360 E. 1st Street, #4126, Tustin, CA 92705 ◊ Phone 714-389-6009
HYDROLOGY STUDY
Montiel Road Proposed Office Buildings & Parking
2375 & 2355 Montiel Road
San Marcos, CA 92069
Report Date: March 12, 2019
Updated: June 29, 2022
EPS Project Number: 20-0567
Prepared For
Jamie Looney
United Pacific, Inc.
550 Laguna Drive, Suite A
Carlsbad, CA 92008
(760) 201-6524
[email protected]
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1.0 Narrative
The proposed project located at 2355 and 2375 Montiel Road, San Marcos, CA 92609, is comprised of a 2- story commercial office building with a footprint area of 16,849 square feet and an at -grade parking lot with 185 parking stalls. The purpose of this document is to determine the sufficiency of the proposed storm water mitigation techniques adopted for the site.
The existing site is mostly unpaved with an approximate paved area of 12,470 square feet . There are two main existing structures on the site with footprint areas of 1,575 and 2,150 square feet respectively. There are also a number of sheds and chicken coops on this site. All existing drainage sheet flows southerly to Caltrans right-of-way (Highway 78) . The proposed site is design ed to capture and retain site flows from DMA1 and DMA3 utilizing tree wells. An excess of 119 cubic feet of runoff from DMA1 will be discharged into Caltrans right -of-way. If necessary, the 119 cubic feet of runoff from DMA1 will be stored and discharged with flow control device to ensure post-development runoff from site into Caltrans right-of-way does not exceed pre-development volume. The remaining site runoff will be diverted to DM A2, which will retain the runoff on site through infiltration. Stormwater runoff from a small portion of the site being comprised of a driveway ramp and stairs sheet flows to the adjoining property on the southeast . The proposed stairs and drive ramp will be used by the adjoining property for access to parking lot. If both the Biofilter and Permeable Pavement system is clogged, an overflow drainage system is available to handle loads. Stromwater runoff from the building roof will be conveyed to surface drainage via downspout and ultimately into the proposed DMA2.
2.0 Hydrology
Hydrology shall be performed per the San Diego County Hydrology Manual (SDCHM), June 2003 as prepared by the Department of Public Works, Flood Control Section. As the site is less than one square mile in size, and is an urban development consisting of small storm drains and drainage structures, the Rational Method shall be used per Section 3 of the SDCHM. Per Section 2.3 of the SDCHM, the design storm frequency shall be the 50-year storm event. For a better understanding on how storm rates affe ct the site, both the 25 - year and 2-year storm event will also be considered.
2.1 The Rational Method
The Rational Method Can be expressed as:
Q = CIA
Where:
Q = peak discharge [cfs]
C = Runoff coefficient, unitless
I = Average rainfall intensity for a duration equal to Tc for the area [in/hr] A = Drainage area contributing to the design location [Acres]
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Because of the conformance of units, acre-in/hr ≈ cfs
The runoff coefficient, C can be determined from the soil type and land use. The soil type is “Group B”. The existing land use is residential, with approximately 2 dwelling units per acre while the final land use is commercial, with approximately 83% impervious land area. Entering the table with Soil Type B the Existing Runoff Coefficient is 0.38 and the proposed Coefficient is 0.80.
The rainfall Intensity can be expressed by the following equation 𝐼 = 7.44 𝑃6 𝐷−0.645
Where:
P6 = Adjusted 6-hour storm rainfall amount (must be between 45% to 65% of the P 24, the 24- hour storm duration)
D = Duration [min], can use TC
From the Isopluvial maps provided in the SDCHM, the following values can be established:
Occurrence Interval 2-Year 10-Year 25-Year 50-Year
6-Hour Storm Event (P6) 1.6 in/hr 2.2 in/hr 2.6 in/hr 3.0 in/hr 24-Hour Strom Event (P24) 2.5 in/hr 4.0 in/hr 5.0 in/hr 5.5 in/hr P6/P24 64% 55% 52% 54.5%
All values of P6 can be accepted as they are within the 45% to 65% threshold.
TC can be defined as follows:
𝑇𝐶 = 𝑇𝑖 + 𝑇𝑡
Where:
Ti = Initial time of concentration
Tt = Travel time
T cannot be taken as less than 5 mins. Due to the relatively small size of the site, T is assumed to always be 5 minutes, which is, at worst, a conservative estimate as longer periods of T would result in less flow
Given these values of P6, D, and C the following equation can be developed:
𝑄 = 𝐶 × (7.44𝑃6 × 𝑇𝐶
−0.645) × 𝐴
Substituting for values, the following can be determined:
Occurrence Interval 2-Year 10-Year 25-Year 50-Year
Proposed Storm Event (QP) 3.37A 4.63A 5.48A 6.32A
Existing Storm Event (QE) 1.60A 2.20A 2.60A 3.00A
Delta Storm Event (ΔQ) +211% +211% +211% +211%
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For the sizing of pipes, the 50-year occurrence shall be used:
𝑄𝑃 [𝑐𝑓𝑠] = 6.32 𝑖𝑛 ℎ𝑟⁄ × 𝐴𝑟𝑒𝑎 [𝐴𝑐𝑟𝑒𝑠]
Tributary Area Area [Sqft] Acres QP-50 [cfs] QI-50 [cfs] ΔQ50 [cfs] Tributary Area 1 42113 0.967 6.12 2.90 3.21
Tributary Area 2 37237 0.855 5.40 2.57 2.83
Tributary Area 3 32.045 0.736 4.65 2.21 2.44
North Offsite Flows 940 0.022 0.14 0.07 0.07
East Offsite Flows 545 0.013 0.08 0.04 0.04
Site Total 112,880 2.591 16.39 7.79 8.59
NOTE: Runoff from the adjoining property on the northwest (APN 228-370-19) through the existing 18-in HDEP, through existing retaining wall near the most northwesterly corner of the subject property calculated based on the assumption of pipe flowing at a velocity of 10 ft/sec. is 17.7 cfs. This existing runoff, discharges to Caltrans’s right-of-way and will remain a pass-through flow to Caltrans right-of-way.
3.0 Hydraulics
Hydraulic analysis will be performed at a later design stage
4.0 Sedimentation
4.1 Near-Term Sedimentation Yield
The SDCHM required the Universal Soil Loss Equation (USLE) as defined as the following:
𝐴𝑆 = 𝑅𝐾𝐿𝑠𝐶𝑃
Where:
AS = Computed Soil Loss in Tons (Dry Weight) [tons/acre/year] R = Rainfall Erosion Index
K = Soil Erodibility Factor
L = Slope Length Factor
s = Slope Gradient Factor
C = Cropping Management (Vegetation) Factor
P = Erosion control practice Factor
The rainfall Erosion Factor (R) can be determined by entering Figure 5-2 of the SDCHM with the 2- year, 6-hour storm event. Entering the graph with the 2 -Year, 6-Hour storm event, t he Erosion Index Factor is found to be approximately 250.
Per the soils report, the topsoil layer of “Sandy Alluvium” is found for the first 5 to 10 feet. It is anticipated that during construction, this soil, along with imported fill, will be encountered most.
Utilizing Table 5-2 of the SDCHM the soil is a coarse sandy loam (VsC) with a K-Factor 0.28.
The Cropping Management Factor (C) shall be taken as 1.00 as there is assumed to be no ground cover, and the surface will be rough and irregular. The Eros ion Control Factor (P) 0.80, as a Sandbags shall be used to contain on-site erosive elements.
The SDCHM required the Universal Soil Loss Equation (USLE) cam be evaluated as follows:
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𝐴𝑆 = 𝑅𝐾𝐿𝑠𝐶𝑃
𝐴𝑆 = 250 × 0.28 × 𝐿𝑠 × 1 × 0.8
𝐴𝑆 = 56𝐿𝑠 𝑇𝑜𝑛𝑠 𝐴𝑐𝑟𝑒 − 𝑌𝑟⁄
Entering Figure 5.5 of the SDCHM with the Tributary Length (L) and the % slope (s) the Topographic factor (Ls) can be determined. Entering the table for each Tributary area yields the following:
Tributary Area Slope Length [ft] Slope [%] Topographic Factor (Ls) Tributary Area 1 435 3 0.5
Tributary Area 2 230 3 0.4
North Offsite Flows 5 9 0.6
East Offsite Flows 11 24 3.0
This yields the following values:
Tributary Area Topographic Factor (Ls) Area [Acres] Soil Loss [Tons/Yr] Tributary Area 1 0.5 1.822 51.0
Tributary Area 2 0.4 0.735 16.5
North Offsite Flows 0.6 0.022 0.7
East Offsite Flows 3.0 0.013 2.2
Total N/A 2.591 70.4
Soil is estimated to weigh 120 lbs/cuft yielding:
70.4 𝑇𝑜𝑛𝑠 𝑌𝑟⁄ × 2000 𝑙𝑏𝑠 𝑡𝑜𝑛𝑠⁄ × 1 𝑐𝑢𝑓𝑡
120 𝑙𝑏𝑠 = 1,173 𝑐𝑢𝑓𝑡
4.2 Long-Term Sedimentation Yield
The Cropping Management Factor for the finished site, which will assume a 100% total grass cover minimum, is 0.01. As no Erosion control devices are utilized, the P factor is 1.0. Otherwise, all other factors remain the same, however exposed areas are dramatically reduced.
The SDCHM required the Universal Soil Loss Equation (USLE) can be evaluated as follows:
𝐴𝑆 = 𝑅𝐾𝐿𝑠𝐶𝑃
𝐴𝑆 = 250 × 0.28 × 𝐿𝑠 × 0.01 × 1
𝐴𝑆 = 70𝐿𝑠 𝑇𝑜𝑛𝑠 𝐴𝑐𝑟𝑒 − 𝑌𝑟⁄
Tributary Area Topographic
Factor (Ls)
Landscaped Area
[sqft]
Landscaped Area
[Acres]
Soil Loss [Tons/Yr]
Tributary Area 1 0.5 12,617 0.290 10.14
Tributary Area 2 0.4 5,396 0.124 3.47
North Offsite Flows 0.6 843 0.019 0.81
East Offsite Flows 3.0 155 0.004 0.75
Total N/A 19,011 0.436 15.17
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Soil is estimated to weigh 120 lbs/cuft yielding:
15.17 𝑇𝑜𝑛𝑠 𝑌𝑟⁄ × 2000 𝑙𝑏𝑠 𝑡𝑜𝑛𝑠⁄ × 1 𝑐𝑢𝑓𝑡
120 𝑙𝑏𝑠 = 252.8 𝑐𝑢𝑓𝑡
5.0 Offsite Catch Basin Analysis
An offsite catch basin within the public right -of-way is impacted by the design of the site. While a full analysis of the catch basin is outside of the scope of the project, the impact of the projected on the catch basin itself can be adequately established.
For the existing condition, none of the improved areas drained to the catch basin. Previously, drainage from the public right -of-way drained onto private property due to the lack of curb and gutter. The improved plan remedies that, as well as draining the additional on-site areas described in section 2.1.
The amount of offsite area that is draining to the proposed catch basin is 3,938 sqft (0.090 acres) of this, only 339.2 sqft of which is pervious, making the porosity only 8.6%. As there is no entry within the Appendix A for the runoff coefficient shall be defined by the following formula, as given in Section 3.1.2 of the SDCHM:
𝐶 = 0.90 × (% 𝐼𝑚𝑝𝑒𝑟𝑣𝑖𝑜𝑢𝑠) + 𝐶𝑃 × (1 − % 𝐼𝑚𝑝𝑒𝑟𝑣𝑖𝑜𝑢𝑠
Where:
CP = Pervious Coefficient Runoff Value for th Soil Type, as undisturbed natural terrain, 0.25 Plugging in our values gives:
𝐶 = 0.90 × (91.4%) + 0.25 × (1 − 91.4%)
𝐶 = 0.84
Plugging this into the rational equation yields:
𝑄 = 𝐶 × (7.44𝑃6 × 𝑇𝐶
−0.645) × 𝐴
𝑄 = 0.84 × (7.44(3.0) × 5−0.645) × 0.09 = 0.598 𝑐𝑓𝑠
Added to this are the North Offsite Flows from the site, and the East offsite Flows:
𝑄 = 0.598 + 0.14 + 0.08 = 0.82 𝑐𝑓𝑠
The incremental increase on this structure is 0.82 cfs for a 50-year storm event. Calculating the current capacity of the catch basin and the current demand of the catch basin is beyond the scope of the project.
This information is provided for the city to estimate the sufficiency of existing drainage s ystems in place.
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DECLERATION OF RESPONSIBLE CHARGE
I HEARBY DECLARE THAT I AM THE ENGINEER OF WORK FOR THIS PROJECT, THAT I HAVE
EXERCISED RESPONSIBLE CHARGE OVER THE DESIGN OF THE PROJECT AS DEFINED IN SECTION 6703 OF TH EBUSINESS AND PROFESSIONS CODE, AND THAT THE DESIGN IS CONSISTANT WITH THE CURRENT STANDARDS.
I UNDERSTAND THAT THE CHECK OF THE PROJECT DRAWINGS, SPECIFICATIONS, AND REPORTS
BY THE GOVERNING MUNICIPALITY(S) IS CONFINED TO A REVIEW ONLY AND DOES NOT RELIEVE ME, THE ENGINEER OF WORK, OF MY RESPONSABILITIES FOR THE PROJECT DESIGN.
_____________________________________________________________________________ _________________ ALI AZIMIJIRSARAI, RCE 48773 DATE
EXPIRY SEPTEMBER 30TH, 2022
06/29/2022
Ali Azimijirsarai
310
11 14
36 11
6 13
13 13
12 416
15 10.00'
Template Version 2.01 180514
0SCALE: 60402010201" = 20'NORTH
PRELIMINARY PLANBUILDING AND PLANNING COMMISSIONAPPROVAL ONLYNOT FOR CONSTRUCTION TA-BMP
TA-BMPTA-BMP
ONON
OEOE
TRIBUTARY AREA TOINFILTRATION BMP (TA-BMP)111,345 SQFT17,665 SQFT (LS) OFFSITE EAST545 SQFT155 SQFT LS
OFFSITE NORTH990 SQFT892 SQFT LS
SITE AREASTRIBUTARY AREASQFTLANDSCAPE [SQFT]TRIBUTARY AREA TOINFILTRATION BMP111,34517,665 OFFSITE NORTH990892 OFFSITE EAST545155 TOTAL112,88018,712 110'
20'20' 10'2
2 EASEMENT NOTES1
33 30'
2 3 TA1
TREE WELL (TYP)
TREE WELL (TYP)
TREE WELL (TYP)TREE WELL (TYP)
SECTION A-ANOTES:
SECTION B-B
C-9B
C-9AC-9A
TREE WELLS FOR RUNOFFDMABOTANICALNAMECOMMONNAMEMATURECANOPYDIAMETER(FT)MATUREHEIGHT(FT)CREDITVOLUMEPER TREE(FT3)REQUIREDSOIL VOLUMEPER TREE(FT3)PROVIDEDSOIL VOLUMEPER TREE(FT3)NUMBERREQUIREDNUMBERPROVIDEDREMAININGDCV (FT3) 3ChilopsisLinearisDesertWillow30304001,32510'X30'X5'(W x L x D)= 1,50034140
TREE WELL (TYP)TREE WELL (TYP)
CALTRANS R/W
PROP. GROUTED RIPRAP CALTRANS R/W PROP. GROUTED RIPRAP PROP. GROUTED RIPRAP CALTRANS R/W
STORMTECH SUBSURFACEINFILTRATION CHAMBERS
EQUALIZER PIPE
TA-BMP TA-BMP
TA-BMP
TA-BMPTA-BMPTA-BMPTA-BMPTA-BMPTA-BMPTA-BMP
TA-BMP TA-BMP
06/29/2022
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/51 /48 /51 /77/105/108/101/115
/78
/69/87
/83
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
F I G U R E5-3Rainfall Distribution Regions in California IAI II
Type IAType IType IIRainfall Distribution Types
SOURCE: Wischmeier, 1977
F I G U R ER Factors based on 2-Year, 6-Hour Storm Event (Intensity)5-22-Yr., 6-Hr. Rain (in inches) 500Mean Annual “R” Factor
SOURCE: Wischmeier, 1977.4.6 1 2 4 6 10100101IIIIA
F I G U R EChart for determining Topographic Factors (L,s)5%6%8%10%12%14%16%20%25%30%50%40%SLOPE4%3%2%1%0.5%