Preliminary Water Quality Management Plan
Supplemental Narrative
For
FORT BRAGG SENIOR APARTMENTS
860 HAZELWOOD STREET
FORT BRAGG, CA
APN: 018-210-29-00
Prepared for:
The Pacific Companies
430 E. State Street, Suite 100
Eagle, ID 83616
Prepared by:
Atlas Civil Design, Inc.
872 Higuera Street,
San Luis Obispo, CA 93401
(619) 307-2749
Prepared: December 2024
Fort Bragg Senior Apartments
WQMP Supplemental Narrative
December 2024 1
TABLE OF CONTENTS
I. PROJECT SUMMARY ...................................................................................................................2 Purpose ................................................................................................................................................. 2 Project Description ............................................................................................................................... 2 II. EXISTING CONDITIONS ................................................................................................................3 Existing Condition Overview ................................................................................................................. 3 Opportunities ........................................................................................................................................ 3 Constraints ............................................................................................................................................ 3 III. Proposed Conditions ...................................................................................................................4 Project Layout Optimization ................................................................................................................. 4 Site Design Measures ............................................................................................................................ 5 Drainage Management Areas ............................................................................................................... 5 IV. HYDROMODIFICATION MITIGATION ............................................................................................7 Overview ............................................................................................................................................... 7 Hydromodification Mitigation Calculations .......................................................................................... 8 V. CONCLUSION ..............................................................................................................................9
PROPOSED BUILDING
HAZELWOOD STREET
(PUBLIC STREET)
LEGEND
( IN FEET )
GRAPHIC SCALE 0 40
1" = 20'
20 20
NOT TO SCALE
BMP 1 - BIORETENTION BASIN (PVT.)
BIORETENTION BASIN NOTES:
SITE INFORMATION
335
335
DMA
X.X
SQ FT
DMA 6
31,600
SQ FT
DMA 2
15,000
SQ FT
DMA 1
59,100
SQ FT
DMA 3
22,800
SQ FT
DMA 4
1,000
SQ FT
DMA 5
900
SQ FT
www.AtlasCivilDesign.com
Civil Engineering Site Optimization
Design Civil Tel: 1-619-307-2749 San Diego Orange County Los Angeles San Luis Obispo Monterey BY REVISIONS DATE SHEET
OF PROJECT SITE MAP 2 860 HAZELWOOD STREET, CITY OF FORT BRAGG, COUNTY OF MENDOCINO STATE OF CALIFORNIA 2PROPOSED CONDITION Fort Bragg Senior Apartments
WQMP Supplemental Narrative
December 2024 2
I. PROJECT SUMMARY
Purpose
This narrative accompanies the Stormwater Control Plan for the Fort Bragg Senior Apartments project, prepared on the template per the County of Mendocino Low Impact Development Standards Manual, dated May 2021. The purpose is to provide additional information to demonstrate compliance with applicable stormwater requirements as set forth in the Phase II Small MS4 General Permit, WQ Order No. 2013-0001-DWQ, NPDES No. CAS000004, and its subsequent amendments. The City of Fort Bragg utilizes the County of Mendocino Low Impact Development Standards Manual to ensure compliance with MS4 requirements.
Project Description
The project is located at 860 Hazelwood Street in the City of Fort Bragg within Mendocino County in the State of California. The 2.99-acre site is bounded by an undeveloped public road to the west, a multifamily housing complex to the north, and vacant lots to the east and south. See the Vicinity Map below.
The project proposes an affordable, senior housing development, consisting of one multifamily housing building with associated parking, hardscape, and landscape improvements.
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II. EXISTING CONDITIONS
Existing Condition Overview
The existing project site consists of one single-family structure at the northwest corner with primarily vegetated groundcover on the remainder of the property. The northwest corner of the site, adjacent to the structure, drains toward Hazelwood Street, which is unpaved fronting the site. The northern portion of the site east and south of the structure slopes at approximately 2% southeast. The southern and eastern portions of the site drain at 5% to 10% toward the southern and eastern property lines.
The site receives no significant offsite run-on from the properties to the north, east, and south.
Hazelwood Street, a public street to the west of the property, is unpaved and contains no drainage infrastructure. In the existing condition, drainage from the right-of-way flows onto the subject property, draining southeast across the property.
Site soils are considered silty fine sand according to the geotechnical report. Groundwater was encountered approximately 10 to 13 feet below existing grade. See the Project Site Map in Attachment 1 for a depiction of the existing conditions and Attachment 6 for the project-specific geotechnical report.
Opportunities
The area of the site is relatively large compared with the footprint needed to support the proposed development. This allows the proposed design to maintain a large area of pervious landscaping. It also provides the space needed for self-retaining areas and bioretention.
The tested infiltration rates onsite appear to support a moderate level of infiltration, allowing retention of some drainage onsite.
Constraints
No public storm drain infrastructure exists in the near vicinity of the site. Existing drainage runs toward private properties. In order to not exacerbate the cross-lot drainage and because there is no public storm drain to tie into, water must be retained onsite to the extent feasible. Infiltration rates are moderate, but not high, so a large infiltration footprint must be maintained in order to retain drainage within appropriate drawdown times.
Groundwater was encountered 10 to 13 feet below existing grade in the geotechnical investigation borings; however, the borings were not taken in the location of the bioretention basin. Further investigation will be required to determine the groundwater level at the location of the bioretention basin and to ensure adequate separation between the infiltrating surface and water table can be met.
The site receives runon of public drainage from Hazelwood Street, which must continue to pass through the site in the proposed design.
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III. Proposed Conditions
Proposed site improvements shall consist of one multifamily housing building, surface parking and associated drive aisles, landscaping, pedestrian hardscape, and outdoor recreation areas.
Project Layout Optimization
The project layout has been optimized per the following low impact design principles.
Minimize Impervious Surfaces
The proposed design utilizes a hammerhead turnaround in lieu of a looped drive aisle around the building in order to reduce impervious surface area. Perimeter parking bays and the turnaround are proposed as permeable pavement. Permeable pavement is also proposed at the driveway entrances. Proposed parking counts have been reduced to the extent feasible in order to minimize paving areas.
Preserve Vegetation
The project borders private lots to the south and east, both of which contain dense areas of trees.
The trees within the subject property at the southeast corner of the site will be protected.
Utilize and Conform to Site-Specific Topography
The northern portion of the site is relatively flat with grades at the south and east sloping more steeply toward the southerly and easterly property lines. The proposed site has been laid out consistently with the existing topography. The main development footprint is proposed on the norther portion of the site. The southern portion and eastern edge of the site will consist of pervious slopes drainage toward to southerly and easterly property lines as in the existing condition.
Replicate the Site’s Natural Drainage Patterns
The site maintains the existing drainage patterns to the extent feasible. Site grading will match existing drainage direction with relatively flat slopes on the northern portion of the site and perimeter slope grading to the south and east. Detention and infiltration features are provided to mitigate increases in peak flow per the project Runoff Mitigation Plan.
Detain and Retain Runoff Throughout the Site
The overall site limits the ratio of impervious to pervious area to less than 2:1. The project will implement self-retaining landscaped areas and self-retaining permeable pavement throughout the site to maximize infiltration of runoff. Self-retaining planter areas will allow for three inches of ponding below area drains. Permeable pavement will include nine inches of gravel storage below the subdrain. Drainage below the area drains and pavement subdrain will infiltrate.
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Impermeable parking stalls, drive aisles and sidewalks will be directed toward the permeable pavement and self-retaining planter areas.
The site will include an unlined bioretention basin, designed to biofilter, detain, and infiltrate runoff. The basin includes 24-inches of gravel storage below the subdrain. Storage below the subdrain will infiltrate.
Roof drains will outlet at grade in landscaped areas where feasible. These areas are not designed as self-retaining areas, as ponding against the building is not advisable. The planter areas will allow for some infiltration of the roof drainage before drainage reaches the area drains.
The public drainage from Hazelwood Street will continue to flow onto the property as it does in the existing condition. It will flow through a gently graded vegetated swale to encourage infiltration of the public drainage.
Site Design Measures
The Phase II MS4 Permit and the Mendocino LID prioritize site design measures with the goal of retaining the 85th percentile, 24-hour storm event to the extent feasible. The manual prioritizes the creation of selfretaining areas, designed to retain a minimum of 3-inches of water with a maximum ratio of tributary impervious area to pervious self-retaining area of 2:1. According to the manual:
“In Mendocino County the 85th percentile, 24-hour storm event is equivalent to approximately one inch. When using a 2:1 impervious to pervious ratio for the calculation of the self-retaining area, the area must be able to retain the first 2-inches of stormwater runoff from the impervious areas and the first inch that falls on the self-retaining area itself. This is why the self-retaining area should be depressed in order to achieve this 3-inch retention requirement.” The manual requires that projects reduce runoff by draining impervious areas to pervious self-retaining areas. If the 2:1 maximum ratio of impervious to pervious area cannot be met, a treatment control BMP must be implemented, with bioretention being the priority.
The proposed project implements site design measures throughout the site. Permeable pavement has been proposed within the perimeter parking bays and the hammerhead turnaround. The permeable pavement is considered to be self-retaining as it will have greater than 3-inches of gravel storage below the subdrain. Self-retaining landscaped areas throughout the site have been proposed.
Where self-retaining areas cannot completely retain the runoff based on the exceedance of the 2:1 impervious to self-retaining area ratio, bioretention has been implemented. The bioretention facility was sized as 4-percent of the tributary area after the application of runoff factors based on surface type.
Self-retaining and bioretention areas are depicted on the Project Site Map in Attachment 1. Calculations are provided in Attachment 3.
Drainage Management Areas
The site has been delineated into six drainage management areas (DMAs) as described below. DMAs were delineated based on the area flowing to each site design feature.
Fort Bragg Senior Apartments
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December 2024 6
DMA 1 – Area Draining to a Bioretention Facility
DMA 1 encompasses the multifamily building with surrounding landscaped areas, as well as the drive aisle and parking area south of the building. Roof drainage outlets at grade where possible and is captured in landscape swales surrounding the building. Runoff will enter the private storm drain system through area drains. Runoff from the drive aisles, parking stalls, and carport roofs will surface flow onto the permeable pavement in the south parking bay. Runoff below the paver subdrain will infiltrate. When the paver section is full, runoff will enter the subdrain.
As the DMA exceeds the 2:1 ratio of impervious to self-retaining area, bioretention has been provided. Runoff from the paver subdrain and area drains will be piped to a bioretention basin referred to as BMP 1. BMP 1 serves to treat, infiltrate, and detain runoff. Drainage within the bioretention basin will infiltrate below the subdrain. As the water level reaches the subdrain, runoff will exit through an orifice at the subdrain connection to the outlet structure. As the water level rises above the graded bottom of basin, runoff will enter a mid-flow orifice in the outlet structure. In an overflow condition, runoff would enter the grated inlet at the top of the outlet structure. From the outlet structure, drainage is piped to a level spreader outlet at the southeastern corner of the site. The level spreader will dissipate energy, and drainage will spill over evenly toward the property line at the southeastern corner of the site. Drainage leaving the level spreader will travel through existing vegetation to be preserved onsite before leaving the site in the same direction as drainage in the existing condition.
DMA 2 – Area Draining to Self-Retaining Area / Self-Retaining Area DMA 2 includes the sidewalk, drive aisle, carports and parking east of the building. Drainage surface flows onto the permeable pavement in the parking and hammerhead turnaround.
Drainage below the paver subdrain infiltrates. When the paver section is full, runoff will exit through the subdrain. The subdrain will be piped to the level spreader at the southeast corner of the property. DMA 2 has been designed with an impervious to pervious self-retaining area ratio of less than 2:1; therefore, site design requirements have been met.
DMA 3 – Area Draining to Self-Retaining Area / Self-Retaining Area DMA 3 includes the sidewalk, drive aisle, carports and parking north of the building. Drainage surface flows onto the permeable pavement in the parking as well as into a self-retaining landscaped area designed to allow minimum 3-inches of ponding. Drainage below the subdrain of the pavers and below the raised area drains of the self-retaining area will infiltrate. When the water level reaches the subdrain of the pavers and the area drains in the self-retaining area, drainage will enter the private storm drain, from where it will be piped to a level spreader along the eastern edge of the property. This mimics existing conditions, in which the northeast corner of the site drains toward the eastern property line. DMA 3 has been designed with an impervious to pervious self-retaining area ratio of less than 2:1; therefore, site design requirements have been met.
Fort Bragg Senior Apartments
WQMP Supplemental Narrative
December 2024 7
DMAs 4 and 5 – Area Draining to Self-Retaining Area / Self-Retaining Area DMAs 4 and 5 consist of the two driveway entrances to the site that will flow offsite into Hazelwood Street. Permeable pavement will be installed within the driveway entrances in order to intercept the drainage to the extent feasible. Drainage below the subdrain will infiltrate.
When the water level reaches the subdrain, drainage will be piped toward the eastern property line for DMA 4 and toward BMP 1 for DMA 5. DMAs 4 and 5 have been designed with an impervious to pervious self-retaining area ratio of less than 2:1; therefore, site design requirements have been met.
DMA 6 – Self-Treating Area
DMA 6 includes the pervious graded area surrounding the site that is not tributary to the proposed drainage infrastructure. This area will be planted and does not require treatment or detention as it contains no impervious area and will match the existing conditions. Runoff that is not intercepted in the soil and vegetation will flow toward the south and eastern property lines as in the existing condition.
Offsite
The project proposes the development of a portion of public Hazelwood Street with asphalt pavement, curb, gutter, and sidewalk. The street will be sloped east as in the existing condition.
It will be intercepted in the gutter and directed to a proposed vegetated swale. The swale will flow from the public right-of-way onto the subject property and flow along the southerly property line. The swale will allow for the interception of drainage in the soil as infiltration and vegetation as evapotranspiration. Drainage that is not intercepted will flow toward the southeast corner of the site and exit the property as in the existing condition. Treatment of public, offsite drainage has not been tabulated in this analysis.
IV. HYDROMODIFICATION MITIGATION
Overview
According to Section E.12.f of the Phase II MS4 Permit, regulated projects within the California Coastal Ranges shall implement hydromodification management measures, by demonstrating that post-project runoff shall not exceed estimated pre-project runoff for the 2-year, 24-hour storm. The LID Manual notes that if a project has been designed with the 2:1 impervious to self-retaining area ratio for the entire site and that the site supports infiltration of greater than 1 inch per hour, the hydromodification mitigation requirement has been met.
The LID Manual does not provide guidance for how to meet the requirement if the project infiltrates at less than 1 inch per hour or does not meet the required ratio of impervious to self-retaining area for the entire site. The proposed project does not support infiltration of 1 inch per hour with a factor of safety applied to the tested rate and does not comply with the 2:1 ratio for the entire site; therefore, the project has performed hydrology and hydraulic routing calculations for the 2-year, 24-hour storm event to demonstrate compliance.
Fort Bragg Senior Apartments
WQMP Supplemental Narrative
December 2024 8
Hydromodification Mitigation Calculations
In order to demonstrate that the proposed 2-year, 24-hour peak runoff does not exceed that of the existing condition, detention in the permeable pavement and bioretention must be taken into account.
Detention analysis requires time distribution of rainfall over a particular storm duration; therefore, the National Resources Conservation Services (NRCS), formerly Soil Conversation Service (SCS), hydrologic procedure was followed.
Calculations described below were performed in Autodesk’s Storm Sanitary Analysis program (SSA). SSA input and output can be found in Attachment 4.
NRCS Type IA, 24-hour storm distribution was selected based on the geographic region. Distribution IA is appropriate for the northern California coast per NRCS Technical Release 55, Figure B-2. The storm distribution was applied to the 2-year, 24-hour precipitation depth per NOAA Atlas 14. See reference material in Attachment 5.
Each DMA was modeled as a subbasin in SSA based on area, time of concentration, and curve number.
Time of concentration was based on NRCS TR-55 method with calculations occurring directly in SSA. Time of concentration was calculated for the existing condition and for proposed DMA 6, which contains only pervious area. The remaining DMAs were modeled with the minimum time of concentration of five minutes, as they are relatively small with relatively high impervious area.
Composite curve numbers were calculated directly in SSA based on the input impervious and pervious areas for each DMA. Impervious areas were modeled with a curve number of 98, and pervious areas with a curve number of 58. The pervious curve number is from TR-55, Table 2-2c, from the value for meadow with soil type B. This ground cover matches the existing state. Although the NRCS does not list a hydrologic soil group, type B is the most consistent with the soil description and tested infiltration rates.
The existing condition was modeled by running the 2-year, 24-hour storm through the existing subbasin with the above parameters. The model results in a peak flow at the overall discharge point.
For the proposed condition, detention in the permeable pavement and bioretention basin needed to be modeled. The bioretention basin was modeled for DMA 1, and the permeable pavement was modeled for DMAs 2 through 5. The bioretention parameters are per the detail shown on the Project Site Map in Attachment 1. The pavement sections were assumed to have 9-inches of gravel below the subdrain.
The bioretention basin and each paver area were added as a storage node in SSA. A staged storage curve was added representing the depth to volume values for each storage node. Staged storage calculations considered 0.4 porosity for the gravel layers and 0.2 porosity for the basin soil layer.
Infiltration from the basin and paver sections was modeled. The design infiltration rate was calculated by applying a factor of safety of 3 to the tested infiltration rate per the geotechnical report. The design infiltration rate was multiplied by the bottom area of the basin and pavers to obtain an infiltration flow rate for each area. The infiltration flow rate was applied to each storage node based on its bottom area.
Subdrains for the basin and pavers were modeled as orifices in SSA. The paver sections were each modeled with a 4-inch subdrain. The orifice size of the bioretention basin was iterated until the peak 2- year, 24-hour flow rate did not exceed that of the existing condition. The result was an orifice of 1.375- Fort Bragg Senior Apartments
WQMP Supplemental Narrative
December 2024 9
inches in diameter. A mid-flow orifice was added on the outlet structure above the graded basin bottom for detention of larger storm events per the project Runoff Mitigation Plan. The water level remains below the mid-flow orifice in the 2-year, 24-hour storm event.
The basin and pavers were modeled with overflow structures; however, the sizing resulted in water levels that never reached the overflow for any storage node. For the basin, the grated overflow was modeled as a weir at the grate elevation. For the pavers, the overflow was modeled as a long weir to reflect surface flow leaving the paver area in an overflow condition.
The 2-year, 24-hour storm event was run through the proposed subbasins with runoff then being routed through the storage nodes, orifices, and to the overall outfall. The model results in a peak flow and total at the overall discharge point after considering detention and infiltration in the basin and pavers.
The table below summarizes the results of the hydrologic and hydraulic analysis:
2-Year, 24-Hour Runoff
DMA Peak Flow (CFS)
Existing 0.09
1 0.06
2 0
3 0
4 0
5 0
6 0.02
Total Proposed 0.08
The analysis shows that the project detention features are sufficient to reduce the proposed peak flow leaving the site in the 2-year, 24-hour storm event to less than that of the existing condition. Therefore, hydromodification mitigation requirements have been met.
V. CONCLUSION
The proposed project has been designed to address site design requirements of the Phase II MS4 Permit as enforced within the Mendocino County LID Manual through the use of self-retaining landscaped areas, permeable pavement, and a bioretention basin. Hydrologic and hydraulic analysis shows that these features are sufficient for compliance with the hydromodification mitigation requirement with the inclusion of a 1.375-inch orifice on the bioretention basin subdrain.
ATTACHMENT 3
Water Quality Calculations
Fort Bragg Senior Apartments - DMA Summary
1 28200 0.65 30900 0.71 59100 1.36 1100 0.03 0 0.00 1100 0.03 25.6 No 15002 8100 0.19 6900 0.16 15000 0.34 4600 0.11 0 0.00 4600 0.11 1.8 Yes 03 13100 0.30 9700 0.22 22800 0.52 4700 0.11 4100 0.09 8800 0.20 1.5 Yes 04 650 0.01 350 0.01 1000 0.02 350 0.01 0 0.00 350 0.01 1.9 Yes 05 550 0.01 350 0.01 900 0.02 350 0.01 0 0.00 350 0.01 1.6 Yes 06 0 0.00 31600 0.73 31600 0.73 0 0.00 0 0.00 0 0.00 N/A Yes 0 Total 50600 1.16 79800 1.83 130400 2.99 11100 0.25 4100 0.09 15200 0.35 Existing 2300 0.05 0 0.00 130400 2.99
LID Req.
met?
Paver
Area (SF)
Bioretention
Area (SF)
Paver
Area (AC)
SRA
Landscape
(SF)
SRA
Landscape
(AC)
Total SRA
(SF)
Total SRA
(AC)
Ratio
Impervious
to SRA
DMA Area Summary Site Design Measures
DMA Impervious
Area (SF)
Impervious
Area (AC)
Pervious
Area (SF)
Pervious
Area (AC)
Total
Area (SF)
Total
Area (AC)
Stormwater Control Plan for Regulated Projects
64 | Page
Table 2. Area Calculations of Self-retaining Areas Used to Treat Impervious Areas 1 DMA Name
2 Area (sq. ft.)
Tables 4-6 below should be used to quantify the amount of runoff that is reduced by using site design measures. Using the tables in chronological order will calculate the minimum size for your bioretention facility in order to meet the MS4 permit requirements. Several iterations may be need to size facilities according to the site design.
Table 4. Area draining to self-retaining areas
1 DMA Name
(must correspond to
area on the site map
and on Table 1)
2 DMA Area
(sq. ft.)
(Table 1)
3 Type of Surface
(Runoff Factor
Table 3)
4 Surface with
Runoff Factor
Column 2 X
Column 3
5 Area of Self-retaining
Area Receiving the Runoff
(sq. ft.)
(Table 2, Col. 2)
6 Ratio
Col. 4 : Col. 5
Not to exceed 2:1 ratio
(if number exceeds 2:1 use table 5 - 6 to
reduce tributary area and recalculate or go
directly to Table 7)
Example 700 Roof (1.0) 700 100 7:1 (must use site design measures, bioretention or both)
Table 3. Runoff Factor (surface type)
Roofs and Paving 1.0
Landscaped Area 0.1
Bricks or solid pavers- grouted 1.0
Bricks or solid Pavers-on sand base 0.5
Pervious Concrete Asphalt 0.1
Turfblock or gravel 0.1
Open or Porous pavers 0.1
1 (Self-retaining)
2 (Self-retaining)
3 (Self-retaining)
4 (Self-retaining)
5 (Self-retaining)
28,200
8,100
13,100
650
550
Roof, paving (1.0)
Paving (1.0)
Paving (1.0)
Paving (1.0)
Paving (1.0)
28,200
8,100
13,100
650
550
1,100
4,600
8,800
350
350
25.6:1
1.8:1
1.5:1
1.9:1
1.6:1
1,100
4,600
8,800
350
350
1 (Impervious)
2 (Impervious)
3 (Impervious)
4 (Impervious)
5 (Impervious)
Stormwater Control Plan for Regulated Projects
65 | Page
Table 5. Tree Planting and Preservation (if not planting trees, go to Table 6) 1 DMA Name
(must correspond to
area on the site
map)
2 DMA sq. ft.
(from Table
4. Col. 6)
3 Deciduous
(Input 100 for each
deciduous tree)
4 Evergreen
(Input 200 for each
evergreen tree)
5 Total Tree Credit
(Col. 3 + Col. 4)
(DMA runoff reduction)
6 New DMA Area
Col. 2 – Col. 5
(for use in Table 6 - 8)
Example 700 -------- 200 200
500 (new DMA size that must
be treated with methods
below Table 6-7)
Table 6. Rain Barrels and Cisterns (if not using site design measures, go to Table 8) 1 DMA Name
(must correspond to
area on the site
map)
2 New DMA sq. ft.
(Table 5, Col. 7 or, if
no trees used, value
from Table 4, Col. 2)
3 Number
of Rain
Barrels
4 Runoff Reduction from using a standard 55
gallon Rain Barrel = 88 sq. ft.
Use the following if size is other than the
standard
(for every gallon of storage, approx. 1.6 sq.
ft. of reduction is achieved)
5 Col. 3 X Col. 4
(DMA runoff
reduction)
6 New DMA Area
Col. 2 - Col. 5
Example 500 1 88 88 412 (go to Table 7 to
recalculate Ratio)
Stormwater Control Plan for Regulated Projects
66 | Page
Table 7. New Tabulation of areas draining to self-retaining area after use of site design measures (must achieve a 2:1 ratio; if not achievable, use table 8 to calculate the size of bioretention required) 1 DMA Name
(must correspond to area on the
site map)
2 New Square footage of
DMA
(Col 6, Table 4,5,6)
3 Area of Self-retaining Area
Receiving the Runoff
(Table 2, Col. 2)
4 Ratio
Column 2 : Column 3
Not to exceed 2:1
Example 412 (Table 6) 100
4.12:1(still exceeds 2:1 go back, add more trees, rain
barrels, or use bioretention – example uses
bioretention, Table 8)
Table 8. Tabulation of areas draining to Bioretention Facility 1 DMA Name
(must
correspond to
area on the site
map)
2 DMA sq. ft.
(Table 1, Col 2
or new DMA sq. ft.
Table 7, Col. 2)
3 Runoff Factor
Table 6
(skip if coming
from Table 1)
5 DMA Area
Col. 2 x Col.
3 6
Standard
Sizing
Factor
Minimum facility size
Col. 5 X Col. 6
If site does not allow for the minimum
size, recalculate DMA using additional
Site Design Measures to further reduce
the tributary size
Example 300
1 (already
calculated in
steps above,
300 0.04 12 sq. ft. (proposed facility size on site plans) 1 (Impervious)
28,200 1,100 25.6:1
Stormwater Control Plan for Regulated Projects
67 | Page
for this
example)
0.04 0.04
0.04 0.04
Table 9. Runoff Factors
Roofs and Paving 1.0
Landscaped Area 0.1
Bricks or solid pavers- grouted 1.0
Bricks or solid Pavers-on sand base 0.5
Pervious Concrete Asphalt 0.1
Turfblock or gravel 0.1
Open or Porous pavers 0.1
G. Operation and Maintenance in Perpetuity
Indicate whether an Operation and Maintenance Plan is accompanying this document (Appendix 9).
Yes No
H. Stormwater Control Plan
A Stormwater Control Plan is required for all Regulated Projects. This worksheet is designed to be the SCP if all requested descriptions and site plans have been attached. This document will be used by the plan checker to confirm that adequate stormwater control measures are being implemented on the project.
Indicate whether all supporting descriptions and worksheets are accompanying this document, Stormwater Control Plan Yes No
28,200 Impervious
30,900 Pervious
1,500 SF provided.
Minimum size achieved.
1 1
0.1 28,200 SF
3,090 SF
1,128 SF
124 SF
1,252 SF
Total
ATTACHMENT 4
Hydromodification Mitigation Calculations
Fort Bragg Senior Apartments - Hydromodification Mitigation Summary 2-year, 24 hour
Results
1 28200 0.65 30900 0.71 59100 1.36 1100 1500 0.062 8100 0.19 6900 0.16 15000 0.34 4600 0 03 13100 0.30 9700 0.22 22800 0.52 4700 0 04 650 0.01 350 0.01 1000 0.02 350 0 05 550 0.01 350 0.01 900 0.02 350 0 06 0 0.00 31600 0.73 31600 0.73 0 0 0.02 Total 50600 1.16 79800 1.83 130400 2.99 0.08
Existing 2300 0.05 0 0.00 130400 2.99 0.09
Note: Pavement detention not modeled for DMA 1. Self-retaining landscaped area not modeled for DMA 3.
Paver
Area (SF)
Bioretention
Area (SF)
Peak Flow
(CFS)
DMA Area Summary Detention Features
DMA Impervious
Area (SF)
Impervious
Area (AC)
Pervious
Area (SF)
Pervious
Area (AC)
Total Area
(SF)
Total
Area (AC)
Fort Bragg Senior Apartments - Design Infiltration Rate Test1
Infiltration Rate
(in/hr)
P-1 1.03
P-2 0.94
Average 0.985
1 Per geotechnical infiltration testing results.
Average
Infiltration
Rate (in/hr)
0.985 in/hr
FS 3
Design
infiltration
rate
0.33 in/hr
Fort Bragg Senior Apartments Existing Condition, 2-year, 24-Hour Page 1 of 5 Project Description
24-043 Hazelwood Fort Bragg - Existing TR20.SPF
Project Options
CFS
Elevation
SCS TR-20
SCS TR-55
Kinematic Wave
YES
YES
Analysis Options
00:00:00 0:00:00
00:00:00 0:00:00
00:00:00 0:00:00
0 days
0 01:00:00 days hh:mm:ss
0 00:05:00 days hh:mm:ss
0 00:05:00 days hh:mm:ss
15 seconds
Number of Elements
Qty
4 1
1 0
1 0
0 0
0 0
0 0
0 0
0 0
0 Rainfall Details
SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall ID Source ID Type Units Period Depth Distribution
(years) (inches)
1 Rain Gage-01 Time Series TS-04 Intensity inches User Defined 2 Rain Gage-03 Time Series TS-02 Cumulative inches 0.00 3 Rain Gage-04 Time Series TS-03 Cumulative inches 0.00 4 Rain Gage-05 Time Series TS-04 Intensity inches User Defined Antecedent Dry Days ..................................
File Name ..................................................
Flow Units .................................................
Elevation Type ............................................
Hydrology Method ......................................
Time of Concentration (TOC) Method ..........
Link Routing Method ...................................
Enable Overflow Ponding at Nodes ..............
Skip Steady State Analysis Time Periods ......
Start Analysis On ........................................
End Analysis On .........................................
Start Reporting On ......................................
Storage Nodes .....................................
Runoff (Dry Weather) Time Step ..................
Runoff (Wet Weather) Time Step .................
Reporting Time Step ...................................
Routing Time Step ......................................
Rain Gages ................................................
Subbasins..................................................
Nodes........................................................
Junctions .............................................
Outfalls ...............................................
Flow Diversions ...................................
Inlets ...................................................
Outlets ................................................
Pollutants ..................................................
Land Uses ..................................................
Links..........................................................
Channels .............................................
Pipes ...................................................
Pumps .................................................
Orifices ...............................................
Weirs ..................................................
Fort Bragg Senior Apartments Existing Condition, 2-year, 24-Hour Page 2 of 5 Subbasin Summary
SN Subbasin Area Peak Rate Weighted Total Total Total Peak Time of ID Factor Curve Rainfall Runoff Runoff Runoff Concentration Number Volume
(ac) (in) (in) (ac-in) (cfs) (days hh:mm:ss)
1 E1 2.99 484.00 58.67 3.29 0.40 1.18 0.09 0 00:20:02
Fort Bragg Senior Apartments Existing Condition, 2-year, 24-Hour Page 3 of 5 Subbasin Hydrology
Subbasin : E1
Input Data
Area (ac) ....................................................................... 2.99 Peak Rate Factor ........................................................... 484 Weighted Curve Number ................................................ 58.67 Rain Gage ID ................................................................. Rain Gage-01 Composite Curve Number
32 Area Soil Curve
Soil/Surface Description (acres) Group Number
Meadow, non-grazed 2.94 B 58
Paved parking & roofs 0.05 B 98
Composite Area & Weighted CN 2.99 58.67
Time of Concentration
TOC Method : SCS TR-55
Sheet Flow Equation :
Tc = (0.007 * ((n * Lf)^0.8)) / ((P^0.5) * (Sf^0.4))
Where :
Tc = Time of Concentration (hr)
n = Manning's roughness
Lf = Flow Length (ft)
P = 2 yr, 24 hr Rainfall (inches)
Sf = Slope (ft/ft)
Shallow Concentrated Flow Equation :
V = 16.1345 * (Sf^0.5) (unpaved surface)
V = 20.3282 * (Sf^0.5) (paved surface)
V = 15.0 * (Sf^0.5) (grassed waterway surface)
V = 10.0 * (Sf^0.5) (nearly bare & untilled surface)
V = 9.0 * (Sf^0.5) (cultivated straight rows surface)
V = 7.0 * (Sf^0.5) (short grass pasture surface)
V = 5.0 * (Sf^0.5) (woodland surface)
V = 2.5 * (Sf^0.5) (forest w/heavy litter surface)
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hr)
Lf = Flow Length (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
Channel Flow Equation :
V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n
R = Aq / Wp
Tc = (Lf / V) / (3600 sec/hr)
Where :
Tc = Time of Concentration (hr)
Lf = Flow Length (ft)
R = Hydraulic Radius (ft)
Aq = Flow Area (ft²)
Wp = Wetted Perimeter (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
n = Manning's roughness
Fort Bragg Senior Apartments Existing Condition, 2-year, 24-Hour Page 4 of 5 Subarea Subarea Subarea
Sheet Flow Computations A B C
Manning's Roughness : 0.3 0 0
Flow Length (ft) : 100 0 0
Slope (%) : 2 0 0
2 yr, 24 hr Rainfall (in) : 3.4 0 0
Velocity (ft/sec) : 0.1 0 0
Computed Flow Time (min) : 16.55 0 0
Subarea Subarea Subarea
Shallow Concentrated Flow Computations A B C
Flow Length (ft) : 100 130 0
Slope (%) : 1.5 4 0
Surface Type : Grass pasture Grass pasture Unpaved
Velocity (ft/sec) : 0.86 1.4 0
Computed Flow Time (min) : 1.94 1.55 0
Total TOC (min) ..................20.04
Subbasin Runoff Results
Total Rainfall (in) ........................................................... 3.29 Total Runoff (in) ............................................................ 0.4 Peak Runoff (cfs) ........................................................... 0.09 Weighted Curve Number ................................................ 58.67 Time of Concentration (days hh:mm:ss) ......................... 0 00:20:02 Fort Bragg Senior Apartments Existing Condition, 2-year, 24-Hour Page 5 of 5 Subbasin : E1
Rainfall Intensity Graph
Time (hrs)
95908580757065605550454035302520151050
R ainfall (in/hr)
0.8 0.75
0.7 0.65
0.6 0.55
0.5 0.45
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
Runoff Hydrograph
Time (hrs)
95908580757065605550454035302520151050
R unoff (cfs ) 0.095
0.09 0.085
0.08 0.075
0.07 0.065
0.06 0.055
0.05 0.045
0.04 0.035
0.03 0.025
0.02 0.015
0.01 0.005
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 1 of 31 Project Description
24-043 Hazelwood Fort Bragg - Proposed TR-20 Mitigated.SPF Project Options
CFS
Elevation
SCS TR-20
SCS TR-55
Kinematic Wave
YES
YES
Analysis Options
00:00:00 0:00:00
00:00:00 0:00:00
00:00:00 0:00:00
0 days
0 01:00:00 days hh:mm:ss
0 00:05:00 days hh:mm:ss
0 00:05:00 days hh:mm:ss
15 seconds
Number of Elements
Qty
2 6
6 0
1 0
0 5
11 0
0 0
6 5
0 0
0 Rainfall Details
SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall ID Source ID Type Units Period Depth Distribution
(years) (inches)
1 Rain Gage-01 Time Series TS-03 Intensity inches User Defined 2 Rain Gage-02 Time Series TS-02 Cumulative inches 0.00 Antecedent Dry Days ..................................
File Name ..................................................
Flow Units .................................................
Elevation Type ............................................
Hydrology Method ......................................
Time of Concentration (TOC) Method ..........
Link Routing Method ...................................
Enable Overflow Ponding at Nodes ..............
Skip Steady State Analysis Time Periods ......
Start Analysis On ........................................
End Analysis On .........................................
Start Reporting On ......................................
Storage Nodes .....................................
Runoff (Dry Weather) Time Step ..................
Runoff (Wet Weather) Time Step .................
Reporting Time Step ...................................
Routing Time Step ......................................
Rain Gages ................................................
Subbasins..................................................
Nodes........................................................
Junctions .............................................
Outfalls ...............................................
Flow Diversions ...................................
Inlets ...................................................
Outlets ................................................
Pollutants ..................................................
Land Uses ..................................................
Links..........................................................
Channels .............................................
Pipes ...................................................
Pumps .................................................
Orifices ...............................................
Weirs ..................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 2 of 31 Subbasin Summary
SN Subbasin Area Peak Rate Weighted Total Total Total Peak Time of ID Factor Curve Rainfall Runoff Runoff Runoff Concentration Number Volume
(ac) (in) (in) (ac-in) (cfs) (days hh:mm:ss)
1 P1 1.36 484.00 77.12 3.29 1.28 1.75 0.37 0 00:05:00
2 P2 0.34 484.00 80.35 3.29 1.50 0.51 0.12 0 00:05:00
3 P3 0.52 484.00 81.08 3.29 1.55 0.80 0.19 0 00:05:00
4 P4 0.02 484.00 84.00 3.29 1.71 0.03 0.01 0 00:05:00
5 P5 0.02 484.00 82.00 3.29 1.54 0.03 0.01 0 00:05:00
6 P6 0.73 484.00 58.00 3.29 0.37 0.27 0.02 0 00:12:06
Runoff from subbasins
before storage nodes
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 3 of 31 Node Summary
SN Element Element Invert Ground/Rim Initial Surcharge Ponded Peak Max HGL Max Min Time of Total Total Time ID Type Elevation (Max) Water Elevation Area Inflow Elevation Surcharge Freeboard Peak Flooded Flooded Elevation Elevation Attained Depth Attained Flooding Volume Attained Occurrence
(ft) (ft) (ft) (ft) (ft²) (cfs) (ft) (ft) (ft) (days hh:mm) (ac-in) (min) 1 Out-01 Outfall 108.00 0.08 108.00
2 Stor-01 Storage Node 108.00 114.00 108.00 0.00 0.37 111.57 0.00 0.00 3 Stor-02 Storage Node 116.00 119.00 116.00 0.00 0.12 116.29 0.00 0.00 4 Stor-03 Storage Node 116.00 119.00 116.00 0.00 0.18 116.53 0.00 0.00 5 Stor-04 Storage Node 116.00 119.00 116.00 0.00 0.01 116.26 0.00 0.00 6 Stor-05 Storage Node 116.00 119.00 116.00 0.00 0.01 116.22 0.00 0.00 Peak flow from total site
after detention
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 4 of 31 Link Summary
SN Element Element From To (Outlet) Length Inlet Outlet Average Diameter or Manning's Peak Design Flow Peak Flow/ Peak Flow Peak Flow Peak Flow Total Time Reported ID Type (Inlet) Node Invert Invert Slope Height Roughness Flow Capacity Design Flow Velocity Depth Depth/ Surcharged Condition Node Elevation Elevation Ratio Total Depth
Ratio
(ft) (ft) (ft) (%) (in) (cfs) (cfs) (ft/sec) (ft) (min) 1 Orifice-01a Orifice Stor-01 Out-01 108.00 108.00 1.375 0.062 Orifice-01b Orifice Stor-01 Out-01 108.00 108.00 3.000 0.003 Orifice-02 Orifice Stor-02 Out-01 116.00 108.00 4.000 0.004 Orifice-03 Orifice Stor-03 Out-01 116.00 108.00 4.000 0.005 Orifice-04 Orifice Stor-04 Out-01 116.00 108.00 4.000 0.006 Orifice-05 Orifice Stor-05 Out-01 116.00 108.00 4.000 0.007 Weir-01 Weir Stor-01 Out-01 108.00 108.00 0.008 Weir-02 Weir Stor-02 Out-01 116.00 108.00 0.009 Weir-03 Weir Stor-03 Out-01 116.00 108.00 0.00 10 Weir-04 Weir Stor-04 Out-01 116.00 108.00 0.0011 Weir-05 Weir Stor-05 Out-01 116.00 108.00 0.00 Peak flow leaving storagenodes from orifices andweirs
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 5 of 31 Subbasin Hydrology
Subbasin : P1
Input Data
Area (ac) ....................................................................... 1.36 Peak Rate Factor ........................................................... 484 Weighted Curve Number ................................................ 77.12 Rain Gage ID ................................................................. Rain Gage-01 Composite Curve Number
32 Area Soil Curve
Soil/Surface Description (acres) Group Number
Paved parking & roofs 0.65 B 98
Meadow, non-grazed 0.71 B 58
Composite Area & Weighted CN 1.36 77.12
Time of Concentration
User-Defined TOC override (minutes): 5
Subbasin Runoff Results
Total Rainfall (in) ........................................................... 3.29 Total Runoff (in) ............................................................ 1.28 Peak Runoff (cfs) ........................................................... 0.37 Weighted Curve Number ................................................ 77.12 Time of Concentration (days hh:mm:ss) ......................... 0 00:05:00 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 6 of 31 Subbasin : P1
Rainfall Intensity Graph
Time (hrs)
95908580757065605550454035302520151050
R ainfall (in/hr)
0.8 0.75
0.7 0.65
0.6 0.55
0.5 0.45
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
Runoff Hydrograph
Time (hrs)
95908580757065605550454035302520151050
R unoff (cfs )
0.4 0.38
0.36 0.34
0.32 0.3
0.28 0.26
0.24 0.22
0.2 0.18
0.16 0.14
0.12 0.1
0.08 0.06
0.04 0.02
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 7 of 31 Subbasin : P2
Input Data
Area (ac) ....................................................................... 0.34 Peak Rate Factor ........................................................... 484 Weighted Curve Number ................................................ 80.35 Rain Gage ID ................................................................. Rain Gage-01 Composite Curve Number
32 Area Soil Curve
Soil/Surface Description (acres) Group Number
Paved roads with curbs & sewers 0.19 B 98
Meadow, non-grazed 0.15 B 58
Composite Area & Weighted CN 0.34 80.35
Time of Concentration
User-Defined TOC override (minutes): 5
Subbasin Runoff Results
Total Rainfall (in) ........................................................... 3.29 Total Runoff (in) ............................................................ 1.5 Peak Runoff (cfs) ........................................................... 0.12 Weighted Curve Number ................................................ 80.35 Time of Concentration (days hh:mm:ss) ......................... 0 00:05:00 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 8 of 31 Subbasin : P2
Rainfall Intensity Graph
Time (hrs)
95908580757065605550454035302520151050
Rainfall (in/hr) 0.85
0.8 0.75
0.7 0.65
0.6 0.55
0.5 0.45
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
Runoff Hydrograph
Time (hrs)
95908580757065605550454035302520151050
Runoff (c fs ) 0.125
0.12 0.115
0.11 0.105
0.1 0.095
0.09 0.085
0.08 0.075
0.07 0.065
0.06 0.055
0.05 0.045
0.04 0.035
0.03 0.025
0.02 0.015
0.01 0.005
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 9 of 31 Subbasin : P3
Input Data
Area (ac) ....................................................................... 0.52 Peak Rate Factor ........................................................... 484 Weighted Curve Number ................................................ 81.08 Rain Gage ID ................................................................. Rain Gage-01 Composite Curve Number
32 Area Soil Curve
Soil/Surface Description (acres) Group Number
Paved parking & roofs 0.3 B 98
Meadow, non-grazed 0.22 B 58
Composite Area & Weighted CN 0.52 81.08
Time of Concentration
User-Defined TOC override (minutes): 5
Subbasin Runoff Results
Total Rainfall (in) ........................................................... 3.29 Total Runoff (in) ............................................................ 1.55 Peak Runoff (cfs) ........................................................... 0.19 Weighted Curve Number ................................................ 81.08 Time of Concentration (days hh:mm:ss) ......................... 0 00:05:00 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 10 of 31 Subbasin : P3
Rainfall Intensity Graph
Time (hrs)
95908580757065605550454035302520151050
R ainfall (in/hr) 0.85
0.8 0.75
0.7 0.65
0.6 0.55
0.5 0.45
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
Runoff Hydrograph
Time (hrs)
95908580757065605550454035302520151050
Runoff (c fs )
0.2 0.19
0.18 0.17
0.16 0.15
0.14 0.13
0.12 0.11
0.1 0.09
0.08 0.07
0.06 0.05
0.04 0.03
0.02 0.01
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 11 of 31 Subbasin : P4
Input Data
Area (ac) ....................................................................... 0.02 Peak Rate Factor ........................................................... 484 Weighted Curve Number ................................................ 84 Rain Gage ID ................................................................. Rain Gage-01 Composite Curve Number
32 Area Soil Curve
Soil/Surface Description (acres) Group Number
Paved parking & roofs 0.01 B 98
Meadow, non-grazed 0.01 B 58
Composite Area & Weighted CN 0.02 84
Time of Concentration
User-Defined TOC override (minutes): 5.00
Subbasin Runoff Results
Total Rainfall (in) ........................................................... 3.29 Total Runoff (in) ............................................................ 1.71 Peak Runoff (cfs) ........................................................... 0.01 Weighted Curve Number ................................................ 84 Time of Concentration (days hh:mm:ss) ......................... 0 00:05:00 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 12 of 31 Subbasin : P4
Rainfall Intensity Graph
Time (hrs)
95908580757065605550454035302520151050
R ainfall (in/hr) 0.85
0.8 0.75
0.7 0.65
0.6 0.55
0.5 0.45
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
Runoff Hydrograph
Time (hrs)
95908580757065605550454035302520151050
Runoff (c fs) 0.009
0.009
0.008
0.008
0.007
0.007
0.006
0.006
0.005
0.004
0.004
0.004
0.003
0.003
0.002
0.002
0.001
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 13 of 31 Subbasin : P5
Input Data
Area (ac) ....................................................................... 0.02 Peak Rate Factor ........................................................... 484 Weighted Curve Number ................................................ 82 Rain Gage ID ................................................................. Rain Gage-01 Composite Curve Number
32 Area Soil Curve
Soil/Surface Description (acres) Group Number
Paved parking & roofs 0.01 B 98
Meadow, non-grazed 0.01 B 58
Composite Area & Weighted CN 0.02 82
Time of Concentration
User-Defined TOC override (minutes): 5
Subbasin Runoff Results
Total Rainfall (in) ........................................................... 3.29 Total Runoff (in) ............................................................ 1.54 Peak Runoff (cfs) ........................................................... 0.01 Weighted Curve Number ................................................ 82 Time of Concentration (days hh:mm:ss) ......................... 0 00:05:00 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 14 of 31 Subbasin : P5
Rainfall Intensity Graph
Time (hrs)
95908580757065605550454035302520151050
Rainfall (in/hr) 0.85
0.8 0.75
0.7 0.65
0.6 0.55
0.5 0.45
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
Runoff Hydrograph
Time (hrs)
95908580757065605550454035302520151050
R unoff (c fs ) 0.008
0.008
0.007
0.007
0.006
0.006
0.005
0.004
0.004
0.004
0.003
0.003
0.002
0.002
0.001
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 15 of 31 Subbasin : P6
Input Data
Area (ac) ....................................................................... 0.73 Peak Rate Factor ........................................................... 484 Weighted Curve Number ................................................ 58 Rain Gage ID ................................................................. Rain Gage-01 Composite Curve Number
32 Area Soil Curve
Soil/Surface Description (acres) Group Number
Meadow, non-grazed 0.73 B 58
Composite Area & Weighted CN 0.73 58
Time of Concentration
Subarea Subarea Subarea
Sheet Flow Computations A B C
Manning's Roughness : 0.3 0 0
Flow Length (ft) : 80 0 0
Slope (%) : 3.5 0 0
2 yr, 24 hr Rainfall (in) : 3.4 0 0
Velocity (ft/sec) : 0.12 0 0
Computed Flow Time (min) : 11.07 0 0
Subarea Subarea Subarea
Shallow Concentrated Flow Computations A B C
Flow Length (ft) : 50 0 0
Slope (%) : 1.3 0 0
Surface Type : Grass pastureGrass pasture Unpaved
Velocity (ft/sec) : 0.8 0 0
Computed Flow Time (min) : 1.04 0 0
Total TOC (min) ..................12.11
Subbasin Runoff Results
Total Rainfall (in) ........................................................... 3.29 Total Runoff (in) ............................................................ 0.37 Peak Runoff (cfs) ........................................................... 0.02 Weighted Curve Number ................................................ 58 Time of Concentration (days hh:mm:ss) ......................... 0 00:12:07 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 16 of 31 Subbasin : P6
Rainfall Intensity Graph
Time (hrs)
95908580757065605550454035302520151050
Rainfall (in/hr) 0.85
0.8 0.75
0.7 0.65
0.6 0.55
0.5 0.45
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
Runoff Hydrograph
Time (hrs)
95908580757065605550454035302520151050
Runoff (c fs) 0.021
0.02 0.019
0.018
0.017
0.016
0.015
0.014
0.013
0.012
0.011
0.01 0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 17 of 31 Storage Nodes
Storage Node : Stor-01
Input Data
108.00
114.00
6.00 108.00
0.00 0.00
0.00 Infiltration/Exfiltration
0.011
Storage Area Volume Curves
Storage Curve : Storage-01
Stage Storage Storage
Area Volume
(ft) (ft²) (ft³)
0 0 0
2 1200 1200
3.5 942.86 1650
4 900 1800
6 1980.67 5942
Evaporation Loss ...............................................................
Constant Flow Rate (cfs) ...................................................
Invert Elevation (ft) ............................................................
Max (Rim) Elevation (ft) .....................................................
Max (Rim) Offset (ft) ..........................................................
Initial Water Elevation (ft) ..................................................
Initial Water Depth (ft) .......................................................
Ponded Area (ft²) ...............................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 18 of 31 Storage Area Volume Curves
Storage Area Storage Volume
Storage Area (ft²)
1,8001,6001,4001,2001,0008006004002000
Storage Volume (ft³)
5,0004,0003,0002,0001,0000
S tage (ft)
6 5.8
5.6 5.4
5.2 5
4.8 4.6
4.4 4.2
4 3.8
3.6 3.4
3.2 3
2.8 2.6
2.4 2.2
2 1.8
1.6 1.4
1.2 1
0.8 0.6
0.4 0.2
0 S tage (ft)
6 5.8
5.6 5.4
5.2 5
4.8 4.6
4.4 4.2
4 3.8
3.6 3.4
3.2 3
2.8 2.6
2.4 2.2
2 1.8
1.6 1.4
1.2 1
0.8 0.6
0.4 0.2
0 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 19 of 31 Storage Node : Stor-01 (continued)
Outflow Weirs
SN Element Weir Flap Crest Crest Length Weir Total Discharge ID Type Gate Elevation Offset Height Coefficient
(ft) (ft) (ft) (ft)
1 Weir-01 Trapezoidal No 113.00 5.00 6.00 1.00 3.33
Outflow Orifices
SN Element Orifice Orifice Flap Circular Rectangular Rectangular Orifice Orifice ID Type Shape Gate Orifice Orifice Orifice Invert Coefficient Diameter Height Width Elevation
(in) (in) (in) (ft)
1 Orifice-01a Side CIRCULAR No 1.38 110.00 0.61
2 Orifice-01b Side Rectangular No 3.00 6.00 112.50 0.63 Output Summary Results
0.37 0.37
0.06 0.66
111.57
3.57 109.24
1.24 0 21:23
2.359
0 0
0 Peak Inflow (cfs) ...............................................................
Peak Lateral Inflow (cfs) ....................................................
Peak Outflow (cfs) .............................................................
Peak Exfiltration Flow Rate (cfm) ........................................
Total Flooded Volume (ac-in) .............................................
Total Time Flooded (min) ...................................................
Total Retention Time (sec) .................................................
Max HGL Elevation Attained (ft) ..........................................
Max HGL Depth Attained (ft) ..............................................
Average HGL Elevation Attained (ft) ....................................
Average HGL Depth Attained (ft) ........................................
Time of Max HGL Occurrence (days hh:mm) .......................
Total Exfiltration Volume (1000-ft³) ....................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 20 of 31 Storage Node : Stor-02
Input Data
116.00
119.00
3.00 116.00
0.00 0.00
0.00 Infiltration/Exfiltration
0.035
Storage Area Volume Curves
Storage Curve : Storage-02-03
Stage Storage Storage
Area Volume
(ft) (ft²) (ft³)
0 0 0
0.75 3680 1380
1.08 3690.74 1993
1.42 2807.04 1993
2 1993 1993
Invert Elevation (ft) ............................................................
Max (Rim) Elevation (ft) .....................................................
Max (Rim) Offset (ft) ..........................................................
Initial Water Elevation (ft) ..................................................
Initial Water Depth (ft) .......................................................
Ponded Area (ft²) ...............................................................
Evaporation Loss ...............................................................
Constant Flow Rate (cfs) ...................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 21 of 31 Storage Area Volume Curves
Storage Area Storage Volume
Storage Area (ft²)
3,5003,0002,5002,0001,5001,0005000
Storage Volume (ft³)
2,0001,8001,6001,4001,2001,0008006004002000
S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 22 of 31 Storage Node : Stor-02 (continued)
Outflow Weirs
SN Element Weir Flap Crest Crest Length Weir Total Discharge ID Type Gate Elevation Offset Height Coefficient
(ft) (ft) (ft) (ft)
1 Weir-02 Trapezoidal No 118.00 2.00 100.00 1.00 3.33
Outflow Orifices
SN Element Orifice Orifice Flap Circular Rectangular Rectangular Orifice Orifice ID Type Shape Gate Orifice Orifice Orifice Invert Coefficient Diameter Height Width Elevation
(in) (in) (in) (ft)
1 Orifice-02 Side CIRCULAR No 4.00 116.75 0.61
Output Summary Results
0.12 0.12
0 2.1
116.29
0.29 116.02
0.02 0 10:05
1.229
0 0
0 Peak Lateral Inflow (cfs) ....................................................
Peak Outflow (cfs) .............................................................
Peak Exfiltration Flow Rate (cfm) ........................................
Max HGL Elevation Attained (ft) ..........................................
Max HGL Depth Attained (ft) ..............................................
Average HGL Elevation Attained (ft) ....................................
Peak Inflow (cfs) ...............................................................
Average HGL Depth Attained (ft) ........................................
Time of Max HGL Occurrence (days hh:mm) .......................
Total Exfiltration Volume (1000-ft³) ....................................
Total Flooded Volume (ac-in) .............................................
Total Time Flooded (min) ...................................................
Total Retention Time (sec) .................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 23 of 31 Storage Node : Stor-03
Input Data
116.00
119.00
3.00 116.00
0.00 0.00
0.00 Infiltration/Exfiltration
0.036
Storage Area Volume Curves
Storage Curve : Storage-02-03
Stage Storage Storage
Area Volume
(ft) (ft²) (ft³)
0 0 0
0.75 3680 1380
1.08 3690.74 1993
1.42 2807.04 1993
2 1993 1993
Invert Elevation (ft) ............................................................
Max (Rim) Elevation (ft) .....................................................
Max (Rim) Offset (ft) ..........................................................
Initial Water Elevation (ft) ..................................................
Initial Water Depth (ft) .......................................................
Ponded Area (ft²) ...............................................................
Evaporation Loss ...............................................................
Constant Flow Rate (cfs) ...................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 24 of 31 Storage Area Volume Curves
Storage Area Storage Volume
Storage Area (ft²)
3,5003,0002,5002,0001,5001,0005000
Storage Volume (ft³)
2,0001,8001,6001,4001,2001,0008006004002000
S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 25 of 31 Storage Node : Stor-03 (continued)
Outflow Weirs
SN Element Weir Flap Crest Crest Length Weir Total Discharge ID Type Gate Elevation Offset Height Coefficient
(ft) (ft) (ft) (ft)
1 Weir-03 Trapezoidal No 118.00 2.00 100.00 1.00 3.33
Outflow Orifices
SN Element Orifice Orifice Flap Circular Rectangular Rectangular Orifice Orifice ID Type Shape Gate Orifice Orifice Orifice Invert Coefficient Diameter Height Width Elevation
(in) (in) (in) (ft)
1 Orifice-03 Side CIRCULAR No 4.00 116.75 0.61
Output Summary Results
0.18 0.18
0 2.16
116.53
0.53 116.1
0.1 0 18:06
2.786
0 0
0 Max HGL Elevation Attained (ft) ..........................................
Max HGL Depth Attained (ft) ..............................................
Average HGL Elevation Attained (ft) ....................................
Average HGL Depth Attained (ft) ........................................
Time of Max HGL Occurrence (days hh:mm) .......................
Total Exfiltration Volume (1000-ft³) ....................................
Peak Inflow (cfs) ...............................................................
Peak Lateral Inflow (cfs) ....................................................
Peak Outflow (cfs) .............................................................
Peak Exfiltration Flow Rate (cfm) ........................................
Total Flooded Volume (ac-in) .............................................
Total Time Flooded (min) ...................................................
Total Retention Time (sec) .................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 26 of 31 Storage Node : Stor-04
Input Data
116.00
119.00
3.00 116.00
0.00 0.00
0.00 Infiltration/Exfiltration
0.0027
Storage Area Volume Curves
Storage Curve : Storage-04-06
Stage Storage Storage
Area Volume
(ft) (ft²) (ft³)
0 0 0
0.75 280 105
1.08 281.48 152
1.42 214.08 152
2 152 152
Initial Water Elevation (ft) ..................................................
Initial Water Depth (ft) .......................................................
Ponded Area (ft²) ...............................................................
Evaporation Loss ...............................................................
Constant Flow Rate (cfs) ...................................................
Invert Elevation (ft) ............................................................
Max (Rim) Elevation (ft) .....................................................
Max (Rim) Offset (ft) ..........................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 27 of 31 Storage Area Volume Curves
Storage Area Storage Volume
Storage Area (ft²)
280260240220200180160140120100806040200
Storage Volume (ft³)
1501401301201101009080706050403020100
S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 28 of 31 Storage Node : Stor-04 (continued)
Outflow Weirs
SN Element Weir Flap Crest Crest Length Weir Total Discharge ID Type Gate Elevation Offset Height Coefficient
(ft) (ft) (ft) (ft)
1 Weir-04 Trapezoidal No 118.00 2.00 15.00 1.00 3.33
Outflow Orifices
SN Element Orifice Orifice Flap Circular Rectangular Rectangular Orifice Orifice ID Type Shape Gate Orifice Orifice Orifice Invert Coefficient Diameter Height Width Elevation
(in) (in) (in) (ft)
1 Orifice-04 Side CIRCULAR No 4.00 116.75 0.61
Output Summary Results
0.01 0.01
0 0.16
116.26
0.26 116.01
0.01 0 09:26
0.066
0 0
0 Peak Inflow (cfs) ...............................................................
Average HGL Depth Attained (ft) ........................................
Time of Max HGL Occurrence (days hh:mm) .......................
Total Exfiltration Volume (1000-ft³) ....................................
Total Flooded Volume (ac-in) .............................................
Total Time Flooded (min) ...................................................
Total Retention Time (sec) .................................................
Peak Lateral Inflow (cfs) ....................................................
Peak Outflow (cfs) .............................................................
Peak Exfiltration Flow Rate (cfm) ........................................
Max HGL Elevation Attained (ft) ..........................................
Max HGL Depth Attained (ft) ..............................................
Average HGL Elevation Attained (ft) ....................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 29 of 31 Storage Node : Stor-05
Input Data
116.00
119.00
3.00 116.00
0.00 0.00
0.00 Infiltration/Exfiltration
0.0027
Storage Area Volume Curves
Storage Curve : Storage-04-06
Stage Storage Storage
Area Volume
(ft) (ft²) (ft³)
0 0 0
0.75 280 105
1.08 281.48 152
1.42 214.08 152
2 152 152
Evaporation Loss ...............................................................
Constant Flow Rate (cfs) ...................................................
Invert Elevation (ft) ............................................................
Max (Rim) Elevation (ft) .....................................................
Max (Rim) Offset (ft) ..........................................................
Initial Water Elevation (ft) ..................................................
Initial Water Depth (ft) .......................................................
Ponded Area (ft²) ...............................................................
Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 30 of 31 Storage Area Volume Curves
Storage Area Storage Volume
Storage Area (ft²)
280260240220200180160140120100806040200
Storage Volume (ft³)
1501401301201101009080706050403020100
S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 S tage (ft)
2 1.9
1.8 1.7
1.6 1.5
1.4 1.3
1.2 1.1
1 0.9
0.8 0.7
0.6 0.5
0.4 0.3
0.2 0.1
0 Fort Bragg Senior Apartments Proposed Mitigated Condition, 2-year, 24-Hour Page 31 of 31 Storage Node : Stor-05 (continued)
Outflow Weirs
SN Element Weir Flap Crest Crest Length Weir Total Discharge ID Type Gate Elevation Offset Height Coefficient
(ft) (ft) (ft) (ft)
1 Weir-05 Trapezoidal No 118.00 2.00 15.00 1.00 3.33
Outflow Orifices
SN Element Orifice Orifice Flap Circular Rectangular Rectangular Orifice Orifice ID Type Shape Gate Orifice Orifice Orifice Invert Coefficient Diameter Height Width Elevation
(in) (in) (in) (ft)
1 Orifice-05 Side CIRCULAR No 4.00 116.75 0.61
Output Summary Results
0.01 0.01
0 0.16
116.22
0.22 116.01
0.01 0 09:11
0.05 0
0 0
Peak Inflow (cfs) ...............................................................
Peak Lateral Inflow (cfs) ....................................................
Peak Outflow (cfs) .............................................................
Peak Exfiltration Flow Rate (cfm) ........................................
Total Flooded Volume (ac-in) .............................................
Total Time Flooded (min) ...................................................
Total Retention Time (sec) .................................................
Max HGL Elevation Attained (ft) ..........................................
Max HGL Depth Attained (ft) ..............................................
Average HGL Elevation Attained (ft) ....................................
Average HGL Depth Attained (ft) ........................................
Time of Max HGL Occurrence (days hh:mm) .......................
Total Exfiltration Volume (1000-ft³) ....................................
ATTACHMENT 5
References
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ATTACHMENT 6
Geotechnical Report