hydrologic and hydraulic analyses for fema leve

7/23/2019 HYDROLOGIC AND HYDRAULIC ANALYSES FOR FEMA LEVE

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21st Century Dam Design

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011


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On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the regions

imported water supplies.The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

United States and tallest roller compacted concrete dam raise in the world.

The information contained in this publication regarding commercial projects or firms may not be used for

advertising or promotional purposes and may not be construed as an endorsement of any product or

from by the United States Society on Dams. USSD accepts no responsibility for the statements made

or the opinions expressed in this publication.

Copyright 2011 U.S. Society on Dams

Printed in the United States of America

Library of Congress Control Number: 2011924673ISBN 978-1-884575-52-5

U.S. Society on Dams

1616 Seventeenth Street, #483

Denver, CO 80202

Telephone: 303-628-5430

Fax: 303-628-5431

E-mail: [email protected]

Internet: www.ussdams.org

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Vision

To be the nation's leading organization of professionals dedicated to advancing the role of dams

for the benefit of society.

MissionUSSD is dedicated to:

Advancing the knowledge of dam engineering, construction, planning, operation,

performance, rehabilitation, decommissioning, maintenance, security and safety;

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resources systems;

Providing public awareness of the role of dams in the management of the nation's water

resources;

Enhancing practices to meet current and future challenges on dams; and

Representing the United States as an active member of the International Commission onLarge Dams (ICOLD).


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FEMA Levee Certification 1247

HYDROLOGIC AND HYDRAULIC ANALYSES FOR FEMA LEVEE

CERTIFICATION IN SAN BERNARDINO COUNTY, CALIFORNIA

Daniela Todesco, P.E.1

Dragoslav Stefanovic, P.E., Ph.D.2

Darren Bertrand

3

Mark Seits, P.E.4

Yunjing Zhang, P.E.5

ABSTRACT

This paper summarizes hydrologic and hydraulic analyses performed for the SanBernardino County Levee Certification Project (Project) and the challenges encountered

during the course of the study. The Project included forty-three non-Federal levees on

the national levee inventory list, currently mapped by FEMA as Provisionally AccreditedLevees (PALs). WEST Consultants performed detailed hydrologic/hydraulic analyses for

thirty-five Category 2 levees in order to determine whether they are in compliance withFEMAs regulatory requirements for certification as identified in Title 44 of the Code of

Federal Regulations, Section 65.10 (44 CFR 65.10). In most cases, a completehydrologic model (HEC-HMS) was built for the contributing watershed to

determine/verify the 100-year base flood. Detailed hydraulic models (HEC-RAS) were

built for the majority of the levees, some of them with split flow components, lateral spillstructures, and unsteady routing. Each levee presented its own challenge in terms of data

availability and modeling approach. The results of the hydrologic and hydraulic analyses

were eventually used to verify levee freeboard, evaluate sedimentation and potentialscour problems at the levee toes, and address any interior flooding issues.

INTRODUCTION

WEST Consultants, Inc. (WEST) has conducted hydrologic, hydraulic, and scouranalyses for the San Bernardino County Levee Certification Project (Project) in

cooperation with HDR Engineering, Inc. (HDR) and the San Bernardino County Flood

Control District (SBCFCD). The Project includes forty-three non-Federal levees on the

national levee inventory list, currently mapped by the Federal Emergency ManagementAgency (FEMA) as Provisionally Accredited Levees (PALs). The Project was divided

into three phases. Phase I classified levees into three categories based on their

certification status: Category 1 - levees meet 44 CFR 65.10 and all data and completedocumentation for certification is available; Category 2 levees may meet 44 CFR 65.10,

but additional data or documentation is needed; Category 3 levees do not currently meet

44 CFR 65.10 without remediation measures. Category 3 also includes structures that arenot classified as levees (such as side ditches) or levees that are entrenched (i.e. when

1Senior Engineer, WEST Consultants, Inc., 503-946-8536, [email protected] Manager, WEST Consultants, Inc., 858-487-9378, [email protected], WEST Consultants, Inc., 858-487-9378,[email protected] Manager, HDR, 858-712-8312, [email protected] Engineer, HDR, 858-712-8355, Vicky.Zhang @hdrinc.com


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21st Century Dam Design Advances and Adaptations1248

the landside toe of the levee is above the 100-year water surface elevation). Ten levees

were found to be Uncertifiable (Category 3) and nineteen did not meet the FEMAsdefinition of a levee (part of Category 3 as well). The remaining fourteen levees had

adequate freeboard, but additional geotechnical analyses are needed to confirm that they

meet stability and/or seepage requirements (and therefore are part of Category 2). If a

levee is deemed certifiable, FEMA will remove the PAL status on the Flood InsuranceRate Map (FIRM). If the levee is not certifiable, the area behind the levee will be

mapped as Special Flood Hazard Area (SFHA). During Phase II, WEST closely worked

with SBCFCD to collect topographic, hydrologic, and hydraulic information for alllevees in order to determine whether they are in compliance with the regulations 44 CFR

65.10. The study results aided in preparing a certification package (which included

hydrologic, hydraulic, and geotechnical analyses, as-built plans, and Operation andMaintenance manual) for the levees found in compliance. For those levees not in

compliance, San Bernardino County (County) will determine whether it is feasible to

upgrade the levees into compliance and/or provide an outreach program (especiallyregarding flood insurance) for the residents and business owners behind the levees. The

County will upgrade the selected levees during Phase III (design and construction).Table 1 presents a summary of the levees analyzed during the Project.

Table 1. List of Levees and Methods.

LEVEE

/SYSTEMHYDROLOGIC ANALYSIS

HYDRAULIC

ANALYSIS

Donnell Basin

(86a, 86b, 86c,

87)

FEMA flow available. SBCFCD

flow used and verified performing

Unit Hydrograph (UH) analysis

Steady HEC-RAS

model

Day Creek

(107b)

FEMA flow not available. Flow from

UH analysis

Steady HEC-RAS

model

Ely Basins

(29a, 29b, 29c)FEMA flow available and used

Steady HEC-RAS

model. Split flow

Cable Creek

(61,69)

FEMA flow not available. Base

Flood Elevations (BFEs) available.SBCFCD flow used

Steady HEC-RAS

model

Mill Basin (59) FEMA flow available and usedSteady HEC-RAS

model

Patton Basin

(12)

FEMA flow available. SBCFCDflow available. Flow from UH

analysis

Steady HEC-RASmodel

East Badger

Basin (57a)

FEMA flow available and verifiedperforming UH analysis

Steady HEC-RASmodel

Daley Basin

(58)

FEMA flow not available. Flow fromUH analysis

Steady HEC-RASmodel

MacQuiddy-

Severance

Channel (57b)

FEMA flow available. Flow from

UH analysis

Steady HEC-RAS

model

Macy Basin

(3)

FEMA flow not available. SBCFCD

flow used

Broad crested weir

calculations

Reche Canyon

(7)


flow used

Steady HEC-RAS

model


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LEVEE

/SYSTEMHYDROLOGIC ANALYSIS

HYDRAULIC

ANALYSIS

South Badger

Basin (63)

FEMA flow not available. BFEsavailable. Flow from UH analysis

Steady HEC-RASmodel

29th StreetBasins (81a) FEMA flow not available. Flow fromSBCFCD used Steady HEC-RASModel

Lynwood

Basins (55, 56,

81b)


flow available. Flow from Rational

Method analysis

Steady HEC-RAS

model

Plunge Creek

(13a, 13c, 14)

FEMA flow not available. Flow fromSBCFCD

Steady HEC-RASmodel

Wilson Basins

(90)


UH analysis

Steady and

unsteady RASmodels

Mojave River

(88,100,118)FEMA flow used

Steady HEC-RAS

model

Mojave River

(1a/104)

FEMA flow usedSteady HEC-RAS

modelMojave River

(98)FEMA flow used

Steady HEC-RASmodel

Chris Basin

(28c)

FEMA flow not available. SBCFCDflow used

Steady HEC-RASmodel

Banana Basin

(103)


flow used

Steady HEC-RAS

model with splitflow and lateral

structures

Rich Basin

(93, 94)


flow available. Flow from UH

analysis

Steady HEC-RAS

model

Devil Creek

(79)


UH analysis

Steady HEC-RAS

modelMill Creek

(44b)


flow used

Steady HEC-RAS

model

Santa Ana

River (111)


flow used

Steady HEC-RAS

model

San Sevaine

Basins (34)


UH analysis

Steady HEC-RAS

model

City Creek

(20)FEMA flow used

Steady HEC-RAS

model

DATA COLLECTION

The layout and location of the levees, as well as the flooding sources, are shown in theFIRM Panels for San Bernardino County. Discharge information, as well as flood

profiles and Base Flood Elevations (BFEs) when available, were found in the FloodInsurance Study (FIS) for San Bernardino County (FEMA, 2008). Additional flow

information was provided by SBCFCD in the form of hydrologic studies, as-built plans,

master plan of drainage for the study area, comprehensive storm drain plans, or hand

calculations.


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Digital Elevation Models (DEMs) for the contributing watersheds were downloaded from

the U.S. Geological Service (USGS) National Map Seamless Server(http://seamless.usgs.gov/) as National Elevation Dataset (NED) in digital format.

Survey data for the levees and/or basins were provided by David Evans & Associates

(DEA) in digital format. SBCFCD also provided as-built plans for the levees and nearby

basins and, when available, aerial survey data. Additional topographic data in thevicinity of some levees were obtained from Intermap Technologies, Inc. in digital format.

HYDROLOGIC ANALYSES

In some cases, the hydrologic data found in FEMA FIS studies or provided by the

SBCFCD were acceptable such that further hydrologic analysis was not deemednecessary. In many cases (where no flow information was available or the existing

information was questionable), a complete hydrologic model was built for the watershed

contributing to the basin/levee to determine the 100-year (base) flow. The modelingapproach followed the SBCFCD Hydrology Manual (1986) procedures for the Rational

Method (for Lynwood Basin No.1) or the Unit Hydrograph Method (for all other cases).

When the Unit Hydrograph Method was used, the contributing subbasins were delineatedusing the ArcGIS Hydrologic Engineer Center (HEC) Geospatial Hydrologic Modeling

Extension (GeoHMS) based on the USGS DEM. Gridded precipitation data was

obtained from an online version of the National Oceanic and Atmospheric Administration(NOAA) Atlas 14 (http://hdsc.nws.noaa.gov) and an average precipitation value for each

subbasin was calculated. NOAAs 25-year precipitation data was used to estimate the

100-year peak flow. The reason for using the 25-year rainfall rather than the 100-yearrainfall is because the SBCFCD found that NOAAs 25-year precipitation data at the 85-

percent confidence level corresponds to FEMAs 100-year 50-percent confidence levelfor estimating the 100-year peak flow. The lag was estimated for each subbasin based on

stream lengths, centroidal stream lengths, and drainage slopes using the empirical

formula presented in the Manual. Hydrologic soil maps were downloaded from theNational Resources Conservation Service (NRCS). The soil cover was estimated based

on aerial photographs and the 2001 National Land Cover Database (NLCD) data. The

percent impervious for each subbasin was determined based on the 2001 NLCD Percent

Impervious data and aerial maps. Curve numbers and maximum loss rates (calculated foreach soil group and land cover type) were based on the Hydrology Manual. Antecedent

Moisture Conditions (AMC) II were generally assumed. Initial abstractions were omitted

for conservative purposes. S-graphs based on the watershed location and conditions(Desert, Valley, Mountain, or Foothill) were chosen per the Manual guidelines.

Figure 1 and Figure 2 show two characteristic examples of drainage area delineation.The watershed contributing to Wilson Basins is fairly undeveloped and the computed

HEC-GeoHMS watershed boundaries were found to be acceptable. In the case of Patton

Basin, the storm drain master plan contributing areas needed to be incorporated into thepreliminary subbasin boundaries generated by HEC-GeoHMS to correctly identify the

flow contributing area.


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Figure 1. Wilson Basins Watershed Map.

Figure 2. Patton Basin Watershed Map.

Wilson

Basins

Patton

Basin


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For flow routing purposes, either the Lag or the Muskingum-Cunge methods were

chosen. The velocity, cross section, slope, and roughness data were obtained from theHEC River Analysis System (RAS) hydraulic model of the analyzed stream. The input

data were entered into the HEC Hydrologic Modeling System (HMS) model (Version

3.3) to obtain the 100-year peak discharges at key locations. In cases where the system

was composed of several basins in series (such as Badger Basins and Lynwood Basins),the basins were incorporated into the HEC-HMS model to account for routing effects.

The elevation-discharge curves for the basins were obtained from the HEC-RAS model

of the system, while the elevation-area data was obtained from ArcGIS. The HEC-HMSmodel was also used to evaluate the storage capacity of the basins for different

configurations of gates and/or culverts present in the system.

HYDRAULIC ANALYSES

HEC-RAS steady-state models were built for most of the levees analyzed. Some leveesrequired more complicated split-flow models (e.g., Ely Basins), lateral spill structures

(e.g., Banana Basin), and unsteady flow models (e.g., Wilson Basins). One levee (MacyBasin) required only simple weir calculations to determine the available freeboard. Table

1 contains a summary of the hydraulic analyses performed for each system. Threesystems (Ely Basins, Wilson Basins, and Banana Basin) were chosen as characteristic

examples in this paper.

WEST utilized the ArcGIS program (Version 9.1) to extract the cross section profiles

from the survey data in order to develop the hydraulic model of each stream/basin. Cross

sections were cut from the Triangular Irregular Network (TIN) generated from thetopographic data and then imported into HEC-RAS, Version 4.0. Model parameters such

as roughness values, inline/lateral structure data, discharge coefficients, ineffective flowlimits, and levee locations were obtained from field reconnaissance and/or as-built plans.

A subcritical flow regime was set for all hydraulic analyses. The downstream boundary

condition was set at the Base Flood Elevation (BFE) obtained from the current FIS whenavailable, or set as the normal depth. Levees were introduced in the model cross sections

in order to evaluate available freeboard or to identify entrenchment (entrenchment refers

to the channel condition where the landside toe of the levee is above the 100-year water

surface elevation). The landside toe elevation was selected as the lowest groundelevation within approximately 50 feet of the levee landside hinge point.

Ely Basins

Ely Basins are located near the intersection of South Vineyard Avenue and Pomona

Freeway in Ontario, California. The three basins in a series are connected by boxculverts and spillways. The inflow comes from the West Cucamonga Channel at the

northwest corner of the first basin. The most downstream basin (Basin 3) features a

lateral spillway and an outlet box culvert which discharge into two separate rectangularconcrete channels that merge about 700 ft downstream of the outlet box. Figure 3 shows

a layout of the outlet and spillway configuration for Basin 3. Because the lateral spillway

for Basin 3 and the box culvert under Philadelphia Street were modeled as separate


9/18


reaches, a junction was introduced to connect the main reach (labeled Ely Basins in the

model) with the lateral reach (labeled Spillway in the model).

Figure 3. Ely Basins HEC-RAS Model Layout (Basin 3 and Outlet Channel).

The basins are leveed along three sides (eastern, western, and southern). The northernside of the basins was found to be entrenched because the terrain behind them is higher

than the water surface elevation in the basins. Based on the hydraulic analysis, the 100-

year flow is contained within the basins providing a minimum freeboard of 3 ft above the

computed water surface elevations (4 feet within 100 ft of structures or constrictions,tapering to 3.5 ft at the upstream end of the levees), therefore satisfying FEMAs

regulatory requirements for certification.

Banana Basin

Banana Basin is located near the intersection of Foothill Boulevard and Interstate 15 in

the City of Fontana, California. The basin is situated just north of the California

Speedway and the Burlington Northern Santa Fe (formerly Atchison, Topeka & Santa Fe)

Railway, between Cherry Avenue and Etiwanda Avenue. The major source of inflow isthe West Fontana Channel. Additional inflow comes from three storm drain outlets

located on the north side of the basin, collecting flow from the residential andcommercial areas to the north. The basin outlet structure on the western side features a100 feet wide concrete spillway. Banana Basin is leveed along the southern, eastern, and

northern sides.

WEST evaluated the capacity of the West Fontana Channel (whose nominal flow is 5,542

cfs) upstream of Banana Basin using theHydraulic Design Uniform Flowmodule in

HEC-RAS. The maximum flow that the channel is able to convey without spilling into

Vineyard Avenue Culverts

Ely Basins Reach

Spillway Reach

Philadelphia Street


10/18


the overbanks is around 400 cfs. Flow spilling in the left (south) overbank would run

west towards Banana Basin in the low area between the West Fontana Channel and therailroad tracks (just south of the basin levee, see Figure 4), potentially affecting the

landward toe of the levee.

Figure 4. Schematic of Split Flow for Banana Basin.

A spilt flow reach (see Figure 4) was created parallel to the Banana Basin reach to

evaluate the hydraulics of the area. Several lateral structures were added to the model to

connect the two reaches and to simulate flow spilling in the overbanks. A layout of the

HEC-RAS model with cross section locations is shown in Figure 5 and Figure 6.

Banana Basin Reach

(West Fontana Channel)

Split ReachRailroad

Tracks

Flow

Spilling


11/18


Figure 5. HEC-RAS Model Layout (Upstream of Banana Basin).

Figure 6. HEC-RAS Model Layout (Banana Basin and Downstream of the Basin).

WEST assumed that the total inflow of 5,542 cfs is reaching the West Fontana Channeldownstream of Cherry Avenue. A series of lateral weirs were added (from Sta. 4493 to

Sta. 3723) to evaluate how much of this flow would spill into the left overbank and leave

the system downstream of Cherry Avenue.

Cherry

Avenue

Calabash

Avenue


12/18


The split reach starts about 1,000 ft downstream of Cherry Avenue, where the topography

shows the beginning of a defined depression sloping west towards the basin. The reachends 600 ft upstream of Calabash Avenue, where the SBCFCD 2001 topographic data

ends. The reach rejoins the West Fontana Channel just upstream of Calabash Avenue via

three pipe culverts.

The 100-year peak flow of 5,542 cfs was used in the HEC-RAS model at the upstream

boundary of the Banana Basin reach (Sta. 4493). The flow was increased by 176 cfs at

Sta. 2419 to account for additional inflow from the local storm drain system. Anadditional increase of 825 cfs (reaching a total flow of 6,543 cfs) was applied to Sta. 872

to account for the contributing flow from the split reach.

Based on topographic and aerial data, WEST estimated that flow from the lateral

structures at Sta. 4492 and Sta. 4012 would run south and would not rejoin the main flow

from the West Fontana Channel. Out of 5,542 cfs reaching the West Fontana Channeldownstream of Cherry Avenue, only 418 cfs remains in the channel (equal to the current

capacity of the channel) and ultimately reaches Banana Basin. The majority of the inflow(4,297 cfs) would spill over the left channel bank towards the railroad and would not

return into the system. A portion of the flow (827 cfs) would flank the levee on its southside. Table 2 shows a summary of the computed flows for the system.

Table 2. Summary of Flows for Banana Basin.

Location Flow (cfs)

Flow at the upstream end of West Fontana Channel 5,542

Flow in West Fontana Channel reaching Banana Basin 418

Flow in Banana Basin from storm drain system 176

Flow in Split Channel behind the levee 827Flow spilled south (leaving the system) 4,297

Flow in Banana Basin (including storm drains) 594

The HEC-RAS model showed that the portion of the south levee that surrounds BananaBasin (from Sta. 2475 to Sta. 1515) and the entire north levee are entrenched. The model

also showed that the 100-year flow currently reaching Banana Basin is contained withinthe basin, providing minimum freeboard of 3 feet above the computed water surface

elevations for the south levee. The flow depths in the split channel (behind the levee)

vary from 1.2 ft to 3.8 ft, while velocities range from 3.8 ft/s to 8.1 ft/s. The levee

sections upstream and downstream of the basin were not certified.

Wilson Basins

Wilson Basins are a series of five recharge basins connected by culverts (the fifth basin is

a spreading grounds area). The basins are located adjacent to Wilson Creek in the City of

Yucaipa (see Figure 7 ). They are bounded by Fremont Street on the eastern, BryantStreet on the western, and by a residential development on the northern and southern


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sides. The basins are leveed on the north side (north levee, red line) and on the south

side (south levee, blue line).

Figure 7. Wilson Basins Layout.

The five basins are connected by arch culverts and reinforced concrete pipes (RCPs).

The spreading grounds are connected to Wilson Creek by a corrugated metal pipe (CMP).

Wilson Creek flows under Bryant Street in a 12-ft by 10-ft reinforced concrete box(RCB). The system also features 3 concrete pipes with flap gates that allow flow from

Wilson Creek to discharge into Basins No. 2, 3, and 4. The crown elevation of each inlinelevee (that separates the basins) is constant (the levees are horizontal). The south leveeand north levee slope downstream, connecting the inline levees.

Due to the size of the basins and the absence of uncontrolled spillways between them, the

effect of routing needed to be taken into account to correctly model the maximum watersurface elevations in the basins and corresponding levee freeboard. Therefore, an

unsteady flow model (HEC-RAS, Version 4.0) was developed to perform hydraulic

routing (flood propagation) in the basins along the levees, as well as to quantify thediversion of flow from Wilson Creek into the basins. The model is comprised of two

reaches: the Wilson Basins reach and the Wilson Creek reach (see Figure 8).

Fremont

Street

Bryant

Street

Wilson

CreekBasin

No. 1

Basin

No. 2Basin

No. 3

Basin

No. 4

Basin No 5

- Spreading

Grounds

InletStructure to

Basins


14/18


Figure 8. HEC-RAS Model Overview of Wilson Basins.

The Wilson Creek reach starts about 250 ft upstream of Fremont Street and it ends about

400 ft downstream of Bryant Street. The Wilson Basins reach starts at the inlet structurefrom Wilson Creek (connecting with the Wilson Basins reach by means of a lateral

structure) and it ends just downstream of Bryant Street. The two reaches reconnect at

Bryant Street by a lateral culvert. The model features a series of inline and lateralstructures. The creek is connected with the basins at the downstream ends of Basin No.

2, Basin No. 3, and Basin No. 4 by 3-ft RCPs with flap gates that only allow flow from

Wilson Creek into Wilson Basins (reversed flow is prevented).

The four basins (No. 1 through No. 4) were modeled using cross section geometry and

inline structures. The spreading grounds (Basin No. 5) were modeled using a set of inlineweirs. The spreading grounds are connected to Wilson Creek by a circular pipe in the

northwestern corner (the pipe was represented by a lateral structure just upstream of

Bryant Street). It was assumed that this pipe was functional to ensure model stability.

However, an intentionally high Mannings ncoefficient of 0.5 was specified inside thepipe to suppress its flow because the pipe was found filled with sediment during field

reconnaissance. The most downstream portion of the spreading grounds was modeled as

a bridge with a very small culvert opening (0.1 feet in diameter).

To promote model convergence, the downstream boundary condition for the Wilson

Basins reach was set at a low constant elevation representing sheet flow towards BryantStreet. This condition did not affect the computed water surface elevations either in the

spreading grounds or the upstream basins. The upstream boundary condition in the

Wilson Basins reach was an inflow of 5 cfs to add computational stability during low

flows.

Wilson Creek

reach

Wilson

Basins reach


15/18


The unsteady model experienced difficulties during low flows. This was overcome by

creating a hot-start file with an artificially high initial water surface elevation at thedownstream boundary of the Wilson Creek reach. As the simulation progressed, the

boundary water surface elevation was gradually reduced to obtain a stable initial

condition (hot-start).

It was found that only 710 cfs (out of 4,250 cfs in Wilson) is diverted into the basins

during the 100-year flood. Results from the unsteady HEC-RAS model of the Wilson

Basins reach show that this portion of the total 100-year flow is contained within thebasins providing freeboard of minimum 3 ft for the entire south levee. Therefore, the

Wilson Basins south levee in Basins No. 1, 2, 3, and 4 satisfies FEMAs regulatory

requirements. The model also showed overtopping of Bryant Street during the 100-yearflood event. Sufficient freeboard was not found available near the downstream end of the

spreading grounds (Basin No. 5), which therefore does not qualify for certification.

Figure 9 shows the maximum water surface elevation in the basins during the unsteadyflow simulation.

0 500 1000 1500 2000 2500

2780

2800

2820

2840

2860

2880

2900

2920

Wilson Basins Plan: Unsteady Flow 100-yr Hydrograph

Main Channel Distance (ft)

Elevation

(ft)

Legend

WS Max WS

Crit Max WS

Ground

Left Levee

59

132

200

250

300

370

450

500

550

650

759

869

945

1003

1100

1272

1405

1486

1600

1746

1807

1869

1939

2007

2100

2246

2335

2388

2468

2544

2615

Wilson Basins Basins

Figure 9. Maximum Water Surface Elevations in the Wilson Basins Reach.

It is important to note that all the inline culverts (through the inline levees) between thebasins were assumed fully functional in order to provide minimum freeboard. Therefore,

these culverts need to be inspected regularly to ensure their unobstructed operation.

Also, the inline levees were found necessary for the peak flow attenuation (routing)between the basins.


16/18


CONCLUSION

WEST Consultants, Inc. has conducted hydrologic, hydraulic, and scour analyses for the

San Bernardino County Levee Certification Project. Each levee presented its own

challenge in terms of data availability and study approach. A detailed hydrologic

analysis (using HEC-HMS) of the watershed contributing to each watercourse along thelevee was in most cases necessary to estimate the 100-year (base) flow and/or to verify

existing flow information. The hydraulic analyses (performed using HEC-RAS) were

important to verify levee freeboard or identify levees that are entrenched. The results ofthe hydrologic and hydraulic analyses were also used to evaluate sedimentation and

potential scour problems at the levee toes, address any interior flooding issues, and

provide necessary input for geotechnical stability analyses (depth and duration of flowagainst the levee). All the analyses were required to identify levees that are in

compliance with Title 44 of the Code of Federal Regulations, Section 65.10 in order to

remove their Provisionally Accredited Status (PAL). The levees that did not meetFEMAs regulations will be either repaired or decertified. The consequences of levee

decertification include mandatory flood insurance, floodplain management requirements,and change in property value and tax bases for properties behind the levee.

REFERENCES

Flood Emergency Management Agency (FEMA). Flood Insurance Study, SanBernardino County, California, and Incorporated Areas. January 17, 1997.

Hydrologic Engineering Center (HEC). Geospatial Hydrologic Modeling ExtensionHEC-GeoHMS Users Manual. Version 1.1, U.S. Army Corps of Engineers,

Hydrologic Engineering Center, Davis, California, December 2003.

Hydrologic Engineering Center (HEC). HEC-GeoRAS GIS Tools for support of HEC-

RAS using ArcGIS Users Manual. Version 4, U.S. Army Corps of Engineers,Hydrologic Engineering Center, Davis, California, September 2005.

Hydrologic Engineering Center (HEC). HEC-HMS Hydrologic Modeling System

Users Manual. Version 3.3, U.S. Army Corps of Engineers, Hydrologic EngineeringCenter, Davis, California, September 2008.

Hydrologic Engineering Center (HEC). HEC-RAS River Analysis System UsersManual. Version 4.0, U.S. Army Corps of Engineers, Hydrologic Engineering Center,

Davis, California, March 2008.

National Archives and Records Administration. Code of Federal Regulations, Title 44

Emergency Management and Assistance.Revised October 1, 2003.

http://www.access.gpo.gov/cgi-bin/cfrassemble.cgi?title=200344

San Bernardino County Flood Control District (SBCFCD).Hydrology Manual. August

1986.


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WEST Consultants (2009). San Bernardino County FEMA Levee Certification Non-

Federal Levees Phase II, Hydraulic Report, Banana Basin Levee ID 103,prepared forHDR Engineering, August 2009.

WEST Consultants (2009). San Bernardino County FEMA Levee Certification Non-

Federal Levees Phase II, Hydraulic Report, Ely Basins Levee IDs 29a, 29b, and 29c,prepared for HDR Engineering, August 2009.

WEST Consultants (2009). San Bernardino County FEMA Levee Certification Non-Federal Levees Phase II, Hydraulic Report, Wilson Basins Levee ID 90,prepared for

HDR Engineering, August 2009.


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hydrologic and hydraulic analyses for fema leve

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