HYDROLOGY STUDY REPORT - Lynwoodlynwood.ca.us/wp-content/uploads/2018/07/Hydrology-Study... ·...

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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14


15


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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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