PC - Item 3B - Exhibit F - Appendix H Preliminary Hydrology Study1
PRELIMINARY HYDROLOGY STUDY
VESTING TENTATIVE TRACT MAP No. 83705
8601 Mission Drive
Rosemead, California
Project Address:
8601 Mission Drive
Rosemead, California 91770
Prepared For:
Mission Villas, LLC.
Attn: Mitch Gardner
11766 Wilshire Boulevard, Suite 820
Los Angeles, CA 90025
(310) 582-1991
Prepared By:
C&V Consulting Inc.
9830 Irvine Center Dr.
Irvine, CA 92618
Ryan Bittner, P.E.
Contact: Marco Midence
Project Manager
(949) 916-3800
Prepared:
February 2022
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TABLE OF CONTENTS
SECTION PAGE
1.0 SITE DESCRIPTION: 4
2.0 PURPOSE OF STUDY: 4
3.0 EXISTING CONDITIONS: 4
4.0 PROPOSED CONDITIONS: 4
5.0 METHODOLOGY: 5
6.0 RESULTS: 6
7.0 CONCLUSIONS: 7
8.0 DESIGN ASSUMPTIONS: 7
9.0 REFERENCES: 7
APPENDIX A: Hydrology Maps
Existing Conditions Preliminary Hydrology Map
Proposed Conditions Preliminary Hydrology Map
APPENDIX B: Hydrology Calculations
Existing Conditions Hydrology Calculations (25-year, 50-year, & 100-year Storm Events)
Proposed Conditions Hydrology Calculations (25-year, 50-year, & 100-year Storm Events)
Off-site Hydrology Calculations (25-year, 50-year, & 100-year Storm Events)
APPENDIX C: Isohyet Map
APPENDIX D: Hydraulic Calculations
APPENDIX E: As-Builts & References
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Preliminary Hydrology Study
for
Vesting Tentative Tract Map No. 83705
Mission Villas Rosemead
ACKNOWLEDGEMENT AND SIGNATURE PAGE
This Preliminary Hydrology Study was prepared by C&V Consulting, Inc. under the supervision of
Ryan J. Bittner, P.E.
___________________________________ ____________
Ryan J. Bittner, R.C.E. 68167 Date
Principal, C&V Consulting, Inc.
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1.0 SITE DESCRIPTION:
The proposed development encompasses three (3) parcels consisting of approximately 3.44 gross acres
and 3.38 net acres. The site is bounded by existing residential lots to the north and the east, Mission
Drive to the south, and power lines owned by Southern California Edison to the west.
2.0 PURPOSE OF STUDY:
The preliminary hydrology study will determine the amount of stormwater runoff generated from the
project site in the existing and proposed conditions. This study will anticipate whether detention or
other peak flow mitigation methods will be required by comparing the proposed and existing condition
peak flow rates for the 25, 50 and 100-year storm events.
3.0 EXISTING CONDITIONS:
All three parcels are currently a vacant lot. Besides two small service roads in southern and eastern
portions of the site the site is all pervious coverage. The site is surrounded by alternating masonry block
wall, wooden fence, and chain link fence around the entire property. There is existing public sidewalk
and driveway entrances along Mission Drive. There are no existing storm drain facilities located on the
site and no storm drain facilities located in the adjacent portion of Mission Drive along the project
frontage.
The existing drainage pattern of the site consists of one drainage area labeled XA1 on the Pre-Existing
condition hydrology map located in Appendix A. The entire site sheet flows over the vacant land in a
southerly direction. Stormwater appears to simply sheet flow over existing driveway entrances and
enters Mission Road. Once stormwater enters Mission Rd. it flows in a southeasterly direction and
enters a Los Angeles County Flood Control District (LACFCD) owned catch basin approximately 500’
downstream of our site. This catch basin is connected to a LACFCD 30” RCP (LACFCD Project RDD
250) which flows into the Eaton Wash channel then into the Rio Hondo Channel. The Rio Hondo
Channel then connects to the Los Angeles River then ultimately the Pacific Ocean. The existing area
was determined to be 98.5% pervious based on the ALTA/Topographic Survey prepared by C&V
Consulting, Inc. dated October 2021.
Since all the existing onsite stormwater runoff ultimately reaches one location, the existing site was
analyzed as one drainage area (XA1) to approximately quantify the runoff based on the longest
hydraulic path from the most remote high point to low point.
Refer to the “Existing Conditions Hydrology Map” located within Appendix A of this study for more
information.
4.0 PROPOSED CONDITIONS:
The proposed project consists of 8 duplex units and 29 single family homes over approximately 3.38
acres. The proposed development includes drive aisles, parking, landscaping, walkways, patios, and
common open space areas. The site will be graded to collect runoff at one low point to control the
amount of imported fill during grading and maintaining the existing site drainage pattern. The proposed
development will utilize onsite catch basins, infiltration systems, and a detention pipe system to capture
and treat stormwater. Stormwater up to the design capture volume will be infiltrated by a proposed
onsite drywell system.
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Stormwater runoff will be conveyed via proposed onsite gutter and directed to one sump area equipped
with a curb inlet catch basin. There will be two additional catch basins located along the site’s main
spine in flow by conditions to convey stormwater into the underground storm drain system. The sump
catch basin will be located at the end of the drive aisle at the southern property line. All on-site catch
basins will be connected by storm drain pipe to the drywell infiltration system for water quality
treatment. During larger storm events, stormwater runoff will back up the drywell system which is
connected to the underground detention system. Larger storm events will bypass the infiltration system
and overflow into a proposed grate inlet catch basin which is attached to a parkway culvert. Once
stormwater enters the parkway culvert it will drain into Mission Road and follow the existing drainage
pattern. For emergency overflow, runoff will spill out of the proposed curb inlet catch basin, topple
over the proposed driveway entrance, and sheet flow into Mission Road. Refer to separately prepared
Preliminary Grading and Utility Plans for site design information.
In an event where the proposed onsite storm drain system is at its full capacity or clogged, stormwater
will pond up at the lowest proposed onsite sump area and excess stormwater will top over the grade
break at the site’s entrance and continue to flow out through proposed driveway and into Mission Rd.
During final engineering, water surface elevation will be analyzed and provided to verify all habitable
structures will have at least a 1 foot of freeboard during the 100-year storm event.
According to the Federal Emergency Management Agency (FEMA), FIRM rate map Number
06037C1675F, revised September 26, 2008, the site is located within the flood zone as follows: Zone
X – “Areas determined to be outside the 0.2% annual chance floodplain”
The “Proposed Conditions Preliminary Hydrology Map” is included in Appendix A for reference.
5.0 METHODOLOGY:
The site was analyzed using the Los Angeles County Department of Public Works Hydrology Manual.
The initial subarea was analyzed for acreage, land-use, soil type, peak flow rate and time of
concentration according to the Rational Method described in the manual.
In this preliminary hydrology study, the proposed condition impervious area percentage values were
conservative estimation from the LA County Hydrology Manual. During final engineering, impervious
areas will be calculated in more detail to refine all peak flow rates.
In accordance with the Los Angeles County Department of Public Works Hydrology Manual all
habitable structures must have a finished floor elevation to allow 1 ft of freeboard during the 100-year
storm event. Catch basin, pipe sizing and 100-year water surface elevation calculations will be provided
during final engineering.
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6.0 RESULTS:
Hydrology Summary
Pre-Developed Hydrology Summary
Area ID
Total
Area
(AC)
Pervious
Area
(AC)
Pervious
(%)
Impervious
Area (AC)
Impervious
(%)
Q25
(CFS)
Q50
(CFS)
Q100
(CFS)
XA1 3.38 3.33 98.5% 0.05 1.5% 7.21 9.08 11.27
Post-Developed Hydrology Summary
Area ID
Total
Area
(AC)
Pervious
Area
(AC)
Pervious
(%)
Impervious
Area (AC)
Impervious
(%)
Q25
(CFS)
Q50
(CFS)
Q100
(CFS)
A1 3.38 0.47 14.0% 2.91 86.0% 7.97 9.65 10.87
Percent Decrease:
∆25-year peak storm flow = 7.97/7.21 = increase of 10.5%
∆50-year peak storm flow = 9.65/9.08 = increase of 6.3%
∆100-year peak storm flow = 10.87/11.27 = decrease of 3.5%
Refer to Appendix A & B of this report for additional information shown in the LACDPW HydroCalc
output data, as well as the pre-developed and post-developed hydrology maps.
Detention Sizing
Compared to the existing condition the proposed development will, on average, match the existing
condition. As there is a net change in overall storm event volume, detention may be required to mitigate
the proposed condition peak flow rates. However, detention is required for the on-site infiltration
system. This system will be designed to allow the retention of the entire Stormwater Quality Design
Volume (SWQDCv) of 8,630 CF. This retention volume will offset any increase in peak flow from the
proposed development. The detention system will utilize Bioclean’s Urbanpond Detention system and
sizing calculations can be found in the separately prepared Preliminary LID Report.
Catch Basin Sizing
Catch basin Sizing will be analyzed for the 50-year storm event peak flow rates and will be provided
during final engineering.
Pipe Sizing
Pipe Sizing will be analyzed using WSPG software to verify hydraulic grade line (HGL) based on the
100-year storm event peak flow rates and will be provided during final engineering for proposed onsite
conveyance pipe.
100-Year Water Surface Elevations
Water surface elevations for the 100-year storm event peak flow rates will verify that the proposed
finish floor elevations are set at least 1’ above the water surface elevation and will be calculated and
provided during final engineering.
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7.0 CONCLUSIONS:
The results from this preliminary hydrology study utilizing Los Angeles County Department of Public
Works Hydrology Manual demonstrate that the proposed condition peak flow rates compared to the
existing condition peak flow will stay the same as indicated in the hydrology summary results in
Section 6 of this report. This is mainly due to change in grading and elongation of the proposed
condition’s flow path. During final engineering, impervious area for proposed conditions will be
calculated in more detail based on the finalized landscape plan. The proposed peak flow rates will be
re-evaluated to reflect the actual proposed conditions. However, the proposed development is likely to
generate lower peak flows.
The proposed development will be graded to allow for one low point on the site equipped with a curb
inlet catch basin, this catch basin will be connected to the drywell infiltration system for treatment. The
storm drain system will also have detention system to retain the entire SWQDCv. In the event the storm
drain system becomes clogged, the proposed grading will facilitate emergency overflow by ponding at
the grate inlet catch basin, toppling over the curb and sheet flow into the Mission Rd.
8.0 DESIGN ASSUMPTIONS:
1. The property is in the City of Rosemead, Los Angeles County rainfall region.
2. 100-year storm event flood level protection analysis required for habitable structures per the
requirements of the Los Angeles County Department of Public Works Hydrology Manual
3. According to the Los Angeles County Department of Public Works Hydrology Manual 50-Year
24-Hour Isohyet Map 1-H1.20, the drainage area is in Soil Group 006, the site receives 6.7 inches
of rainfall over a 24-Hr storm (Q50).
4. The LACDPW HydroCalc was utilized to determine the time of concentration, run-off flow rate
and run-off volume for site.
5. The site was analyzed for a 25, 50 and 100-year storm events per the requirements of the January
2006 Los Angeles County Department of Public Works Hydrology Manual. The Rational Method
Analysis was performed, and the appropriate calculations are provided herein.
6. The existing project site is currently a vacant lot that is 98.5% pervious based on the aerial survey
performed by C&V Consulting, Inc.
7. The proposed site was assumed to be approximately 86% based on the LACDPW Hydrology
Manual for “Low-Rise Apartments, Condominiums, and Townhouses” land use type.
9.0 REFERENCES:
1. Los Angeles County Department of Public Works, “Hydrology Manual”, January 2006.
2. Los Angeles County Department of Public Works, “HydroCalc” Outputs and Data
3. Hydraflow Express Extensions for Civil 3D 2021.
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4. Preliminary Grading & Drainage Plan for this project by C&V Consulting, Inc. February 2022
APPENDIX A
HYDROLOGY MAPS
Existing Conditions Preliminary Hydrology Map
LEGEND:Pre-Developed Hydroligic SummaryArea IDTotal Area(AC)PerviousArea (AC)Pervious(%)ImperviousArea (AC)Impervious(%)Q25 (CFS)Q50 (CFS)Q100 (CFS)XA13.383.3398.48%0.051.52%7.219.0811.27
Proposed Conditions Preliminary Hydrology Map
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//LEGEND:Post-Developed Hydroligic SummaryArea IDTotal Area(AC)PerviousArea (AC)Pervious(%)ImperviousArea (AC)Impervious(%)Q25 (CFS)Q50 (CFS)Q100 (CFS)A13.380.4714.00%2.9186.00%7.979.6510.87
APPENDIX B
HYDROLOGY CALCULATIONS
Existing Conditions Hydrology Calculations (25, 50, &
100-year Storm Events)
Peak Flow Hydrologic Analysis
File location: P:/B/BORS-003/Admin/Reports/Hydrology/Hydrology/Preliminary/Appendix B - Hydrology Calculations (Hydro Calc)/BORS-003 - XA1 - 25yr.pdf
Version: HydroCalc 1.0.3
Input Parameters
Project Name BORS-003
Subarea ID XA1
Area (ac)3.38
Flow Path Length (ft)705.0
Flow Path Slope (vft/hft)0.01
50-yr Rainfall Depth (in)6.7
Percent Impervious 0.02
Soil Type 6
Design Storm Frequency 25-yr
Fire Factor 0
LID False
Output Results
Modeled (25-yr) Rainfall Depth (in)5.8826
Peak Intensity (in/hr)2.6625
Undeveloped Runoff Coefficient (Cu)0.7986
Developed Runoff Coefficient (Cd)0.8006
Time of Concentration (min)9.0
Clear Peak Flow Rate (cfs)7.205
Burned Peak Flow Rate (cfs)7.205
24-Hr Clear Runoff Volume (ac-ft)0.3725
24-Hr Clear Runoff Volume (cu-ft)16223.9942
Peak Flow Hydrologic Analysis
File location: P:/B/BORS-003/Admin/Reports/Hydrology/Hydrology/Preliminary/Appendix B - Hydrology Calculations (Hydro Calc)/BORS-003 - XA1 - 50yr.pdf
Version: HydroCalc 1.0.3
Input Parameters
Project Name BORS-003
Subarea ID XA1
Area (ac)3.38
Flow Path Length (ft)705.0
Flow Path Slope (vft/hft)0.01
50-yr Rainfall Depth (in)6.7
Percent Impervious 0.02
Soil Type 6
Design Storm Frequency 50-yr
Fire Factor 0
LID False
Output Results
Modeled (50-yr) Rainfall Depth (in)6.7
Peak Intensity (in/hr)3.2051
Undeveloped Runoff Coefficient (Cu)0.8367
Developed Runoff Coefficient (Cd)0.8379
Time of Concentration (min)8.0
Clear Peak Flow Rate (cfs)9.0777
Burned Peak Flow Rate (cfs)9.0777
24-Hr Clear Runoff Volume (ac-ft)0.4585
24-Hr Clear Runoff Volume (cu-ft)19970.4404
Peak Flow Hydrologic Analysis
File location: P:/B/BORS-003/Admin/Reports/Hydrology/Hydrology/Preliminary/Appendix B - Hydrology Calculations (Hydro Calc)/BORS-003 - XA1 - 100yr.pdf
Version: HydroCalc 1.0.3
Input Parameters
Project Name BORS-003
Subarea ID XA1
Area (ac)3.38
Flow Path Length (ft)705.0
Flow Path Slope (vft/hft)0.01
50-yr Rainfall Depth (in)6.7
Percent Impervious 0.02
Soil Type 6
Design Storm Frequency 100-yr
Fire Factor 0
LID False
Output Results
Modeled (100-yr) Rainfall Depth (in)7.5174
Peak Intensity (in/hr)3.829
Undeveloped Runoff Coefficient (Cu)0.87
Developed Runoff Coefficient (Cd)0.8706
Time of Concentration (min)7.0
Clear Peak Flow Rate (cfs)11.2669
Burned Peak Flow Rate (cfs)11.2669
24-Hr Clear Runoff Volume (ac-ft)0.5535
24-Hr Clear Runoff Volume (cu-ft)24109.2086
Proposed Conditions Hydrology Calculations (25, 50, &
100-year Storm Events)
Peak Flow Hydrologic Analysis
File location: P:/B/BORS-003/Admin/Reports/Hydrology/Hydrology/Preliminary/Appendix B - Hydrology Calculations (Hydro Calc)/BORS-003 - A1 - 25yr.pdf
Version: HydroCalc 1.0.3
Input Parameters
Project Name BORS-003
Subarea ID A1
Area (ac)3.38
Flow Path Length (ft)714.0
Flow Path Slope (vft/hft)0.009
50-yr Rainfall Depth (in)6.7
Percent Impervious 0.86
Soil Type 6
Design Storm Frequency 25-yr
Fire Factor 0
LID False
Output Results
Modeled (25-yr) Rainfall Depth (in)5.8826
Peak Intensity (in/hr)2.6625
Undeveloped Runoff Coefficient (Cu)0.7986
Developed Runoff Coefficient (Cd)0.8858
Time of Concentration (min)9.0
Clear Peak Flow Rate (cfs)7.9716
Burned Peak Flow Rate (cfs)7.9716
24-Hr Clear Runoff Volume (ac-ft)1.3208
24-Hr Clear Runoff Volume (cu-ft)57536.2233
Peak Flow Hydrologic Analysis
File location: P:/B/BORS-003/Admin/Reports/Hydrology/Hydrology/Preliminary/Appendix B - Hydrology Calculations (Hydro Calc)/BORS-003 - A1 - 50yr.pdf
Version: HydroCalc 1.0.3
Input Parameters
Project Name BORS-003
Subarea ID A1
Area (ac)3.38
Flow Path Length (ft)714.0
Flow Path Slope (vft/hft)0.009
50-yr Rainfall Depth (in)6.7
Percent Impervious 0.86
Soil Type 6
Design Storm Frequency 50-yr
Fire Factor 0
LID False
Output Results
Modeled (50-yr) Rainfall Depth (in)6.7
Peak Intensity (in/hr)3.2051
Undeveloped Runoff Coefficient (Cu)0.8367
Developed Runoff Coefficient (Cd)0.8911
Time of Concentration (min)8.0
Clear Peak Flow Rate (cfs)9.6539
Burned Peak Flow Rate (cfs)9.6539
24-Hr Clear Runoff Volume (ac-ft)1.5093
24-Hr Clear Runoff Volume (cu-ft)65744.1478
Peak Flow Hydrologic Analysis
File location: P:/B/BORS-003/Admin/Reports/Hydrology/Hydrology/Preliminary/Appendix B - Hydrology Calculations (Hydro Calc)/BORS-003 - A1 - 100yr.pdf
Version: HydroCalc 1.0.3
Input Parameters
Project Name BORS-003
Subarea ID A1
Area (ac)3.38
Flow Path Length (ft)714.0
Flow Path Slope (vft/hft)0.009
50-yr Rainfall Depth (in)6.7
Percent Impervious 0.86
Soil Type 6
Design Storm Frequency 100-yr
Fire Factor 0
LID False
Output Results
Modeled (100-yr) Rainfall Depth (in)7.5174
Peak Intensity (in/hr)3.5961
Undeveloped Runoff Coefficient (Cu)0.8611
Developed Runoff Coefficient (Cd)0.8945
Time of Concentration (min)8.0
Clear Peak Flow Rate (cfs)10.8731
Burned Peak Flow Rate (cfs)10.8731
24-Hr Clear Runoff Volume (ac-ft)1.6989
24-Hr Clear Runoff Volume (cu-ft)74006.0329
APPENDIX C
Isohyet
APPENDIX D
Hydraulic Calculations
To be provided during Final Engineering
APPENDIX E
As-builts & References