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City of Lynwood, CA
Atlantic Housing Development
HYDROLOGY STUDY REPORT
PREPARED BY:
3060 Saturn Street, Suite 250
Brea, CA 92821
(714) 940-0100
I.E. Project No. 6045.23
July 2018
___________________________________________________________
Ray Abassi, P.E. Date: 07/23/2018
California P.E. Number: 48091
Atlantic Housing Development, City of Lynwood, CA Hydrology Study Report
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TABLE OF CONTENTS
1.0 Introduction ........................................................................................................ 5
2.0 Rainfall ................................................................................................................. 6
3.0 Methodology and Design Parameters .......................................................... 7
4.0 Existing Drainage Systems/Network ............................................................... 9
5.0 Hydrology/Hydraulics Analysis ...................................................................... 10
6.0 Conclusion ....................................................................................................... 11
7.0 Appendices ..................................................................................................... 12
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Figures
Figure 1: Project Site ................................................................................................. 6
Figure 2: Project Location ...................................................................................... 7
Figure 3: Rainfall Data ............................................................................................ 8
Figure 4: Existing Strom Drain System per LACFCD ............................................ 9
Tables
Table 1: Tc Data Input Summary .............................................................................. 5
Table 2: Peak Intensity Hydrologic Analysis for Atlantic Housing
Development ........................................................................................................... 6
Table 3: Peak Flow Hydrologic Analysis for Atlantic Housing Development
.................................................................................................................................. 10
Table 4: Summary of HydroCalc Outputs ......................................................... 11
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Appendences
Appendix A: HydroCalc Peak Flow Hydrologic Analysis ...................................... 13
Appendix B: South Gate 50-Year 24-Hour Isohyet Hydrologic Map ............. 17
Appendix C: FEMA/FIRM Maps ........................................................................... 19
Appendix D: Soil Identification Table ................................................................. 22
Appendix E: Runoff Coefficient Curve Soil Type No. 006 ............................... 24
Appendix F: Proportion Impervious Data .......................................................... 26
Appendix G: City of Lynwood Zoning Map ...................................................... 28
Appendix H: Storm Drain As-Built Drawing No. 275-543-D3.11 ....................... 30
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ABBREVIATIONS/ACRONYMS
A Area of the Basin
Ar Flow Area
B Curb Opening
C composite Run-off Coefficient
I Rainfall
LACFCD Los Angeles County Flood Control District
m Cross-Section Slope
N Manning’s Roughness Coefficient
OS Open Space
Q Peak Discharge Rate
R Hydraulic Radius
RCB Reinforced Concrete Box
S Channel Slope
T Gutter Width
Y Curb Height
Yg Gutter Height
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1.Introduction
The Project consists of multiple properties located on the west side of Atlantic
Avenue between Agnes Avenue and Lavinia Avenue. The site is comprised of ten
parcels and is approximately 38,350 square feet in area. The purpose of the
development is to address the need for affordable low-income housing within the
City of Lynwood. The proposed Affordable four-story Multi-Family Housing
Development Project includes the development of 70 multi-family units of which
66 will be affordable. The entire project is built within multiple properties acquired
by the City of Lynwood in the years 2005 and 2006. The existing site is unoccupied
with the current zoning being labeled as Open Space (OS). The proposed
development will be a 70-unit apartment complex and shall be re-zoned as
Multifamily (R-3) upon the development’s completion. The percent impervious is
expected to increase due to the proposed development to 86% according to
code 1125 in the LA County Hydrology Manual.
The proposed drainage and roof runoff will need to be collected and treated
through effective uses of BMPs such as a series of Flow-Through Planters. The
project site is located on the Los Angeles County Hydrologic Map 1-H1.9
(Appendix B). The map designates the site to have a Soil Classification of 006, with
a 50-year 24-hour isohyet of 6.2 inches of rainfall. The 25-year and 50-year
reduction factors are 0.878 and 1.000, respectively. The following table
summarizes data that was used to run the time of concentration (Tc) calculations.
Soil Type 006
24-Hour Rainfall Depth (25-year) 5.4436 in
24-Hour Rainfall Depth (50-year) 6.2000 in
Table 1: Tc Data Input Summary
The purpose of this Hydrology Study is to analyze and compare the existing and
proposed drainage patterns of the Atlantic Housing Development site in the City
of Lynwood, California. The hydrology calculations in this study are based on the
County of Los Angeles Hydrology Manual.
Infrastructure Engineers is responsible for the preparation of the Hydrology and
Hydraulic analysis report. This report is to provide a detailed drainage analysis
based on the proposed development. Figures 1 and 2 depict the Project Site and
the Project Location.
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Figure 1: Project Site
2.0 Rainfall Intensity
In accordance with the LACFCD GIS database, the 50-year rainfall amount in the
project area is estimated to be 6.2 inches, see Figure 3, Rainfall Data. LACFCD
recommends the 50-year storm frequency rainfall intensity for Capital Flood
protection. Peak rainfall Intensities were calculated through LA County’s
HydroCalc for both 25-year and 50-year design storm frequencies which are
shown in Table 2 below.
Storm Event
Pre-Developed Post-Developed
I (In/hr.) I (in/hr.)
25-year 2.7727 2.9811
50-year 3.3953 3.3953
Table 2: Peak Intensity Hydrologic Analysis for Atlantic Housing Development
Project Site
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Figure 2: Project Location
3.0 Methodology and Design Parameters
In accordance with the LACFCD Hydrology Manual, the Modified Rational
Method will be utilized for the hydrology analysis to determine site storm
discharges as the drainage area totals to less than 40 acres.
1. The LACDPW HydroCalc Calculator Program was used to determine Tc,
peak run-off flow rate, and run-off volumes for the subarea.
2. The property is located in the “South Gate” rainfall region, per Figure 1-H1.9
of the Hydrology Manual. 6.2 inches of rainfall was used for the 50-year
storm events while 5.4 inches of rainfall was used for the 25-year storm
events.
3. The site is in the soil classification “006” per figure 1-H1.9 of the Hydrology
Manual.
4. The site was analyzed for 25-year and 50-year storm events. Copies of the
Tc calculator output can be found in Appendix A of this report. The peak
Project Boundary
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runoff values have also been incorporated into the included hydrology
exhibits, which are made a part of this study.
5. On-site storm drains will be sized to convey the 50-year storm per Section
4.2 of the Hydrology Manual.
6. The existing site condition is unpaved and zoned as Open Space which is
10% impervious according to Appendix D of the Los Angeles Hydrology
Manual. The proposed condition will be Medium-Rise Apartments and
Condominiums, which is 86% impervious according to the Manual.
7. The project site is located within Flood Zone X (shaded) as indicated in
Flood Insurance Map 06037C1815F Panels 1815 of 2350, revised 9/26/2008.
8. Detention Calculations were determined by the HydroCalc output data for
the post developed 50-year storm event. Refer to Appendix A of this report
for additional information regarding the detention parameters.
9. Offsite systems currently in place and employed by the existing condition
will continue to accept flow from the site.
Figure 3: Rainfall Data
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4.0 Existing Drainage Systems/Network
In the vicinity of the project area, there is one existing primary storm drain network
which belongs to the LACFCD storm drainage system. The locations of the
drainage pipe system and catch basins were determined by a field visit and the
LACFCD record drawings. Since there will be impacts from the proposed
development on the existing drainage patterns, onsite drainage improvements
must be proposed. The site of the planned development is surrounded by
residential properties and recreational park areas. The project site is located on
the westerly side of the Los Angeles River, which is where the surrounding storm
drain network drains to. An existing catch basin is located on Atlantic Ave. in the
right-of-way of the proposed project’s South-East corner.
The project site is approximately 0.88 acres with approximately 10% being
impervious according to LA County’s proportion impervious data and the City’s
zoning map. The surface runoff from the site sheet-flows south towards the curb
inlets at the corner of Atlantic Ave. and Lavinia Ave. Drainage areas tributary to
Atlantic Avenue are collected and conveyed within an existing 7’-3”X5’-0” RCB
(reinforced concrete box) which flows in the southernly direction, crossing
Lynwood Gardens and discharges into the Los Angeles River.
Figure 4: Existing Storm Drain System per LACFCD
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5.0 Hydrology/Hydraulic Analysis
The Capital Flood is the runoff produced by a 50-year frequency design storm
falling on a saturated watershed (soil moisture at field capacity). A 50-year
frequency design storm has a probability of 1/50 of being equaled or exceeded
in any year. The Capital Flood level of protection applies to all facilities
constructed to drain natural depressions or sumps. These facilities include
channels, closed conduits, retention basins, detention basins, pump stations, and
highway underpasses.
The pre-construction peak runoff rate and the runoff volume for the 50-year, 24-
hour storm event are calculated to be 2.55 cfs and 5,671 cu-ft, respectively. The
hydrology calculations for the existing condition are summarized in Table 3 below.
Complete calculations are shown in Appendix A
The post-construction peak runoff rate and the runoff volume for the 50-year, 24-
hour storm event are calculated to be 2.67 cfs and 15,810 cu-ft, respectively. The
hydrology calculations for the existing condition are summarized in Table 3 below.
Complete calculations are shown in Appendix A
The peak runoff rate will increase due to an increase in impervious area (10%
under the existing condition vs. 86% under the proposed condition). Therefore, the
impact of the proposed development on the downstream storm drain
infrastructure would be affected. However, all flows in the proposed drainage
area ultimately converge with an underground City of Lynwood 7’-3”X5’-0” RCB
storm drain system (Dwg. No.275-543-D3.11dated November 1962) which should
be sufficient to manage the additional flow rate.
Area
(Acres)
Existing or
Proposed
%
Impervious
Design
Storm
Frequency
Time of
Concentration
(min)
Peak
Flow
Rate
(cfs)
24-Hr
Runoff
Volume
(cu-ft)
0.88 Existing 10 25-year 7.0 1.99 4,712
0.88 Proposed 86 25-year 6.0 2.33 13,840
0.88 Existing 10 50-year 6.0 2.55 5,671
0.88 Proposed 86 50-Year 6.0 2.67 15,810
Table 3: Peak Flow Hydrologic Analysis for Atlantic Housing Development
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6.0 Conclusion/Recommendation
A hydrology/hydraulic analysis is a mandatory task in the design of any proposed
project in the City of Lynwood and other associated agencies affected by the
construction of any project. All design calculations were in conformance with the
LACFCD hydrology and hydraulics manual.
Storm Event
Pre-Developed Post-Developed
Q (cfs) Q (cfs)
25-year 1.99 2.33
50-year 2.55 2.67
Table 4: Summary of HydroCalc outputs
In summary, based on the Hydrology calculations presented the site currently in
its predeveloped condition generates 2.55 cfs peak flow based on a 50-year
storm. After applying developed site conditions, the net result of the peak flow
became 2.67 cfs based on a 50-year design storm. Therefore, the design engineer
of the site must propose a drainage facility that will attenuate the difference
between the post-development peak flow and the pre-development peak flow
because any proposed discharge at the offsite must be at or below the
predevelopment peak discharge rate for the design storm. As a part of the site
design, the designer shall incorporate NPDES/SUSMP regulations including water
quality requirements as part of the mandated requirements to prohibit storm
water discharges leaving the site and to minimize the discharge of pollutants.
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APPENDIX A
HydroCalc
Peak Flow Hydrologic Analysis
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APPENDIX B
South Gate
50-Year 24-Hour Isohyet Hydrologic Map
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APPENDIX C
FEMA/FIRM Maps
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Figure 6: FEMA Map
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.
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APPENDIX D
Soil Identification Table
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APPENDIX E
Runoff Coefficient Curve
Soil Type No. 006
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Runoff Coefficient Curve Soil Type No. 006
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APPENDIX F
Proportion Impervious Data
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APPENDIX G
City of Lynwood Zoning Map
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Figure 9: City of Lynwood Zoning Map
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APPENDIX H
Storm Drain As-Built Drawing
No. 275-543-D3.11
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