26_B_01 4.0 20% 13% -29% 42%
27b_B_01 70.1 4% 18% 1% 39%
29_B_01 32.5 17% 135% 7% 200%
29_B_02 3.5 41% 100% 26% 100%
29b_B_01 43.7 0% 4% 6% 37%
30_B_01 177.6 74% 85% 54% 100%
Nominal storage capacity is impounded volume when water surface is at spill crest 10-year 100-year
Table 4-9: Storage basin performance statistics for the 10 and 100 year events.

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5 STORM DRAINAGE CRITERIA USED IN THIS STUDY

5.1 General
This discussion of design criteria is limited to the philosophy and criteria used in this study. Criteria and requirements for implementation of future decisions regarding land development and facility upgrades may be the subject of a future drainage design manual for the District.

Design of improvements to storm drainage facilities should, in order to most effectively reduce flood related risks and damage, be based on more than rote formulas and standard methodologies. The impacts of choices and designs on the behavior of the natural system need to be investigated and included.

One of the most important benefits of numerical simulation in hydrology is the ability to explore and represent details of the natural processes which cannot be captured in fixed formulae and “rules of thumb”. The results of synthetic rainfall analysis and hydrologic simulation of the Canyon del Rey watershed are presented in previous chapters. We consider briefly here, as a prelude to design criteria selection, the interactions between culvert design, detention storage, and increases in runoff due to development.

The HEC-HMS model of the watershed and existing facilities was used to simulate a range conditions, including amount of impervious area (due to development), the discharge capacities of culverts, and the impoundment volumes active during flood events. The results are summarized in Figure 5-1 and Figure 5-2. These figures provide, respectively, peak flows and runoff volumes along the main stem for the 10 year and 100 year events under three alternative conditions. The simulated conditions are: a) existing facilities and existing development, b) existing development but no storage within the basin, and c) no development and no storage within the basin. Condition (a) is the current condition used to determine adequacy of facilities. Condition (b) simulates hydrologic behavior of the watershed and stream system with existing development but no detention storage – as if all of the culverts and outlet structures in the watershed had the capacity to pass any flow rate. Condition (c) represents a watershed with no development and no storage – this is roughly the natural condition before man’s activities modified the watershed.

The effect of storage in the basins, created primarily by culverts under roadways, can be seen by comparing conditions (a) and (b). The basins reduce peak flows by roughly 60% for both the 10 year and 100 year events.

The large step function change in peak flow at the lakes is due to the large flows from the urbanized areas tributary to the lakes. The effect of urbanization on runoff can be seen by comparing conditions (b) and (c). Urbanization increases runoff for the 10 year event by roughly 125% in watersheds other than the intensely urbanized area and by roughly 435% in the intensely urbanized areas. The existing detention storage in the basins reduces peak flows under existing urbanized conditions to the same level as the natural condition for the Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1190, Packet Page 1290

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10 year event (compare conditions (a) and (c)). The existing basins reduce peak flows to 45% below natural conditions for the 100 year event, effectively reducing flood flows below what would occur naturally. An evaluation of runoff volumes in Figure 5-2 leads to similar conclusions.

This analysis demonstrates the importance of detention storage in managing flows during flood events. Any modifications to culverts and outlet works, particularly along the main stem of Canyon del Rey creek, have the potential to change storage performance and alter both peaks flows and runoff volumes during storm events. Such changes could increase flood risks and damages.

Increased flows have the potential to change sediment movement, bank erosion, channel incision, and channel stability. Sediment movement will be discussed in detail in Chapter 8.

Decisions regarding culvert capacity increases, consequently, must consider impacts on detention storage, peak flows, and runoff volume.

Figure 5-1: Peak flow rates along main stem for alternate conditions.

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5.2 Hydrologic analysis approach
Rainfall patterns were developed for specific storms based on regional rainfall frequency analysis, due to a lack of adequate local rainfall records. Refer to Chapter 3 for details.

Due to the highly variable watershed conditions, including soil type, ground cover, slopes, and extent of development, simple hydrologic calculations are not appropriate. While manual unit hydrograph methods have the potential to be sufficiently accurate and locally representative, hydrologic simulation packages such as HEC-HMS are readily available, relatively easy to apply, and sufficiently detailed to represent the highly variable watershed conditions.

Detailed representation of rainfall hyetographs and runoff hydrographs are necessary to properly handle routing through the sub-watersheds, storage in the basins, and discharge from one channel reach to another. In this study HEC-HMS was used to represent the response of the watershed to specific rainfall patterns. Details of the hydrologic modeling methodology and results are provided in Chapter 4.

Figure 5-2: Runoff volumes along main stem for alternative conditions.

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5.3 Hydraulic analysis approach
Hydraulic analyses were performed for each of the culverts listed in Chapter 4 and described further in Chapter 2. These analyses were performed using the Federal Highway Administration HY-8 Culvert Hydraulic Analysis Program. This program, further described in Chapter 6, automates standard calculations used for culvert design.

Hydraulic analyses were not executed for the natural stream channels, except as needed for morphological and sediment transport analyses, described in Chapter 8. The operations of basins were simulated in HEC-HMS using elevation-volume curves for each basin. The results of those simulations, in terms of utilization of available storage volume and effect of storage on outflow hydrographs, are provided in tables in Chapter 4.

5.4 Hydraulic design methodology and criteria
Evaluation of existing culvert adequacy and development of upgrade recommendations is described in Chapter 6. The resultant recommendations are provided in Table 6-3.

California Department of Transportation criteria for design of culverts specify the following:
• The upstream water surface elevation shall not exceed the top of the culvert inlet for the 10 year peak flood flow, and
• The upstream water surface elevation shall not exceed an elevation which would cause objectionable backwater depths or outlet velocities.

California Department of Fish and Game criteria for design of culverts specify the following:
• The upstream water surface elevation shall not exceed the top of the culvert inlet for the 10 year peak flood flow, and
• The upstream water surface elevation shall not exceed 50 percent of the culvert height or diameter above the top of the culvert inlet for the 100 year peak flood flow.

As discussed in section 5.1, the effect of detention storage along the main stem of Canyon del Rey creek is substantial, reducing both peak flows and runoff volumes for both the 10 year and 100 year storm events. The potential impacts of increased culvert capacity on detention storage need to be considered along with improvements in safety, traffic flow during events, and protection of Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1193, Packet Page 1293

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the roadway crossings. A compromise was established for this work, consisting of the following criteria:
• During the 10 year storm event, the water level upstream of the culvert should not exceed the top of the culvert inlet, and
• During the 100 year storm event, the water level upstream of the culvert should not exceed a depth of 0.5 feet above the lowest roadway crest elevation.

The intention of these criteria is to maintain as much detention storage as is feasible while allowing emergency vehicles to traverse the roadway crossing and while reducing the potential for erosion related damage to the culvert and roadway berm. These criteria do not explicitly account for embankment stability during overflow events. They also do not account for the existing condition of each embankment or its suitability to withstand erosion associated with overflow.

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6 HYDRAULIC ANALYSES

6.1 Hydraulic performance calculation methods
Hydraulic capacity calculations were produced for many, but not all, of the culverts in the watershed. Culverts which are on private land and privately maintained were generally not analyzed. Table 6-1 provides design details for the culverts analyzed.

The culvert design details were used in the Federal Highway Administration HY-8 Culvert Hydraulic Analysis Program to determine the hydraulic capacity of each of the culverts. Details of this program, including a quick start guide, can be found at http://www.fhwa.dot.gov/engineering/hydraulics/software/hy8/.

For a particular culvert the following parameters were input:

6.2 Facility capacity estimates
Peak flow rates generated by the HEC-HMS modeling for the reaches associated with each of the culverts were compared with the hydraulic analysis results to determine whether the existing culverts provided adequate conveyance capacity. The bases for that comparison for each of the culverts are provided in Table 6-2. This table provides both the HEC-HMS predicted flow rates and the calculated capacities. The HEC-HMS reaches used for the predicted flow rates are provided in the last column. In some cases, a culvert is located in the middle of a reach or sub-watershed, such that peak flows were prorated based on drainage area contributory to the culvert.

The adequacy of a culvert was determined by calculating the water surface elevation required to drive the 10 year and 100 year peak flow rates through the existing culvert. If the 10 year water surface elevation was higher than the culvert inlet pipe crest or the 100 year water surface elevation was 0.5 ft or more above the lowest point along the crest of the road crossing, then the capacity of the culvert was deemed inadequate.

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Culvert
Culvert
Length
(ft) Diam. (ft)/ W x H Type Shape
Invert
Elev. (ft)
Outlet
Elev. (ft)
Road elev.
(ft)
01_C_01 95.68 18" CMP circular 444.64 443.4 460
01_C_02A 41.20 18" CMP circular 424.39 424.12 426.45
01_C_02B 40.26 18" CMP circular 423.84 423.43 426
01_C_03 74.77 24" CMP circular 424.44 422.36 426.4
02_C_01 105 & 110 28" x 20" double barrel CMP oval 414.52 413.17 417.03 02_C_02 30.00 18" CMP circular 402.59 401.72 404.4
02_C_03 50.88 18" CPE circular 398.21 397.02 400.3
03_C_01 59.17 48" x 30" CMP oval 404.32 404 408.01
03_C_02 55.85 40" HDPE-S circular 394.35 393.02 398.67
04_C_01 71.20 24" CMP circular 395.99 391.67 398.7
04_C_02 59.40 24" HDPE-S circular 388.43 385.64 391.7
05_C_01 60.32 24" CMP circular 384.61 384.47 389.1
06_C_01A 98.25 48" CMP circular 383.64 379.96 388.2
06_C_01B 98.25 30" CMP circular 383.58 382.15 388.2
07_C_01 100.00 24" HDPE-S circular 361.59 347.86 370.3
08_C_02 118.44 52" RCP circular 326.88 325.16 346
09_C_01A 478.83 40" HDPE-S circular 367.01 333.7 40
10_C_01 164.00 36" CMP circular 342.14 339.59 360.4
10_C_02 104.97 60" RCP circular 331.69 330.46 346
10_C_03 745.55 36" RCP/CMP circular 352.68 338.39 361
12_C_01 109.27 48" CMP circular 294.85 293.78 315.5
14_C_01 162.18 48" CMP circular 265.94 262.27 303
15_C_01 124.57 48" CMP circular 225.34 215.33 233.5
16_C_01 56.23 24" CMP circular 271.77 269.51 275.5
17_C_01 35.07 14.4' x 8' Concrete Earth Floor Box w/dome roof 189.22 188.74 204.26 18_C_01 55.36 6' x 4' RCB Box 196.66 196 202.5
21_C_01 65.26 triple 28" x 24" Synthetic Fiberglass Oval 164.16 163.26 169 25_C_01A 130.10 double 48" RCP circular 115.59 112.6 123.5 25_C_01B 130.10 triple 18" RCP circular 113.86 110.09 123.5 25_C_02 120.34 double 48" RCP circular 112.73 111.31 118.53 25_C_03 59.00 14' x 7.7' RCB Box 110.23 110.23 120.65
26_C_01 67.13 36" HDPE-S circular 120.87 117.5 124.63
27_C_01 43.40 14' x 8' RCB Box 108.8 108.45 120.1
27_C_02 27.10 14' x 6.7' RCB Box 109.85 109.55 119.4
27_C_03 379.75 87.6" CMP circular 103.6 98.3 111.7
27_C_04 41.10 6' x 8' RCB Box 88.32 88 98.2
28_C_01 91.50 10.1' x 8' RCB Box 81.9 81.4 91.3
28_C_02 102.8 3' x 3' RCB Box 83.46 79.76 101
29_C_01 67.5 6' x 8' RCB Box 76.52 75.63 84.25
29_C_03 14 user defined Wood w/concrete walls Bridge 62.87 60.68 71 29_C_04 12 user defined Wood w/concrete walls Bridge 59.66 58.44 69.54 29_C_05 13 48" CMP circular 51.1 50.88 55
29_C_05A 10 12' x 8' concrete bridge 55.1 54.88 65
29_C_06 11.7 user defined concrete bridge 45.8 45.6 57.9 29_C_07 40.66 6' x 8.25' RCB Box 42.27 40.1 51
29_C_08 706.3 8' x 8' RCB Box 16.54 12.79 30
30_C_01 136.9 6' x 6' RCB Box 10.77 10.43 46.12
30_C_02 10.75 100' x 7'(in middle) wood arched bridge 10.3 10 14.5 30_C_03 91 double 16' x 7' RCB Box 6.2 6.1 15.3
30_C_03B 17 double 21.36' x 7' RCB Box 6 6 16.57
30_C_04 51.3 double 8' x 6' RCB Box 9.3 9 16.4
30_C_05 768.9 Quad 6' x 6' RCB Box 8.66 7.3 17.1
LS_C_01 2640 15" CPE circular 743.14 507.75 751
Table 6-1: Culvert design parameters used in hydraulic analyses.

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Culverts 30_C_02, 30_C_03, and
30_C_03B are within the lakes
and therefore flow rates are not
computed.

Culvert
Culvert
Capacity
(cfs)
10-year Flow
(cfs)
100-year Flow
(cfs)
Modeled Outflow from:
01_C_01 25 16 55 Prorated
01_C_02A 7 33 115 Prorated
01_C_02B 8 33 115 Prorated
01_C_03 13 4 15 Prorated
02_C_01 31 42 178 Sub_2
02_C_02 7 2 8 Prorated
02_C_03 10 2 8 Prorated
03_C_01 41 95 362 Prorated
03_C_02 65 100 379 Sub_3 + R_02_03
04_C_01 16 12 35 04_B_02
04_C_02 19 2 6 Prorated
05_C_01 20 7 22 05_B_01
06_C_01A
06_C_01B
07_C_01 43 10 36 Prorated
08_C_02 294 76 507 J_08_09
09_C_01A 70 9 61 9_B_01
10_C_01 89 56 389 07_B_01
10_C_02 422 56 372 R_07_08
10_C_03 89 64 199 10_B_04
12_C_01 220 145 660 12_B_02
14_C_01 285 144 481 14_B_01
15_C_01 130 150 483 J_15
16_C_01 21 1 15 Sub_16a
17_C_01 1282 156 489 J_17
18_C_01 198 25 195 J_16b_17_18
21_C_01 105 184 578 J_19
25_C_01A
25_C_01B
25_C_02 236 227 703 25_B_02
25_C_03 1028 236 760 J_25_25c_26
26_C_01 78 13 72 Sub_26
27_C_01 1497 236 760 J_25_25c_26
27_C_02 901 236 760 J_25_25c_26
27_C_03 330 241 769 J_27
27_C_04 450 253 790 J_27 + prorated sub 29b
28_C_01 932 232 756 27b_B_01
28_C_02 230 16 25 Sub_28
29_C_01 334 197 724 29_B_01
29_C_03 450 203 731 J_29_fp_29b
29_C_04 899 203 731 J_29_fp_29b
29_C_05 62 J_29_fp_29b
29_C_05A 935 J_29_fp_29b
29_C_06 885 203 731 J_29_fp_29b
29_C_07 409 203 731 J_29_fp_29b
29_C_08 805 205 694 29b_B_01
30_C_01 898 366 968 29b_B_01 + Prorated 30
30_C_02 3035 30_B_01
30_C_03 740 30_B_01
30_C_03B 1920 30_B_01
30_C_04 537 258 758 30_B_01
30_C_05 1389 258 758 30_B_01
LS_C_01 16.4 9.5 14 LS_B_01
203 731
367 227 703 25_B_02
111 54 378 06_B_01
Table 6-2: Comparison of culvert
capacities with 10 and 100 year
flows.

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6.3 Effects of facilities on existing and future flows
Certain facilities listed in Table 6-1 were included in the HEC-HMS model, where the impounding of water by culvert restrictions has the potential to alter flow rates and volumes along the main stem of the creek. The HEC-HMS model was run for both existing and proposed conditions. Proposed conditions are defined by the recommended upgrades to culverts. Recommended culvert upgrades are discussed in Chapter 8. No changes in watershed development, land use, or storm water collection systems were included.

Recommended upgrades to culverts are shown in Table 6-3. These upgrades are based on the increase in capacity needed to pass the modeled 100 year peak discharge with a maximum depth above the roadway of 0.5 feet. All culverts that were deemed inadequate based on the 10 year peak discharge were also inadequate based on the 100 year peak discharge. In some cases, the resulting depth is lower than these criteria because standard culvert sizes were specified.

Figure 6-1 provides plots of the 10 year and 100 year peak flows predicted by the HEC-HMS model along the main stem of Canyon del Rey Creek from the upper most sub-watershed (at the left of the graph) to the outlet from Lake Roberts. Two sets of predictions appear on the graph: one for existing Culvert
01_C_02
02_C_01
03_C_01
03_C_02
10_C_01
12_C_01
14_C_01
15_C_01
21_C_01
25_C_01
25_C_02
27_C_03
27_C_04
29_C_01
29_C_03
29_C_07
Replacement Recommendations
Replace with 10 ft2 area culvert or ditch. 2' X 5' box culvert.

Replace with 16 ft2 area box culvert. 2' X 8' box culvert.

Replace 18 ft2 area box culvert. 3' X 6' box culvert.

Replace with 24 ft2 are box culvert. 4' X 6' box culvert.

Add 48" concrete pipe or equivalent total capacity of 20 ft2.

Add parallel 60" RCP.

Add parallel 30" RCP, or equivalent total capacity of 18 ft2.

Replace with 40 ft2 area box culvert. High priority.

Replace with 70 ft2 area box culvert. High priority.

Replace with 100 ft2 box culvert. Private facility.

Add parallel 100 ft2 box culvert. 6' X 8' box as suggested in the 1977 report.

Replace with 65 ft2 area box culvert. Will reduce detention storage.

Replace with 81 ft2 area box culvert.

Replace with 90 ft2 area box culvert. 12' X 7.5' box as suggested in the 1977 report.

Add parallel 6' X 8' box as suggested in the 1977 report.

Add parallel 6' X 8' box as suggested in the 1977 report.

Table 6-3: Recommended upgrades to existing culverts.

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conditions and the other for proposed conditions. Figure 6-2 provides the same plots for total runoff volume.

The impact of changes in storage due to enlargement of culvert capacity at several locations can be seen in these graphs. Reductions in water storage at these locations during both the 10 year and 100 year events result in higher runoff volume. The effect, as would be expected, is more pronounced for the 100 year event. Changes in peak discharge due to enlargement of culverts are more complex, particularly for the 100 year event. Timing of peak flows in the main stem relative to peak flows entering the main stem from sub-basins is altered by reductions in storage volume and passage of higher flows through the affected culverts. For some locations peak flows increase by no more than 20%, while at other locations peak flows decrease by no more than 7%. The overall effect of increases in culvert capacity is to increase peak flows and increase runoff volumes during the storm events, both by relatively modest amounts. Increased flows, in turn, will have some effect on stream bank Figure 6-1: Effect of proposed culvert upgrades on future peak flows.

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stability. Stream morphology and sediment transport will be discussed in Chapter 7.

Figure 6-2: Effect of proposed culvert upgrades on future flow volumes.

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7 EROSION AND SEDIMENTATION

7.1 Purposes of the geomorphic study
Balance staff worked with MPWMD staff and other cooperators to identify sources, loadings, and storage of sediment in the watershed. Questions addressed included:
1) What are the past, present, and expected future sediment loads from the watershed?
2) Where are the sources of the sediment?
3) Are the drainage facilities for new developments that have been required since the 1977 drainage plan was completed functioning as intended?
The questions called for a strategy of using multiple independent lines of evidence which can (a) yield answers with immediate applicability, (b) be expanded upon in the future, and (c) attempt to quantify existing baseline conditions, such that the success of the 2014 master drainage plan update can be assessed and refined in the future. Following field reconnaissance, we initially approached these questions with a tiered study plan, as follows. • First, develop an archival or historical assessment of sediment erosion and deposition, and then attempt to quantify the rates of deposition in basins constructed at known times or eroded from migrating, incising channels over known dates
• Second, as conditions allow, measure sediment transport at the MPWMD gage and at the mouth of Roberts Lake,
• Analyze particle-size distributions of sediment in Laguna Grande and Roberts Lake, plus those at the two sediment-transport gages, and • Develop estimates of sediment loadings into and out of the Laguna Grande/Roberts Lake complex, over a range of year types and watershed conditions, and
• Make informed recommendations regarding dredging and beach augmentation.

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INITIAL PARADIGM FOR SEDIMENT TRANSPORT
The 1977 drainage plan identified the Canyon del Rey watershed as a naturally pervious landscape, particularly north of stream. South of the stream, soils were also sandy and transmissive, but thinner and less pervious. The primary sediment sources are often south of the stream, particularly when sedimentation from episodic events (see below) are factored in. The geomorphic evidence suggests that most sediment is mobilized and transported to the main channels at high rainfall intensities during periods of watershed saturation.

In addition, regarding Roberts Lake and Laguna del Rey, the 1977 report indicates sand is possibly transported from the watershed to the ocean via the lagoon. This implies the lagoon fills with sediment pulses which continue on to the beach and ocean, over the outlet weir at Roberts Lake and under the highway through the culvert.

To estimate sediment loading, the authors of the 1977 report used indirect methods to estimate annual sediment yield. One method looked at the lake levels versus mean sea level, which yielded high results and was qualified as a less accurate estimate. Acknowledging the uncertainties with the estimated sediment load from the lake bed elevation estimates, a method of similitude was used to estimate annual yield. The similarly-sandy Colma Creek watershed in the Bay Area was used for similitude and comparison, using the 1970 water year, one with near-average rainfall and rainfall intensities. The resulting estimated annual sediment yield in the 1977 report was 50 tons per square mile, or 700 tons per year at the MPWMD gage site.

Regarding depositional zones, the 1977 drainage plan identified two significant deposition zones:
• Upstream of Fremont Blvd (Safeway center) on lower watershed • Roberts Lake and Laguna del Rey
Since 1977, watershed conditions have changed significantly due to residential and industrial development.

GEOMORPHIC EVOLUTION
Sediment transport and deposition through the main channels of the Canyon del Rey hydrographic network are complex products of both pre-historic and historic processes and events. For most readers, it will prove very helpful to review the effects of the past 20,000 to 100,000 years and of the past 200 years (see next two sections) to better understand why certain segments of the channel are deeply incised while others have been depositional reaches for centuries, if not longer. Many of these insights are new, developed during this study. They also point toward changes in drainage and sediment Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1202, Packet Page 1302

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management which may be needed in the foreseeable future of perhaps 50 to 100 years.

The geomorphic history of the past 100,000+ years contributes to development of discrete sedimentation chambers in the Canyon del Rey watershed. Key processes and events contributing to the semi-segregated chambers include:
a. Late-Pleistocene Valley Incision and Refilling: Sea-level decline and the cooler, wetter climate of the last two major glaciations led to sea level declines to a maximum about 412 feet in the Monterey Bay area. Most major streams, including Canyon del Rey incised down to the glacial age sea level, leaving steep-walled canyons much deeper than the current valley floors, with thalwegs typically 160 to 180 feet below sea level at the modern coastline. In all likelihood, the glacial longitudinal profile of Canyon del Rey of 20,000 years ago was steeper, with depths tapering upstream to meet today’s valley floor. If Canyon del Rey were like several other Monterey Bay Area streams, such as Scott and Waddell Creeks or the Salinas River, the beds of the glacial-age channels would taper upstream to meet the present-day profiles at elevations of roughly 200 to 240 feet, corresponding to the location of the Laguna Seca turnoff from Highway 68. We believe that almost all of the valley floor west of the Laguna Seca turnoff is younger than 20,000 years; additionally, there are areas where much younger deposits have re-configured the valley floor.
b. Work Park Landslides: Much of the northern side of Canyon del Rey Valley between Fremont Blvd and Frog Pond Wetland Preserve is a complex of ancient rotational landslides heading at the top of bluff above Highway 218 (Clark and others, 1997). Bedrock in this area is Monterey shale, buried at depths of only several tens of feet beneath terrace deposits and sand dunes. As in most valleys which were incised into the shale bedrock during the glacial sea-level minima, large landslide complexes filled the valleys which undermined the shaley slopes3 (see Appendix G, Figure G-5). Canyon del Rey remains steeper in this reach, where the rubble within the large slides has tended to stabilize the valley floor, leaving a steeper longitudinal slope (see Figure 7-1).

3 The closest analogy for large landslides in the shales may be the southern side of the Carmel Valley, particularly between Las Garzas and Potrero Creeks, where similar large valley-lateral slope failures form most of the southern slope of the valley. Other shale valley slopes which failed in valleys cut down to the lowered sea level are massive landslides in the lower portions of Potrero, San Jose, and (in Santa Cruz County) Majors, Scott and Waddell Creeks (Hecht and Rusmore, 1973). Almost all streams in the region which drain to the Pacific or Monterey Bay exhibit these near-coast landslide complexes, if they are underlain by shale or mudstone bedrock (generally mapped as Monterey formation or Santa Cruz mudstone) have large similar near-coast landslide complexes.

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c. Landslides from the southern side of the valley: Large landslides, again with Monterey-shale bedrock underpinnings, have added sediment to the valley floor, (a) through erosion of the slide mass (e.g., Boots Road), (b) formation of large alluvial fans which discharge to the Canyon del Rey main channel and/or are being eroded at their toes, and (c) deposition behind landslide debris or natural levees which temporarily (in a geomorphic sense) dam the channels and impose deposition.

Examples of each type of deposition are shown in Appendix G, keyed to the (a), (b) (c) description above.

Sometimes, it is not entirely clear what processes have created the deposition which creates an irregular (or ‘crenulated’) longitudinal profile. For example, Professor Doug Smith of CSUMB has shared with us a well log for a valley-floor well near Highway 218 on the parcel east of General Jim Moore Drive (GJMD, formerly called the North-South Road), between Frog Pond and Tarpy’s Corner (Table 7-1):

Depth (feet) Description Interpretation
0’ to 3’
3’ to 15’
15’ to 46’
46’
sand
black clay
sand and gravel
shale
Historical deposition (?)
Pre-historical ponding and deposition
Pleistocene deposition
Bedrock or landslide deposits, possibly reworked by proto-Canyon del Rey channel The bedrock underlying the valley here is at a depth of (at least) 46 feet, overlain by deposits of sand and gravel, which likely were deposited as basal alluvium of an ancient Canyon del Rey main channel draining to a much lower Monterey Bay (see Table 7-1). Fifteen feet of black clay likely represents Table 7-1: Well Log for abandoned well east of GJMD and east of Frog Pond.

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deposits of suspended sediment ponded behind a landslide presumably in Work Park, or deposits in a drowned valley gradually filling in as sea level rose following the maximum extent of glaciation (see Figure 7-1 and Figure 7-2). The three feet of sand at the top of the well log may be sediment from historic changes in the Canyon del Rey channel, or they might also include some prehistoric but geomorphically recent deposits as the channel finally filled the ponded area -- perhaps because deposition had finally filled the available storage area in that compartment, and that sediment from upstream could now be moved downstream to begin filling it even further.

Figure 7-1: Profile and geomorphic interpretation of lower reach of Canyon del Rey, Monterey County, California. Aggrading and degrading (incising) reaches shown schematically only. Profile data from 2007 LiDAR map from AMBAG.

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The geomorphic evidence seems clear that the Canyon del Rey watershed has been progressively (and probably continuously, in a geologic sense) depositing sediment near the axis of the valley.

According to Cain (1995), Laguna Grande was originally constructed in the late 1870s by excavating an 11-acre marsh for a recreational lake, as part of construction of Hotel del Monte (see Figure 7-3). Over time, the lake has been dredged. The original intent of dredging is not clearly known, but it is speculated that development from 1950 to 1970 created a large pulse of legacy sediment that ultimately ended up trapped above Fremont Avenue and in the lakes. This prompted a dredging operation to deepen the lakes for capacity and to deter vegetation growth in shallow water.

Figure 7-2: Profile and geomorphic interpretation of upper reaches of Canyon del Rey Monterey County, California.

Aggrading and degrading (incising) reaches shown schematically only. Profile data from 2007 LiDAR map from AMBAG. Vertical exaggeration is about half as shown for lower reaches.

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REVISIONS NEEDED BASED ON FIELD OBSERVATIONS
Field evidence shows that runoff and sediment-transport regime is highly defined by a compartmentalized system and episodic (high flow) events that mobilize sediment and may reconfigure or re-align the channel.

A key observation is that there seems to be minimal deposition in Roberts Lagoon and Laguna Grande since the 1983 dredging, based on bathymetry measurements and acoustical profiling by CSUMB. Both impoundments are relatively unchanged since the last dredging operation in 19834. The CSUMB bathymetric survey was commissioned as part of the current report. The resulting output is shown on Figure 2-4. The highly accurate survey actually shows treads and tracks from heavy equipment, presumably a remnant from original dredging operation in 1983, and likely an indication of less than 1 or 2 inches of deposition over the past 30 years 5. This leads to a finding that

4 The date of the last dredging of the lakes is thought to be 1983. 5 Deposition in the lakes includes not only fluvial sediment delivered from the watershed, but also other sources, such as sand blowing in from the beach and the dunefield to the northwest. This is further discussed in the sedimentologic analysis below.

Figure 7-3: Historic Photo of Laguna del Rey showing original lake and Hotel del Monte, circa 1880 per Cain (2005).

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sediment from the watershed is not appreciably delivered to the beach and ocean through Laguna Del Rey and Roberts Lake.

The results of the CSUMB study are supported by three other lines of evidence:
• The delta at the head of Laguna Grande has prograded very slowly into the lake. If significant silt, sand, and fine gravel were being transported by Canyon del Rey, a discernible expansion of the delta into the lake would be observed (see Figure 7-4). Other coastal channels with low rates of sediment delivery show evidence of distinct and quantifiable delta growth into their respective downstream lakes; one example is Antonelli’s Lagoon in Santa Cruz, where sediment yields of 50 tons/sq mi/yr (similar to what was estimated for Canyon del Rey in the 1977 report) defined a discernible delta (see Hecht, 1980).
• No discernible deposition in other locations of near-zero flow, such as backwater corners of Laguna Grande, or in the lees of islands. • Little change in the particle sizes of samples taken from the bed surfaces of the two lakes; downstream fining would be expected if substantial sediment loads were passing through the system.

In addition, this past water year, a hopeful time for storm measurements, has been very dry. The largest storms occurred in December 2012 in the beginning of the water year. Total rainfall for water year 2013 at San Clemente Dam is estimated as 14.6 inches, compared to 21.3 inches for an average year, about 69 percent of an average, well below the average year. As a result, the original proposed plan to conduct flow and sediment load measurements and produce annual sediment yield based on these measurements was not available. The new approach used indirect measurements and deconstructed sediment transport histories from a study of sediment samples, geomorphology, geology, and system infrastructure. There are distinct points along the Canyon del Rey flow path that are intercepted by hydraulic controls.

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Another observation that shaped our analysis is observation of long incised and confined channel reaches, with evidence of bank slumping and erosion. Road crossing culverts and drop inlet culverts were observed to apparently intercept incision and reset the incision processes.

Also, as built information of sediment basins was limited, if not unavailable. This affected the process estimating sediment transport rates based on basin data, leveraging an indirect approach.

A refocused approach to sediment transport and trapping within the upper watershed was therefore implemented to evaluate upstream conditions.

REVISED APPROACH
The revised approach considers the compartmentalized hydrologic system of Canyon del Rey. The sediment transport regime in the current system is a Figure 7-4: Historic progression of delta in Laguna del Rey.

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sequence of filling and spilling sediment compartments, starting in the upper subwatersheds, leading to less sediment in the lower watersheds. Typical processes of sedimentation in a compartment are shown on Figure 7-5. An additional process of aggradation from bankline and in-channel vegetation is shown on Figure 7-5 to amend findings from Brown and Jackson (1973). Heavy vegetation in a flowing channel tends to slow flow and promote additional aggradation and sediment trapping, particularly at the upstream end of a basin near the deltaic deposits.

Episodic events are viewed to fill the upper basin compartments, facilitating sediment transport when the hydraulic structures are overwhelmed. The resulting transport scenario is typically low rates of transport with periodic high sediment loads advancing down the system when a compartment is overwhelmed.

In addition, as sediment is trapped successively in the compartments moving downstream, fluvial processes are influenced by ‘hungry water’ that draws sediment from the channel bed and banks. These reaches are viewed to be in various states of incision, as described by Schumm (1984) and shown on Figure 7-6. The incision is further exacerbated by increased peak runoff rates due to development and increased impervious area, relative to conditions in reported in 1977 and to native geomorphic conditions.

Figure 7-5: Conceptual profile of aggradation process upstream of a dam per Brown and Jackson (1973).

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In terms of sediment source, it was observed that a main source of sediment in the system is material transport from channel bank erosion. There are long reaches of deeply confined, incised channel with low slopes that tend to promote lateral stream movement and subsequent bank failure. Upland sources of sediment typically originate from the south side of the stream, with some pockets of upland sediment from the north side of the stream.

The reaches in the watershed can be described by function in terms of low range and episodic sediment transport. Each segment of stream can be classified as aggrading, degrading, or ‘stable’, with anthropogenic structural controls regulating pulses of sediment.

The Canyon del Rey watershed is unconventional and further watershed studies are recommended to better understand the processes in the individual subwatersheds and further refine estimates of sediment loading. Indirect methods of sediment loading by the subwatershed and compartment basis were applied for this study. For each compartment, the sediment loading was compared to the estimated capacity remaining in the compartment.

Figure 7-6: Conceptual geomorphic cross sections of incision process per Schumm (1984).

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METHODS USED IN ASSESSING COMPARTMENTS
An historical analysis was first performed. This included developing a timeline of historical events and watershed conditions, followed by analysis of aerial photos and historic topographic maps to interpret geomorphic changes in the watershed.

The historic analysis was followed by examination to distinguish compartments within the Canyon del Rey watershed using geology mapping, soils mapping, and structure locations. Field reconnaissance was performed to confirm compartment delineation. We also examined cutbanks in incised reaches of the valley floor to discern depositional patterns over time. Definition of the compartments included the following data and criteria.

7.2 Historical and archival analysis
To further explore background on the CdR watershed several data sources were reviewed to distinguish the history of the watershed and changes since the 1977 stormwater Master Plan. Major events in the watershed were identified. Channel morphology was described using analysis of the stream profile, plan mapping, and field reconnaissance. Analysis of archival data included historic USGS topographic maps and historical aerial photography.

Most archival photography was obtained from UC Santa Cruz’s Special Collections.

MAJOR EVENTS IN THE WATERSHED
The Canyon del Rey watershed has experienced a period of urban growth starting as early as 1880 with the appearance of the Hotel del Monte. The most dynamic period of urbanization was from 1949 to 1953 (see Figure 7-7). By the mid-1970s, the level of urbanization was estimated to be approaching current conditions.

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Figure 7-7: Time line of major sedimentological events in Canyon Del Rey Watershed.

Historic water year rainfall records shown for Monterey area including Del Monte NOAA from 1949 to 1994 and Monterey Airport NWSFO from 1996 to 2011.

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GEOLOGY
A description of the geomorphic history is provided above in this section. To expand on the geomorphic history, geology mapping was used to identify sediment sources, trapping locations or compartments, and transport reaches (see Appendix G). Specifically, alluvial deposits (Qa) were located, as well as the older deposits (Qar/Qoa/Qos/Qm), with the intent to identify the extents of geologically recent deposition areas at the transition from Qa to older deposits.

In addition, locations of Santa Margarita sandstone were located and mapped as potential sediment sources, typically mapped to the south of the Canyon del Rey channel, which follows Chupines fault line. Further definition of sediment sources were locations of landslides, identified as Qls.

The predominant geology of the watershed north of Canyon del Rey channel is permeable Paso Robles formation (Qtp). Sediment source is less likely north of the creek, with the exception of gullies near Laguna Seca and in areas designated as the Badlands soil type. The role of the northern watershed appears to infiltration and interaction with groundwater levels (Yates and others, 2002), which affects base flow, storm flow peaks (higher ground water = higher peak flows and channel shear stress), and slope stability within the deeply incised creek channel (higher ground water and pore water pressure = more prone to bank erosion).

SOILS
Soils data was procured from the USDA (see Appendix G). Compartment transitions were initially estimated by mapped locations of soils classified as Aquic Xerofluvents, Rindge muck, Elder soils, water, Badlands, as associated with locations of deposition and alluvial material. Delineations of compartments based on soils mapping and interpretation were field confirmed.

STRUCTURES
A detailed hydrologic analysis is provided in Chapters 1 through 6, with accounting of structure types and capacities. The sediment loading and transport analysis utilizes the same nomenclature for the structures and identified major compartments and geomorphic changes, as influenced by infrastructure and stream interceptions.

SEDIMENT-DEPOSITION COMPARTMENTS
Based on analysis of mapping and field confirmation of soils, geology, and structures, locations and extents major compartments were defined and mapped (see Table 7-2). In addition, a profile of Canyon del Rey was developed to identify reach slopes, impoundments/compartments, incised reaches, aggrading reaches, and transport or source reaches (see Figure 7-1 and Figure 7-2). The stream profile and capacity of the compartments were estimated used LiDAR data from 2010 (see Figure 7-8, Figure 7-9, Figure 7-10, Figure Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1215, Packet Page 1315

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7-11, Table 7-2, Table 7-3, and Table 7-4). As shown on Table 7-2, eight major compartments were identified to trap sediment. Within compartment 6, we have identified Compartment 0 (see Figure 7-11), which is interpreted to currently contribute less sediment load due highly permeable sand in the area (see Appendix G, Figure G-3). No appreciable sediment transport is estimated in Compartment 0 at its current level of development. However, given potential future development in this area (see Section 7.4), sediment load from upland areas was included in this analysis.

Table 7-2: Summary and description of active major compartments in Canyon del Rey watershed.

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Figure 7-8: Canyon Del Rey watershed sediment sources and sinks (lower watershed).

Sediment compartments have been approximated through a combination of field observation, aerial photo and LiDAR interpretation. Polygons in green identify major current sources of sediment and red polygons identify major compartments of sediment deposition. Photo: Google Earth, 2012 Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1217, Packet Page 1317

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Figure 7-9: Canyon Del Rey watershed sediment sources and sinks (middle watershed).

Sediment compartments have been approximated through a combination of field observation, aerial photo and LiDAR interpretation. Polygons in green identify major current sources of sediment and red polygons identify major compartments of sediment deposition. Photo: Google Earth, 2012 Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1219, Packet Page 1319

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Figure 7-10: Canyon Del Rey watershed sediment sources and sinks (upper watershed).

Sediment compartments have been approximated through a combination of field observation, aerial photo and LiDAR interpretation. Polygons in green identify major current sources of sediment and red polygons identify major compartments of sediment deposition. Photo: Google Earth, 2012.

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Figure 7-11: Compartments mapped on aerial photograph, with subwatershed boundaries, Canyon Del Rey, Monterey County
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Table 7-3: Summary of incised reaches and characteristics based on LiDAR analysis and field observationsa.

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SEDIMENT SOURCES
In the 1977 report, sediment yields were estimated largely on the basis of measurements by Jim Knott (1973) of USGS in sandy soils of similar parent material in the pre-urbanization Colma Creek watershed near San Mateo. We now understand that there are three classes of sediment in the Canyon del Rey watershed which should be distinguished if useful estimates of sediment yields are to be developed:

1. Upland sources, which include rills, gullies, and other surfaces outside of the mapped channels. The unit sediment yield of 50 to 100 tons per square mile from the 1977 appears reasonable, and is consistent with Table 7-4: Summary of compartments, estimated sediment loads, and capacity, Canyon del Rey.

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subsequent work in sandy soils in the region, such as measured rates of deposition in Antonellis Lagoon prior to construction of UCSC (Hecht, 1980), transport rates in non-incising watersheds on the north side of the Carmel Valley (Matthews, 1983), and the sandy watersheds of the northern Santa Lucia Mountains (Hecht and Napolitano, 1995). Maps showing known locations of erosional point sources are included in Appendix G.

2. Channel incision or widening, which occurs in scattered locations throughout the watershed. We developed an estimate of the sediment yield associated with channel stability in Table 7-3, above. These loads were tabulated based on channel reach and then reorganized by compartment and reported in Table 7-4, along with estimated load from upland sources.

3. Sediment from exceptional, episodic events, such as fires, landslides, and truly major storms. Although these rarely occur, episodes can frequently account for half or more of long-term sedimentation in northern Monterey County (e.g., Hecht, 1981; Williams and Mathews, 1986; Hampson, 1995, Hecht, 2000). Episodic sedimentation is often omitted from calculations of sediment yield because such events may not occur during the period in which sediment-transport is typically measured. Based on data from nearby streams and the exposures of the sediments along the Canyon del Rey channels, episodic events have been and are particularly important in this watershed under both pre-European and current conditions. The outcrops showing multiple events which have left beds 3 to 5 feet thick on the floor of Canyon del Rey indicate that drainage and sediment management in this watershed should include plans to manage the roads, channels, and improvements under such conditions.

Estimating the frequency and volume of such events is beyond the current scope of work.

Of perhaps equal importance in choosing future management directions is to quantify the locations and amounts of deposition. Under existing conditions, deposition roughly equals sediment delivery, as accumulation of sediment in Laguna Grande/Roberts Lake is essentially undiscernible. Hence, upland plus incisional erosion by definition equal compartment sedimentation. These factors are further discussed in the Implications section of this chapter, below.

SEDIMENT-TRANSPORT MEASUREMENTS
Due to the very dry water years, no substantial bedload- or suspendedsediment measurements could be made. No major storm events produced sufficient runoff to mobilize sediment in a manner justifying transport measurements. For reference, the hydrograph at the MPWMD gage at Work Park in Del Rey Oaks for water year 2013 is shown on Figure 7-12. The project deadline was extended, making it possible to collect sediment samples through the end of February 2014, but similar very dry conditions prevailed. In lieu of Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1227, Packet Page 1327

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storm measurements, bedload samples were taken at strategic points to estimate characteristics of sediment transported in the system and likely depositional locations.

PARTICLE SIZE ANALYSIS OF BED SAMPLES AND BEACH SAND
Samples of bed material6 were collected and sieved to quantify particle sizes entering and leaving basins/compartments in the lower Canyon Del Rey watershed, and to support other sedimentological metrics to help characterize the system The results of four samples are discussed below. These samples are classified as either upstream or downstream of the basins of interest. See Figure 7-13 for locations of samples sites.

6 Bed material is the composition of the bed of a stream, and differs from bedload, which is sediment in transit, and supported by the bed. Bedload is commonly much smaller than bed material.

Figure 7-12: Flow at MPWMD gage at Del Rey Park for water year 2013. Flow data courtesy of Greg James of the MPWMD.

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Figure 7-13: Locations of sediment sample sites in lower Canyon del Rey.

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7.3 Evidence from the field
The observed conditions differ sufficiently from the initial conceptual framework for the drainage study that reviewing what we can learn from field observations to check the reasonable of the findings, and perhaps to extrapolate what may occur in the future.

EVIDENCE FROM BANK EXPOSURES
CdR has incised into the valley floor at a number of locations between Tarpy’s Corner and Las Laureles Grade, sometimes as deeply as 25 to 30 feet. We looked into what might be seen in these eroding banks, and how it might confirm our inferences regarding sediment source and delivery processes, or redirect how these might be used to plan a sustainable drainage program.

We found that the sediments beneath the valley floor seemed to be predominantly from three sources which we could often (but not always) distinguish:

Overbank deposits from floods: Thinly bedded, generally fine-grained or finesand sediments, often alternating light and dark in color, these were likely deposited prior to incision, when the channel was much more shallow.

Inferred south-side sources: Light-colored deposits, often 1 to 4 feet thick, with visible concentrations or bands of angular chunks of light brown or white shale.

In most cases, these deposits likely originated as debris flows from canyon-like tributaries on the south side of the valley, or possibly from rapidly incising into the canyon floors and channel-less swale which deliver most of the sediment from the south side of the valley.

Inferred north-side sources: Sandy deposits composed of medium and coarse angular sands containing a fair amount of clay and silt. White or greyish in color, these seem to originate from the ‘Badlands Areas’ and other portions of the Paso Robles formation, as well as from debris flows or incising channels typical of the of canyons and channel-less swales of the north side of the valley.

Where exposed on the floor of Canyon del Rey. These light-colored strata are often 1 to 3 or 4 thick.

Further, we observed in these exposures what we interpret as a continuous depositional sequence, with no obvious breaks in the sequence. We found little or no sign of newer channels cutting through the older deposits, or of reworking of the sediments. We also did not find evidence of primary mudflow deposits which had not been eroded and reworked by the channel. Also, we found no sign of bedrock beneath these flat-lying stream deposits, suggesting that the 40 feet plus of valley-floor deposits recorded in the well near Frog Pond (Table 7-1) may be representative of the depth of fill. All of these observations are consistent with a long history of persistent deposition in compartments, where Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1231, Packet Page 1331

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sediment could be deposited in slow-velocity water spread over much of the valley floor. If so, the flat and wide valley floor shown in the earlier topographic maps was largely unchanged from the gradual filling of the valley that had taken place for many thousands of years. In our view, the compartments were kept from filling because the tributary deposition at narrow points in the valley kept raising the base level of the spillways between the compartments. It is also possible to consider the seemingly even-aged stand of oaks on the valley floor east of York Road (Figure 7-6) as having germinated in the deposits left in that compartment after an event of unknown age perhaps a century or two ago.

The date of this event is perhaps not as significant as the extent of its deposits, which seem to have covered much of the valley floor.

In summary, portions of CdR upstream of the Highway 68/218 intersection (Tarpy’s Corners) have been -- for many thousands of years -- a series of naturally-formed compartments, separated by low depositional fans acting as dams. The dams, only a few feet high, are commonly formed where alluvial fans from tributaries have prograded onto the valley floor, often extending all the way across the valley, sometime meeting another fan emanating from a tributary on the other side. Other processes, such as landslides, debris flows, or even logjams, can also lead to impounding rises which form compartments.

Under natural conditions, these compartments accumulate sediment gradually, as relatively low volumes of sediment have been introduced from the uplands except during years of very high rainfall and rainfall intensities, or runoff events following wildland fires. Over the years, each individual compartment has trapped sediment and attenuated flows. Storms passing through individual compartments lost much of their sediment and peak flows were significantly attenuated. The amount of sandy and silty sediment moving downstream to the next compartment was much reduced. The hydrologic modeling described in Chapter 3 shows a very high ratio of 100-year to 10-year peak runoff events, suggesting that on very rare occasions (perhaps only once or twice per century) flows were sufficient to pass through multiple compartments and reach Laguna Grande; however, the extreme events may have been those which built the alluvial fans and other constricting sediment accumulations. Under these natural conditions, only rarely would high, sediment-laden flows pass all the way through the watershed.

This overall pattern continues under existing conditions. Greg James from MPWMD notes that peak flows exceeding 40 to 50 cubic feet per second (cfs) are rare, with only a handful recorded in the 40+ years of measuring peak flows at the MPWMD gage. Yet conditions have changed, such that the Canyon del Rey channel has been incised to depths of 20 feet or more by changes in storm runoff and sediment yields. Where the sequence of accumulated sediments have been incised or exposed in recent years, their composition shows that the individual deposits have originated from both sides of the valley. Further, about half of the sedimentary sequence seems to be composed of beds up to 4 feet thick of alluvium. In all likelihood, these thicker beds have been reworked from episodic events, such as post-wildfire or post-landslide deposition. “Regular flood deposits”, which have deposited thinly-laminated strata, are interleaved through this sequence. This historical pattern will likely continue into the future, Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1232, Packet Page 1332

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modified by human activity. Drainage planning should recognize that largescale deposition will occur from time to time, perhaps at expected recurrences of perhaps 50 to 100 years, or perhaps longer. Additional sedimentation and incision associated with human activity, will also affect deposition, which also may be concentrated in one or more the compartments.

EVIDENCE FROM SEDIMENTOLOGY AND MINERALOGY
The lower portion of the watershed is more altered, where some compartments may not be a clearly bounded. In a sedimentologic reconnaissance effort, bed sediments from the streams and samples from the dunes were used to infer processes and history of the channel, and material which it moves.

Samples from the bed were collected from 4 sites downstream from Tarpy’s Corners:
• From Canyon del Rey about 10 yards upstream from the outlet of Frog Pond
• From the channel approximately 100 yards upstream from culvert beneath the Safeway store at the corner of Highway 218 and Fremont Street
• Upstream from Laguna Grande, about halfway upstream to Fremont Street
• Approximately 50 feet upstream of the outlet weir of Roberts Lake Also collected were samples from:
• Active dunes near the mouth of Canyon del Rey, and
• Paleodunes in the Pebble Beach area, pre-dating any possibility of human influence.

Samples were collected from the upper 6 inches of the bed or dune, at locations upstream from evidence of recent roadwork or deposition. After collection, the sediments were dried, sieved and examined for rounding and gross mineralogy at the Balance laboratory.

Results are presented in Appendix I, and summarized in Figure 7-14. The observations indicate that sediments throughout the lower portion of the creek are angular or subangular, rather than the rounded and subrounded grains characteristic of the dunes. The bed sediments are substantially coarser, as well. White shale chips appeared at the site above Frog Pond, perhaps Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1233, Packet Page 1333

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reflecting relative recent input of shale, possibly from the south side of Hwy 218, where it forms most of the south bank of the channel; the proportion of white shales are higher in the two samples downstream, upstream from Laguna Grande, strongly suggestive of shale input from other sources. At Roberts Lake, the shales have weathered to grey, suggesting much older delivery to the channel. Perhaps most surprising is that the size, angularity, and mineralogy of the sample from near the Roberts Lake outlet weir were overwhelmingly angular, indicating deposition from the stream, with likely little input from the local dunes.

Figure 7-14 shows the results of sieving each sample and plotting the cumulative weight percentage of each size fraction. Results are presented and documented further in Appendix I. Among the key findings are:

1. All four samples within the Canyon del Rey watershed are composed overwhelmingly of stream-transported sediment, demonstrating sorting, particle-size, and angularity/roundness data characteristic of stream sediments.

2. Very little of the sediment at any site had the characteristic size, sorting, mineralogy or roundness typical of dune sands.

3. Particle sizes within the lower half of the watershed (downstream from junction of Highways 218 and 67) are large relative to upper site, suggesting that bed material originates from the banks and bed of the stream in its lower half. It is doubtful that material of the sizes observed could have transported through the Monterra compartment upstream of the junction.

4. Shale chips were predominantly angular, implying that they had not been transported a long distance.

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Figure 7-14: Sediment size distribution of Canyon Del Rey bed-material samples and surrounding dunes downstream from Tarpy's corner.

Fluvial samples are represented with solid lines, dune samples with dashed lines. Note the general downstream coarsening of bed material in Canyon del Rey. The dune samples are more well sorted than the fluvial samples, especially in the coarser fractions. The dune samples are finer, and are coarsely skewed or symmetric, while the fluvial samples are finely skewed. Skewness and sorting variations indicate different transport mechanisms (Prothero and Schwab, 1996). Table 1 provides detailed grain size statistics.

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7.4 Management implications of the sediment study
Two alternative strategies for managing sediment are available to the managing agencies:
a. A longitudinally integrated emphasis, in which sediment is purposely passed downstream through channel segments of varying stability to Laguna Grande, from which it can periodically be dredged, with the predominantly sandy sediment used for beach nourishment or as clean fill for other projects.
b. The native geomorphic solution in which sediment will be managed on a compartment by compartment basis, resulting in much-reduced sediment delivery to the Laguna Grande and Roberts Lake, and moderate attenuation of peak flows as the flat valley floor is used for detention storage.

This section of the report outlines how the observations obtained during this investigation may be applied to implementing the integrated or compartment approaches, or (potentially) a combination of the two.

KEY PHYSICAL FINDINGS
Four key physical findings of this report can shape evaluation of the two alternatives:
• The apparent minimal post-1983 sediment delivery to Laguna Grande and Roberts Lake
• Intrinsic emphasis upon high-recurrence events as demonstrated by modeling and which can be inferred from the gaging record. • A long geomorphic record of episodic deposition – thick sedimentation units exposed in the floor of the valley, principally upstream from York Road – indicating that lenses of sediment 3 to 5 feet thick were periodically deposited in individual compartments.
• Evidence that depositional compartments are a native geomorphic response to the sandy watershed, and are presently effective in inhibiting sedimentation of Laguna Grande.

Sediment will continue to enter the Canyon del Rey valley both gradually and episodically. The chronic sediment generated by routine storms will pass through the stream system, or be retained on the valley floor. Episodic delivery of sediment -- as runoff during very major storms, post-fire runoff, landslides, debris flows and perhaps other rare events -- will enter the valley at discrete Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1237, Packet Page 1337

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locations and then collect on the floor of the compartments. The fate of the sediment, the stability of the channel downstream, and the extent of deposition in Laguna Grande/Roberts will differ between the two approaches.

LONGITUDINALLY INTEGRATED APPROACH
Management of sediment would emphasize passing sediment downstream to Laguna Grande. This management approach would be implemented by breaching barriers and lowering the roughness of channel segments between the compartments. Chronic sediment generated by typical storms will pass through the system. Sediment associated with episodic events would primarily move through the tributaries and main stem as pulses of varying durations, which can vary from a single storm or season to several events. Peak flows will be slightly attenuated. The combination of more sediment moved at higher flows could result in a wider and shallower main-stem channel downstream, possibly resulting in additional bank erosion where the banks are sandy or where deep-rooted vegetation may be insufficient to prevent bank retreat.

Under this approach, management of drainage and sediment will be similar to practices applied on other streams of roughly similar size, such as Toro, Gabilan, or Prunedale creeks.

COMPARTMENT APPROACH
Management of sediment would emphasize retaining it on the valley floor as close to the source as feasible. Some of the sediment will move downstream into the next sequential compartments, but most would remain within the compartment to which it was delivered. Relatively little delivery to Laguna Grande would occur. The intervening channel segments would not be stressed with as much sediment and with slightly lower peak flows. The valley floor would come to gradually be covered by sediment to a depth of up to a foot or more. If the source channel for an episodic event is on south side of the valley, it is possible that Highway 68 may be overtopped or affected in some way.

After the event, sediment can be removed, or left in place, partially exhausting the capacity of the compartment. Inundation levels on the floodplain within the affected compartment may end up increasing by a small amount, requiring re-mapping of the floodplain at intervals similar to those of the episodic events, perhaps several decades to several centuries. Attenuated peak flows and less sediment would enter the main-stem channel downstream from the affected compartment, with likely fewer downstream effects than might be the case with the longitudinally integrated approach.

BLENDED APPROACH
It may prove possible to use a blended approach in which compartments are used in the upper portion of the watershed, with an integrated channel downstream from either the highway bridge just below Frog Pond or the intersection of Highways 218 and 68.

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RECOMMENDATIONS
Recommending one approach over the other goes beyond the scope of this project. The use of a novel – albeit native – drainage management approach (the compartment approach) calls for a broader planning perspective. Effects on habitat, existing policies and regulations, and public safety all warrant greater consideration before a suitable approach can be adopted.

However, it may make sense to keep managing sediment in Canyon del Rey using the conceptual approach of preserving compartment functionality, particularly in portions of the watershed which are less urbanized. We believe that the compartments or blended approach will better serve the Monterey Bay community if the flood mapping and several other institution hurdles can be overcome.

THE ROLE OF DREDGING OF LAGUNA GRANDE AND ROBERTS LAKE
Dredging of Laguna Grande/Roberts Lake is reported to have occurred in 1983.

Additionally, Laguna Grande was deepened and expanded in 1870, presumably using horse-drawn equipment to form ‘Lake Como’ (see Figure 7-3).

We have not heard of nor found evidence of other dredging activities of meaningful scale, nor do the effects of dredging appear in the historical maps and aerial photography presented in Appendix H. Further, minimal deposition is discernible in the high-precision bathymetric mapping conducted by CSUMB (Figure 2-4). A meaningful analysis of sedimentation should usefully account for substantial deposition prior to the 1870 and 1983 events, with very little postdredging deposition.

PAST RESPONSE TO URBANIZATION
Appendix H depicts the rapid growth of Seaside, plus North Monterey and Del Rey Oaks, during the decades following World War II. While 1913 maps show only a handful of homes in the areas surrounding Laguna Grande, the lower watershed had reached more than 75 percent urbanization by the early 1980s, principally through the growth of Seaside and expansion of the Monterey airport. The hydrology of the lower watershed had fundamentally changed by that time, from an area contributing only a few percent of incident rainfall as runoff to most rain being drained to storm drains, many of which discharged to the lower Canyon del Rey watershed. With the additional runoff, much additional sand was transported to the lower portion of the watershed.

LIKELY RESPONSE TO FUTURE URBANIZATION
Given the limited area in the lower watershed which might be further urbanized, the likelihood of another pulse of sedimentation entering the lakes is small, unless a choice is made to accelerate movement of water and sediment through the compartments of the upper watershed (see “Implications” section, Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1239, Packet Page 1339

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below). Measures already taken to reduce downstream effects of urbanization are expected to further reduce delivery to the lakes.

One significant exception is potential delivery from the FORA lands east of General Jim Moore Drive, presently undeveloped. These drain primarily to the “Southside tributary”, a topographic catchment of about 2.8 square miles along and north of the South Boundary Road (see Figure 7-11, Compartment 0, within Compartment 6), and through Frog Pond Wetland Preserve (FPWP) to Canyon del Rey. Projected urbanization or compaction of surfaces in this area can potentially lead to a pulse of sediment entering Canyon del Rey through FPWP. The current FORA plan calls for development of 600 to 700 acres within this area, which is within the Sphere of Influence of Del Rey Oaks. The most current plans (2007), prepared prior to the current economic slowdown, are oriented toward visitor-commercial uses which can generate substantial concentration of storm runoff. Recent evaluation of conditions on either side of South Boundary Road (Geisler and Smith, 2014) demonstrates high vulnerability of soils in that area to culverts draining the road and to runoff from the City of Monterey corporation yard.

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8 RECOMMENDED FACILITY IMPROVEMENTS

8.1 General
Recommended improvements to facilities and the cost of those improvements are provided in this section. The existing facilities are described in Chapter 2, with photographs assembled in Appendix C and field inspection notes provided in Appendix B. Both the photographs and notes are organized sequentially by facility number. The locations of the facilities are shown on the sub-watershed map panels provided in Appendix A. The facility descriptions in Chapter 2 also indicate which map panel shows the particular facility.

Digital file submittals, which are not an integral part of this report, include AutoCAD drawings of each sub-watershed which provide considerably more detail regarding location, orientation, and elevation of each facility. Appendix F provides a summary of the digital files that are available. Recommended improvements
Table 8-1 provides a list of recommended upgrades for specific culverts that were found to be inadequate for safely conveying the 100 year peak flows.

Certain of these recommendations match improvements recommended by the 1977 report. These recommended improvements are proposed primarily to increase high flow carrying capacity, so that roads are not excessively overtopped by high water and safe conditions are maintained. The improvements are specifically not proposed to decrease detention of high flows, as such detention is integral to flood management in the watershed.

8.2 Additional rain and stream gages
The 1977 Plan provided specific recommendations regarding addition of rain gages and stream gages within the watershed, designed to provide more complete hydrologic data suitable for future storm water analyses and planning. While some additional meteorological data is available today in the watershed, hydrologic data remains sparse. The 1977 recommendations mostly remain appropriate and would provide highly useful information. New rain gages could include:
• at Laguna Seca Race Track (elevation 750 feet);
• at a central point along highway 68, if the Pasadera Golf Course weather station does not provide adequate rainfall information; • at the southeasterly boundary of the watershed, perhaps at the junction of Laureles Grade Road and El Toro Road (elevation 850 feet); and • at an accessible location in one of the southwestern sub-watersheds (26 or 23) at moderate elevation (400-600 feet).

Assuming that the Frog Pond gage operated by CSUMB continues to provide usable stream flow data, an additional gage could be usefully sited at the York Road crossing.

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Culvert
01_C_02
02_C_01
03_C_01
03_C_02
10_C_01
12_C_01
14_C_01
15_C_01
21_C_01
25_C_01
25_C_02
27_C_03
27_C_04
29_C_01
29_C_03
29_C_07
Replacement Recommendations
Replace with 10 ft2 area culvert or ditch. 2' X 5' box culvert.

Replace with 16 ft2 area box culvert. 2' X 8' box culvert.

Replace 18 ft2 area box culvert. 3' X 6' box culvert.

Replace with 24 ft2 are box culvert. 4' X 6' box culvert.

Add 48" concrete pipe or equivalent total capacity of 20 ft2.

Add parallel 60" RCP.

Add parallel 30" RCP, or equivalent total capacity of 18 ft2.

Replace with 40 ft2 area box culvert. High priority.

Replace with 70 ft2 area box culvert. High priority.

Replace with 100 ft2 box culvert. Private facility.

Add parallel 100 ft2 box culvert. 6' X 8' box as suggested in the 1977 report.

Replace with 65 ft2 area box culvert. Will reduce detention storage.

Replace with 81 ft2 area box culvert.

Replace with 90 ft2 area box culvert. 12' X 7.5' box as suggested in the 1977 report.

Add parallel 6' X 8' box as suggested in the 1977 report.

Add parallel 6' X 8' box as suggested in the 1977 report.

Table 8-1: Recommended culvert upgrades.

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9 FACILITY IMPROVEMENT COST ESTIMATES

9.1 Cost estimation methodology
Concept-level plans were developed for each of the identified upgrade projects for the purpose of project scoping and cost estimation. The scope of each culvert upgrade is described in Table 9-1, below.

The estimates utilize unit pricing for the major work items such as structures, pipes, inlet/outlet protection, and pavement replacement. Bid item scope, where an “item code” is given, generally follows the 2010 edition of the Caltrans Standard Specifications and Standard Plans. Unit costs for these bid items were developed based on an analysis of costs reported in the Caltrans Contract Cost Database (CCDB) and bid results from similar, selected projects completed in Caltrans Districts 4 and 5. Details of those projects are provided in the separate electronic record submittal.

A concept sketch was developed for each culvert upgrade for the purpose of general project scoping, estimating quantities for the major items of work, and developing budgets for temporary traffic and environmental controls and final landscaping and restoration. These concept sketches and quantity calculations are provided in the separate electronic record submittal.

Temporary controls, including temporary traffic control, construction area signs, staged construction, excavation storing, temporary environmental pollution controls, construction of temporary access roads, and temporary creek diversions, were included as lump sum items. Temporary controls account for between 5% and 35% of the total estimated cost.

Electrical and landscape work were similarly included as lump sum items.

Landscape items are referred to generally in the estimates as one of the following (listed in order of increasing cost): highway planting (basic erosion control and seeding); planting and irrigation (highway planting plus container plants and temporary irrigation); and creek/riparian restoration and planting. In relative terms, highway planting accounts for 1% to 2% of the total project cost, and creek/riparian restoration and planting accounts for between 8% and 15% of the total estimated project cost.

Minor work items, supplemental work and contingencies are each included as a percentage mark-up to the work items identified above. “Minor items” are those items which are not specifically enumerated, due to the preliminary nature of the estimate, but which are anticipated to be included in the final construction documents. Supplemental work is work which could be identified and added to the project after bidding, during the construction process. The “contingencies” category accounts for potential scope items which are not anticipated at this conceptual stage.

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Detailed cost estimates for each of the identified projects are provided in Appendix E.

Plans and bid results for several Caltrans projects of similar scope and cost were obtained and compared to the projects evaluated in this study. This comparison informed selection of unit prices used in the cost estimation within this study. A review of these projects for total project scope and cost, unit costs for individual work items, and costs for temporary environmental and traffic controls and landscape work was included. The bid summaries for these projects are provided in the electronic submittal that accompanies this report.

9.2 Facility improvement costs
The preliminary cost estimates for the identified culvert upgrades are provided in Table 9-1, below.

ID Conceptual Project Scope
Estimate of
Probable
Construction
Cost
01-C02
Remove two (2) existing 18” CMP culverts located in
north shoulder of and running parallel to Highway 68 at Laureles Grade. Construct 5’W x 2’H x 130’L RCB, and
install RSP inlet and outlet protection. Includes traffic signal conduit relocation and reconstruction of existing bus stop.
$ 158,000
02-C01
Remove existing double 20”x28” CMP culvert which
crosses Highway 68 just west of Laureles Grade.

Construct 2’H x 8’W x 110’L RCB, and install RSP inlet and outlet protection. Existing culverts are shallow and
existing utility crossings are anticipated to require similarly shallow RCB. Staged construction.
$ 599,000
03-C01
Remove existing 30”x48” CMP culvert which crosses
under secondary Laguna Seca Recreation Area access
road. Construct 3’H x 6’W x 70’L RCB, and install RSP inlet and outlet protection.
$ 187,000
03-C02
Remove existing 40” HDPE culvert which crosses under
Laguna Seca Recreation Area entrance road. Construct
4’H x 6’W x 80’L RCB, construct concrete wing walls and grade control apron on inlet side, and install RSP outlet protection.
$ 253,000
10-C01
Jack 48” dia x 170’L RCP culvert parallel to existing 36” CMP culvert which crosses Highway 68. Construct
concrete head walls and RSP inlet and outlet protection.

Perform creek restoration and planting.
$ 574,000
Table 9-1: Preliminary construction cost estimates.

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12-C01
Construct temporary roads to access construction areas.

Jack 60” dia x 110’L RCP culvert parallel to existing 48” CMP culvert which crosses Highway 68. Construct
concrete head walls and RSP inlet and outlet protection.

Perform creek restoration and planting.
$ 537,000
14-C01
Construct temporary roads to access construction areas.

Jack 48” dia x 160’L RCP culvert parallel to existing 48” CMP culvert which crosses Highway 68. Construct
concrete head wall at inlet and RSP outlet protection.

Perform creek restoration and planting.
$ 566,000
15-C01
Remove existing 48” CMP culvert which crosses under
emergency access road located off Blue Larkspur Lane.

Construct 5’H x 8’W x 110’L RCB, construct concrete
wing walls on inlet side, and install RSP outlet protection. $ 352,000
21-C01
Remove existing double 24”x28” culvert which crosses
Highway 68. Construct double 5’H x 7’W x 68’L RCB,
head walls, and RSP inlet and outlet protection. Staged construction.
$ 417,000
25-C01
Remove existing 2-48” and 3-18” culverts which cross
under Monterra Ranch entrance road. Construct 6’H x
12’W x 135’L RCB, construct inlet control structure, and install RSP outlet protection.
$ 573,000
25-C02
Remove existing double 48” RCP culvert which crosses
Highway 68 just west of the Highway 218 intersection.

Construct 7’H x 12’W x 120’L RCB, head walls, and RSP
outlet protection. Staged construction.
$ 683,000
27-C03
Remove existing 88” CMP culvert located in the west
shoulder of and running parallel to Highway 218 at the
entrance to Del Rey Gardens Drive. Provide temporary
shoring and creek diversion and construct 7.5’H x 12’W x 380’L RCB, head walls, and RSP inlet and outlet
protection.
$ 1,645,000
27-C04
Construct 8’H x 6’W x 42’L RCB culvert parallel to existing 8’H x 6’W x 42’L RCB culvert which crosses Highway 218
and provide RSP inlet protection. Staged construction.
$ 341,000
29-C01
Construct 8’H x 6’W x 42’L RCB culvert parallel to existing 8’H x 6’W x 42’L RCB culvert which crosses Highway 218
and provide RSP outlet protection. Staged construction. $ 324,000
29-C03
Remove existing privately owned 18’ span wood deck
bridge and concrete abutments. Construct new
abutments and 24’ span wood deck bridge and
concrete abutments and relocate associated private
utilities.
$ 139,000
29-C07
Construct 8’H x 6’W x 42’L RCB culvert parallel to existing 8’H x 6’W x 42’L RCB culvert which crosses Rosita Road
and provide RSP outlet protection.
$ 324,000
Notes:

1. Estimates include supplemental work and contingencies.

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2. Estimates dos not include “soft costs”, i.e., project management, engineering, environmental, permitting, inspections, or testing.

3. Costs are provided in year 2014 dollars. Costs should be escalated for later years.

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10 RECOMMENDATIONS

10.1 Utilization of this report and electronic submittals Large amounts of data were collected, organized and archived electronically during this study. Data sets include:
• rainfall and stream flow gauge locations and records; • geographic information including land use, soils, ground slopes and impervious cover;
• facilities data including location, dimensions, configuration, and physical condition;
• stream channel morphology and condition;
• facilities upgrade information including concept designs, preliminary cost calculations, unit costs;
• HEC-HMS hydrologic model set up and parameter definitions; and • HEC-HMS model predictions of stream flow hydrographs at multiple locations with the watershed and for multiple return period and duration rainfall events.

These data sets have been transmitted to the MPWMD. We recommend that these data sets be placed on an active data server and made available to District and local city staff members as well as engineers and planners working on projects in the watershed. Appendix F, attached to this report, provides a list of available data.

10.2 Lake management and preservation recommendations
Bathymetric surveys of Roberts Lake and Laguna Grande were used to define the extent of sediment deposition in the lakes. These surveys produced clear indications that very little sediment has accumulated in either lake over the past several decades. The lack of sediment deposition in the lakes correlates with the extensive evidence of sediment deposition in the watershed, the low peak flows reaching the lakes, and the lack of sediment transport through the creek.

Changes in the runoff due to fires, urbanization or upgrades to flood facilities have the potential to increase sediment transport into the lakes, accelerate deposition of sediments and change lake conditions. Possible changes include Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1247, Packet Page 1347

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shoaling, impaired water quality, eutrophication, and weed growth. Decisions regarding land development, wild fire management, road improvements, and upgrades to the flood conveyance facilities should be made with full understanding of potential impacts on sediment transport into the lakes.

10.3 Facility improvement priorities and execution
The flood management facility improvements recommended in this plan are estimated to cost, in aggregate, $6.5 million. With limited construction budgets, it is important to focus efforts on the most important improvement actions, based on the magnitude of problems with a structure, the expected efficacy of the improvement, and the cost of the improvement. Highest priority proposed improvements are as follows.
• Culvert 21_C_01 (highway 68 at Ryan Ranch): capacity is sufficient to pass safely 57% of the peak 10 year flow and 18% of the peak 100 year flow
• Culvert 15_C_01 (highway 68 near Laguna Seca Golf Ranch): capacity is sufficient to pass safely 87% of the peak 10 year flow and 27% of the peak 100 year flow; drop inlet design is restrictive
• Culverts 3_C_01 and 3_C_02 (highway 68 at Laguna Seca access road):
capacity is sufficient to pass safely 43-65% of the peak 10 year flow and 11-18% of the peak 100 year flow
• Culvert 27_C_03 (Del Rey Gardens entrance from highway 218): capacity is sufficient to pass safely all of the peak 10 year flow and 43% of the peak 100 year flow; capacity needs to match that of upstream culverts Many other existing culverts have inadequate capacity to safely pass the 100 year peak flow; a few additional culverts (01_C_02A, 01_C_01B, 02_C_01) have inadequate capacity to pass the peak 10 year flow. These projects are the next priority.

10.4 Impact of facility improvements on flood flows
The HEC-HMS hydrologic model was used to simulate flood flows under a hypothetical condition where no restrictions on flow and no detention storage exist in the watershed (see figures 5-1 and 5-2 and associated text). This condition is the extreme case where all culverts and controls on detention basins are removed from the watershed, thus maximizing peak flows and runoff volume. The model predicts that removing all storage would increase peak 10 year and 100 year flow at the discharge to Laguna Grande by approximately 250% and 290%, respectively. Total storm volumes for the 10 year and 100 year events increase by 160% and 150%, respectively.

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Additional model simulations were made to predict the effect of the proposed culvert upgrades on storm related flows in the creek (see figures 6-1 and 6-2 and the associated text). Maximum percentage increases in peak flows are predicted for basin 14_B_01 (120%) during the 100 year event and for basin 29_B_01 (115%) during the 10 year event. These increases are relatively modest because most detention storage is unaffected by the recommended facility improvements.

These model predictions exemplify the potential impact of improving conveyance on storm related flows in the creek and highlight the importance of detention storage within the watershed.

10.5 Flow control measures for future development
Currently applicable runoff management and flood control regulations provide the fundamental means to manage the impacts of development and other changes in the watershed. In addition to the regional regulations, oversight of any modifications to existing detention basins (both created by culverts and designed with outlet works) will be essential to maintaining storage capacity.

Further development in the watershed, particularly in the vicinity of the creek channel, has the potential to disrupt the equilibrium conditions that maintain channel morphology and stability, as well as change the dynamics of sediment deposition and erosion. Such changes should be studied carefully before those changes are permitted. Provided that current detention storage is maintained, there is no obvious need for additional detention storage facilities or channel protection works.

10.6 Recommendations regarding sediment management
Since episodic events, including landslides and wildfires, appear to be a major contributors of sediment to the drainage channels in Canyon del Rey due both to (a) soil properties, which result in much higher unit runoff for 100-year storms than for 10-year events, and (b) geomorphic compartments, which retain sediment near the locations where it enters the valley floor. Exposures of the valley floor show that episodic sedimentation accounted for 50 to 60 percent of sediment accumulating on the valley floor, even before European land-use practices affected the Canyon. Anticipating such events, including providing room for sediment storage or disposal, will likely reduce the frequency of sediment movement in the watershed and, ultimately, into the lakes.

The many engineered and natural detention basins within the watershed serve to both control high flows and trap mobilized sediment. Maintenance and management of these basins will allow continued functioning, as designed, to reduce flooding, prevent channel migration and instability, and limit siltation of the lakes. Such maintenance will be particularly important after large rainfall events, landslides, wildfires, and any other episodic events.

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Watershed management activities designed to reduce erosion, particularly in the steep and poorly vegetated upland areas, can be used to reduce and control sediment production and flow to the detention basins and stream channels. Such activities are likely to reduce detention basin and channel maintenance activities and costs.

Given the limited area in the lower watershed which might be further urbanized, the likelihood of future pulses of sediment entering the lakes is small.

Measures already taken to reduce downstream effects of urbanization are expected to further reduce delivery to the lakes.

One significant exception is potential delivery from the FORA lands east of General Jim Moore Drive, which are presently undeveloped. These lands drain primarily to the “South Boundary tributary”, a topographic catchment along and north of the South Boundary Road, and through Frog Pond Wetland Preserve (FPWP) to Canyon del Rey. Projected urbanization or compaction of surfaces in this area can potentially lead to a pulse of sediment entering Canyon del Rey through the frog pond. Management of development in this area would likely reduce future sediment migration and lake deposition.

Assuming that watershed conditions are managed, no need for dredging of Laguna Grande and Roberts Lake during coming decades is anticipated. In addition, the lakes are not likely to be a regular source of dredged material for beach nourishment. These conclusions are based on the very low and intermittent supply of sediment to the lakes from the watershed.

10.7 Use of this Master Drainage Plan in ongoing planning and design of storm The HEC-HMS model of the Canyon del Rey watershed and creeks, together with the precipitation predictions, provide an easily accessed and relatively simple to use method for predicting the hydrologic impacts of changes to the watershed. The model input files and related spreadsheets are provided in the separate electronic submittals. Changes to input data can be readily made to represent proposed development or changes in flood management facilities.

The HEC-HMS model, when run with the appropriate changes, will provide detailed flow predictions which can be compared with the results shown in this report.

The descriptions of flood management facilities, including the appendices and electronic submissions, contain a wealth of information about each of the flood management facilities, including current condition. A number of the culverts have identified problems with deterioration, erosion of the channel at the culvert outlet, and/or accumulation of sediment deposits that partially or fully occlude the culvert entrance. For culverts that have identified problems (see filed notes in Appendix B and field photographs in Appendix C) but are not going to be upgraded, it is recommended that proper maintenance be performed. In addition, culverts that have identified siltation or erosion Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1250, Packet Page 1350

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problems should be inspected periodically (every 3-5 years) and maintenance performed as needed to maintain full culvert function.

Detention basin functionality depends on maintenance of adequate storage volume. Over time sediment will enter the basins and deposit there, decreasing available storage volume. Detention basins must be regularly inspected for excess sediment deposition and sediment removed to maintain capacity.

Areas within the creek channel system and specific detention basins which are most prone to sediment deposition are detailed in Chapter 8. These designations can be used to inform a detention basin management program.

Reaches of the creek which are designated as eroding in Chapter 8 of this report are likely to suffer continuing creek channel instabilities such as downcutting or channel migration. Periodic inspection of these reaches will be helpful in defining channel changes and the need for restoration.

10.8 Future revisions and updates to this plan
Revisiting and updating of this plan is recommended on a ten year cycle. This will ensure that the plan remains reasonably up to date and useful in planning of further urbanization and flood management facility repairs and upgrades.

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11 REFERENCES
Balance Hydrologics, 2013, Proposal to provide Dry-Season Monitoring services for the Pebble Beach golf links.

Brown, W.M., III, 1973, Erosional processes, fluvial sediment transport, and reservoir sedimentation in a part of the Newell and Zayante Creek basins, Santa Cruz County, California: U.S. Geol. Survey Open-File Report, 29 p.

Brown, W.M., III, and Jackson, L.E.,Jr.,1973, Erosional and depositional provinces and sediment transport in the south and central part of the San Francisco Bay Region, California: U.S. Geological Survey Open-File Report MF-515, 21 p.

Dibblee, T.W., 2007, Geologic map of the Monterey and Seaside Quadrangles, Monterey, California, Santa Barbara Museum ofNatural History, Dibblee Geology Center Map #DF-346
Cain, J., 2005, Monterey’s Hotel del Monte, Arcadia Publishing, 128 p.

California Ocean Plan, 2009. California Environmental Protection Plan State Water Resources Control Board.

Clark, Joseph C., Dupre, William R., and Rosenberg, Lewis I., 1997, Geologic Map of the Monterey and Seaside 7.5-minute Quadrangles, Monterey County, California: A Digital Database: U.S. Geological Survey Open-File Report 97-30.

Clark, J.C., Brabb, E.E., Rosenberg, L.I., Goss, H.V., and Watkins, S.E., 2000, Geologic map and map database of the Spreckels 7.5-minute quadrangle, Monterey County, California: U.S. Geological Survey Miscellaneous Field Studies Map 2349, U.S. Geological Survey, Menlo Park, CA.

Daly, C., et al, Physiographically sensitive mapping of climatological temperature and precipitation across the conterminous United States, Int. J.

Climatology, 2008
Duncan and Jones, 1981, Land-use plan for the Laguna Grande/Roberts Lake Local Coastal Program: Prepared for the City of Seaside and the City of Monterey, California, 97 p.

Federal Emergency Management Agency (FEMA), Flood Insurance Study – Monterey County, CA, 2009
Geisler, E. and Smith, D.O., 2014, Frog Pond Wetland Preserve enhancement and erosion-control plan. Draft of June 27, 2014. 69 p.

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Glysson, G.D., 1977, Sedimentation in Santa Margarita Lake, San Luis Obispo County, California: U.S. Geo. Survey Water-Resources Investigations Report 77-56, 15 p.

Hampson, L., 1997, Sediment transport analysis, Carmel River near Carmel, Water Years 1992 – 1995: Monterey Peninsula Water Management District Technical Memorandum No. 97-03.

Hecht, B., 1980. Water quality in the management of Antonelli’s Lagoon, Santa Cruz, California: H. Esmaili & Associates consulting report prepared for Harvey & Stanley Associates, ecological consultants. 27 p.

Hecht, B., 1981, Sequential changes in physical conditions affecting aquatic habitat in the upper Carmel River, California, following the Marble-Cone fire of August 1977: in Proceedings of the California Riparian Symposium, Davis, CA, Sept. 1981.

Hecht, B., 2000, Drought, fire and geology – Key watershed influences in the northern Santa Lucia Mountains, in Salinia/Nacimiento Amalgamated Terrane, Big Sur costal, central California, R. M. Zatkin complier: Peninsula Geological Society, Spring Field Trip 2000, 11 p.

Hecht, B., and Napolitano, M.N., 1995, Baseline characterization of sediment transport and sedimentation in the Santa Lucia Preserve, Monterey County, California: Balance Hydrologics consulting report prepared for the Santa Lucia Preserve, 16 p + 3 appendices.

Hecht, B., and Rusmore, B.,eds, 1973, Waddell Creek: The Environment Surrounding Big Basin: UCSC Environmental Studies Program report prepared for the Sempervirens Fund. 98 p.+ 65 p. appendix
Hecht, B. and Woyshner, M.R., 1984, Response of headwater channels in sandy soils surrounding the Pajaro Valley to urbanization and other changes in land use.

Hecht, B., Johnson, N.M., and Esmaili, H., 1984. Pajaro Basin Groundwater investigation, HEA, a Division of J.H. Kleinfelder, p. C1-C65.

Jackson, L.E., 1977, Dating and recurrence frequency of prehistoric mudflows near Big Sur, Monterey County, California: U.S. Geological Survey Journal of Research, v. 5, no.1, p. 17 – 32
Knott, J.M., 1973, Effects of urbanization on sedimentation and floodflows in Colma Creek basin, California: U.S. Geol. Survey Open-File Report 54 p.

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Koretsky King Engineers and Monterey County Surveyors, 1977, Monterey County Master Drainage Plan - Canyon Del Rey Watershed Drainage and Erosion, Prepared for Monterey County Flood Control and Water Conservation District, 64 p.

Leopold, L.B., 1968, Hydrology for urban land planning – A guidebook on the hydrologic effects of urban land use: U.S. Geological Survey Circular 554, 18 p.

Leopold.L.B., Wolman, M.G., and Miller, J.P., 1964, Fluvial Processes in Geomorphology: San Francisco, W.F. Freeman & Sons, 522 p. (1964) Matthews, G.R., Discharge and sediment load for tributaries to the Carmel River:

Carmel River Watershed Management Plan Working Paper No. 5. Monterey Peninsula Water Management District report to the California Department of Fish and Game, 95 p.

Matthews, W.V.G., 1983, Discharge and sediment load for tributaries in the Carmel River: Carmel River Watershed Management Plan Working Paper No.

5. MPWMD report to the California Department of Fish and Game, 95 p.

Matthews, W.V.G., and Kondolf, G.M., Assessment of channel-forming discharge for the Carmel River: Prepared for the Monterey Peninsula Water Management District, 13 p.

NOAA, Precipitation-Frequency Atlas of the United States – Volume 6, Version

2.3: California, 2011 (revised 2014)
Nolan, M., and Marron, D., 1988, Stream channel response to the storm in the Santa Cruz Mountains: In Ellen, S.D., and Wieczorek, G.F., eds. Landslides, floods, and marine effects of the storm of January 3 – 5, 1982, in the San Francisco Bay Region, California: U.S. Geol. Survey Prof. Paper 1434, p. 245- 264.

Richmond, Sarah, 2009, Post-fire channel response: A comparison between the 1977 Marble-Cone fire and 2008 Basin Complex fire on the upper Carmel River. MS thesis, Energy and Resources Group, UC Berkeley. 62 p.

Schumm, S.A., Harvey, M.D., and Watson, C.C., 1984, Incised channels:

Morphology, Dynamics, and Control, Water Resources Publications, Littleton, Colorado
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Schumm, S.A., 1999, Causes and controls of channel incision. In: S.E. Darby and A. Simon (eds.), Incised River Channels: Processes, Forms, Engineering and Management. John Wiley & Sons : Chichester, United Kingdom, United Kingdom, pp. 19-33
Seaside Planning Division, in association with PMC, 2013, City of Seaside: Local Coastal Program Land Use Plan, 228 p.

Weber, G., 2008?, Map of landslides along the course of Scott Creek, Santa Cruz County, CA: .Landslide map of the Scott Creek watershed: Unpublished consulting map
Williams, J., and Matthews, W.V.G, 1986, The 1983 erosion event on Tularcitos Creek, Monterey County, and its aftermath: Proceedings of the California Watershed Management Conference, Nov. 18 – 20, 1986, West Sacramento, CA Univ. of Calif. Wildland Resources Center Report No. 11, p. 155 Yates, E.B., Feeney, M., and Rosenberg, L., 2002, Laguna Seca subarea Phase III hydrogeologic update: Consulting report submitted to the Monterey Peninsula Water Management District. 152 p.

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Monterey Peninsula, Carmel Bay, and South Monterey Bay
Integrated Regional Water Management Plan Update
APPENDIX 5-A

INTEGRATED REGIONAL WATER

MANAGEMENT INTER-REGIONAL

COORDINATION SUMMARY REPORT

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Integrated Regional Water Management Plan Update
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Council Regular Meeting, 2/7/2023, Item No. 10., Item Page 1257, Packet Page 1357 Project 5 – Inter-Regional Coordination
Interregional Coordination between the
Greater Monterey County and
Monterey Peninsula, Carmel Bay, and South Monterey Bay IRWM Regions
Summary Report (April 18, 2014)
Project 5. Integrated Regional Water Management Inter-Regional Coordination:

Greater Monterey County and Monterey Peninsula, Carmel Bay, and South Monterey Bay Regions
Abstract: The Greater Monterey County Integrated Regional Water Management (IRWM) region shares a border with the Monterey Peninsula, Carmel Bay, and South Monterey Bay (Monterey Peninsula) IRWM region. Along this border, the 45-square-mile Ord Community is a geographical transition zone containing areas and resources that are managed by many agencies, including some that are in both IRWM Regional Water Management Groups (RWMG). Fundamental challenges are: 1) determining which regional IRWM Plan proposed projects should be described in each IRWM Plan; 2) prioritizing projects in each region; 3) how to cooperate between regions in order to ensure that Ord Community projects do not fall into a “no man’s land” between the regions; and 4) moving projects forward that benefit both regions.

This report describes the relationship between the regions, identifies resource challenges, and outlines areas of potential coordination between the regions.

Prepared by: Denise Duffy & Associates, Inc.

Project Manager: Alison Imamura, AICP
Prepared for:

Monterey Peninsula Water Management District
District Engineer: Larry Hampson
on behalf of the
Regional Water Management Group of the Monterey Peninsula, Carmel Bay, and South Monterey Bay
Version Date: April 18, 2014
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Project 5 – Inter-Regional Coordination