CECW-EH-Y

Engineer 

Pamphlet

1110-2-10

Department of the Army

U.S. Army Corps of Engineers

Washington, DC 20314-1000

EP 1110-2-10

31 July 1994

Engineering and Design

HYDROLOGIC ENGINEERING ANALYSIS

CONCEPTS FOR COST-SHARED FLOOD

DAMAGE REDUCITON STUDIES

Distribution Restriction Statement

Approved for public release; distribution is 

unlimited.
EP 1110-2-10

31 July 94

Foreword

Hydrologic engineering is a civil enginting discipline involving the analysis of water and its systems as

it moves abve, on, through, and beneath the surface of the earth. Water is a critical and integral element in

planning and evaluating flood darnage reduction m~ures and wtions. For these studies, hydrologic engineers have

a major role in defining the flood h-d, and in locating, sizing, and assuring the functional and operational integrity

of the projeets.

This document deseribes the study processes performed by U.S. Army Corps of Engineers hydrologic

engineers for Fedd flood damage reduction studies. me objeetive is to enable Corps staff, the cost-s~

partners, and othm involved in the planning process to gain a &ter understanding of the hydrologic engineering

study scope, strategies, and methods of analysis. It is intended that with this better understanding, the study team

participants will more clearly define and grasp the choices available for the conduct of the hydrologic engineering

analysis and will reaeh a mutual agreement on the study req-ents.

This doctient is applicable to HQUSACE elements, major subordinate commands, dutricts, laboratories,

and field operating activities having civil works responsibilities.

FOR THE COMMANDER:

WIL~IAM D. BROWN

Colonel, Corps of Engineers

Chief of Staff
CECW-EH-Y

Pamphlet

No. 1110-2-10

DEPARTMENT OF THE ARMY

U.S. Army Corps Of Engineers

Washington, DC 20314-1000

EP 1110-2-10

31 July 1994

Engineering and Design

HYDROLOGIC ENGINEERING ANALYSIS CONCEPTS

FOR COST-SHARED FLOOD DAMAGE REDUCTION STUDIES

Table of Contents

Subject Paragraph

Chapter 1

Introduction

Purpose . . . . . . . . . . . . . . . . . ...1-1

Overview of Corps Flood Damage

Reduction Studies . . . . . . . . . . . . 1-2

Hydrologic Engineering . . . . . . . . . 1-3

Chapter 2

Data Needs and Hydrologic Processes

of Floods

Overview . . . . . . . . . . . . . . . . ...2-l

Precipitation Runoff Relationship . . . 2-2

Channel Characteristics . . . . . . . . . . 2-3

Flood Chmctenstics . . . . . . . . . . . 2-4

Frequency Analysis . . . . . . . . . . . . 2-5

Chapter 3

Flood Studies

Overview . . . . . . . . . . . . . . . . ...3-l

Definition of the Flood H~d . ...3-2

Formulation and Evaluation of

Solutions . . . . . . . . . . . . . . . ...3-3

Impact Assessment . . . . . . . . . . . . . 3-4

Chapter 4

Determining Flood Flows by

Frequency Methods

Overview . . . . . . . . . . . . . . . . ...4-l

Analysis for Gaged Areas . . . . . ...4-2

Page

1-1

1-1

1-2

2-1

2-2

2-6

2-7

2-8

Subject Paragraph

Simplified Analysis for Ungaged

Areas . . . . . . . . . . . . . . . . . . ...4-3

Detailed Analysis for Ungaged

Locations . . . . . . . . . . . . . . . . . . .4-4

Chapter 5

Determining Flood Flows by

Precipitation-Runoff Analysis Methods

Introduction . . . . . . . . . . . . . . . ...5-1

Watershed/Subbasin Delineation . . . . 5-2

Analysis Approaches . . . . . . . . . . . . 5-3

Precipitation/Runoff . . . . . . . . . . . . . 5-4

Routing Concepts . . . . . . . . . . . . . . . 5-5

Calibration of the Model . . . . . . . . . . 5-6

Verification of the Model . . . . . . . . . 5-7

Chapter 6

Determination of Flood Elevations

3-1 River Hydraulics . . . . . . . . . . . . . . .6-1

3-1 Development and Use of Rating

Relationships . . . . . . . . . . . . . . . . 6-2

3-3 St~dy-~ow River Hydraulics . . . . . . 6-3

3-4 Unsteady-Flow River Hydraulics . . . . 6-4

Multi-Dimensional River Hydraulics . 6-5

Mobile Boundary Hydraulics . . . . . . . 6-6

Use of Physical Models . . . . . . . . . . 6-7

Comparison of Flood Elevation

Determination Methods . . . . . . . . . 6-8

4-1

4-1

Page

4-2

4-4

5-1

5-1

5-1

5-1

5-7

5-9

5-9

6-1

6-1

6-2

6-4

6-7

6-9

6-10

6-10
EP 1110-2-10

31 Jul 94

Sutriect Paragraph

Chapter 7

Hydrologic Engineering Requirements

for Flood Damage Reduction Measures

Overview . . . . . . . . . . . . . . . . . . . 7-1

Without-Project Conditions . . . . . . . 7-2

Scrmning of Alternatives . . . . . . . . 7-3

Reservoirs . . . . . . . . . . . . . . . . . . .7-4

Local Protwtion Proj&ts . . . . . . . . 7-5

Nonstructural Measures . . . . . . . . . 7-6

Floodplain Management Studies

@MS) . . . . . . . . . . . . . . . . ...7-7

Hydrologic Analysis Requirements

Summary . . . . . . . . . . . . . . . ...7-8

Page

7-1

7-1

7-1

7-1

7-2

7-4

7-5

7-5

Subiecl Paragraph

Chapter 8

Hydrologic Engineering Studies

General . . . . . . . . . . . . . . . . . . . ...8-l

Study Design and Management . . . . . 8-2

Level of Detail/Completeness . . . . . . 8-3

Documentation and Reporting . . . . . . 8-4

Local Sponsor Coordination . . . . . . . 8-5

Summ~ . . . . . . . . . . . . . . . . . ...8-6

Appendix A

References . . . . . . . . . . . . . . . . . .

Page

8-1

8-1

8-1

8-2

8-2

8-2

A-1
EP 1110-2-10

31 Jul 94

Chapter 1

Introduction

1-1. Purpose

a. This publication describes study processes performed by U.S. Army Corps of Engineers (USACE)

hydrologic engineers for Federal flood damage reduction

projeck. The goal is to enable Corps staff and cost-share

partners to gain an understanding of hydrologic enginmring procedures, including the study scope, strategies, and

methods of analysis. With a common understanding, the

study team members can clearly define and grasp the

choices available for performing the hydrologic engineering study and arrive at a mutual agrwment on study

requirements.

b. Appropriate references to other pamphlew, manuals, documents, and texts are included if more detailed

explanations are desired. References shown throughout

this document may be found in Appendix A.

1-2. Applicability

This pamphlet applies to all HQUSACE elements, major

subordinate commands, districts, laboratories, and field

operating activities having civil works responsibilities.

1-3. Overview of Corps Flood Damage Reduction

Studies

The Corps undertakes studies of water and related land

resource problems from directives or authorizations issued

by Congress. Congressional authorities are contained in

public laws or in resolutions. Study authorizations are

either for specific studies or for standing program authorities, such as the continuing authorities program. The

focus of the studies is to determine whether a Federal

flood damage reduction project should be recommended

in accordance with ArrrIy policies. Corps studies for

planning, engineering, and design of flood damage reduction projects are predicated on these legislative requirements and institutional policies (ER 1105-2-100 and

EP 1105-2-10).

a. Planning studies.

(1) Project planning studies are conducted in two

phas~. The fwst phase, resulting in a reconnaissance

report, is fully funded by the Federal Government. It

normally requires 12 months for completion, determines if

there is a Federal interest (benefits of the projat exceed

the costs for at lwt one alternative) and if there is nonFederd support (a local sponsor willing to cost-share).

The hydrologic engineering analysis for the reconnaissance phase should establish existing condition hydrology

if adequate time and funding are available. If the

reconnaissance report is favotile, an Initial Project Management Plan is prepared to detail the time, cost, and

work schedule necessary to complete all facets of the

subsequent feasibility study. A Feasibility Cost-Sharing

Agreement is signed with the local (non-Federal) sponsor.

(2) The feasibility phase is cost-shared equatly

between the Federat Government and non-Federal sponsor. It may take up to 4 years to complete and results in

recommendations to Congress concerning Federal participation in reducing the flood problem identified in the

study. This report contains the detailed hydrologic analysis necessary to determine the severity of the existing

flood problem, and to evaluate the success of various

alternatives in alleviating the problem. Detailed economics, plan formulation, and a baseline cost estimate for the

recommended plan are also necessary in this phase. The

feasibility report typically recommends the project which

provides the maximum net benefits. The Project Management Plan is prepared late in the feasibility study to

determine time and funding requirements for the detailed

engineering design and construction phases following

feasibility. A positive recommendation for Federal p~icipation results in the project proceeding into preconstruction engineering and design (PED). me cost-sharing

requirements for the recommended project and items of

non-Federal sponsor cooperation are to be included in the

feasibilityy report. Additional information concerning

feasibility investigations is referenced in ER 1105-2-100

and ER 1110-2-1150.

b. PED studies.

(1) The PED phase continues design efforts following the feasibility study and encompasses the more

detailed construction planning and engineering necessary

for building the projec~ The major items resulting from

the PED phase are design memoranda and plans and

specifications.

(2) A design memorandum (DM) is the primary

document developed in the PED phase, Detailed engineering and design are documented daring preparation of

the DM leading to construction of the projmt. The DM

emphasis is on areas of structural analysis, soils testing

and exploration, real estate analyses, cost engineering, etc.

Where a projmt is large or has several major components,

more than one design memorandum may be necessary.

1-1
EP 1110-2-10

31 Jul 94

(3) Following completion and approval of a DM,

plans and specifications are prepared, which allow the

project to be bid and construct. For most projects, the

PED phase is ex~ted to be completed within 2 years.

(4) For projects where considerable time has elapsed

since completion of the feasibility report or where conditions have changed enough to require project reformulation, a general design memorandum (GDM) may be

necessary. The intent is to provide sufficient engineering

analysis during feasibility, along with prompt and continuous funding into the PED phase, to p~clude the need

for a general reevaluation report, The feasibility report,

rdong with the engineering appendices, should allow a

smooth progression through PED for most projects.

Additional information on the PED phase is referenced in

ER 1110-2-1150.

c. Construction engineering and design. Once preconstruction engineering and design are complete, any

remaining enginmring and design are accomplished concurrent with construction activities. This phase includes

the design memorandums and plans and sp~itications to

construct any remaining project components. Construction of each project component occurs after completion of

plans and specifications for that component.

d. Continuing authority studies. These standing study

and construction authorities are conducted in a two-phase

process. Additional information on continuing authority

studies is available in ER 1105-2-100.

1-4. Hydrologic Engineering

a, Hydrologic engineering is a critical technical element in the planning of flood damage reduction measm

and actions. It is a civil engineering discipline involving

the analysis of water and its systems as it moves above,

on, through, and beneath the surface of the earth as

defined by the hydrologic cycle (See Figure 2-1). Hydrologic engineers have a major pardcipatory role in defining

the flood hazard, locating and sizing flood damage rduction projects, and determining and assuring the functional

and operational integrity of the project.

b. Hydrologic engineers utilize data such as precipitation and streamflow in the planning, design, and

operation of projects. Analysis twhniquw focus on determining the magnitude and frequency of hydrologic events

(prmipitation and straflow) at locations of interest.

The analysis approaches generally involve relating known

measurements of these phenomena to study areas having

little or no measured data. The techniques used include:

information transfer, simplified methods, statistical computations, and computer program models of the hydrologic

systems.

c. To understand the data requirements and the analytical approaches applied, one must grasp the bmic concepts of flood analysis and data needs. Chapter 2

describes these processes. Subsequent chapters define the

analytid methods used by the Corps to perform flood

h-d analyses for with- and without-project conditions.

1-2
EP 1110-2-10

31 JuI 94

Chapter 2

Data Needs and Hydrologic Processes of

Floods

2-1. Overview

a. Hydrologic cycle. Water WIXS on, in, and over

the surface of the earth in many places, forms, and

phases. The transformation from one phase to another

and motion from one Ioeation to another are referred to as

the hydrologic cycle. Major elements of the hydrologic

cycle are shown in Figure 2-1.

b. Runoff. The process may be conceptualized as

starting with precipitation occuning on the land surface.

A potion of the preeipitation is lost to evapotranspiration,

infdtration, depression storage, and interception. The

portion that is not lost becomes prwipitation excess or

runoff as:

c. Drainage systems. The surface runoff enters

small hillside gullies and ditches, flows to brooks and

creeks, and then to rivers that flow into the oceans.

These systems, as shown by Figure 2-2, consist of a network of flow conveyance channels that mupy the iowe,st

pm of the landscape. The ridge of the land surface, or

rim separating runoff networks, is called the drainage

divide. The area of the land that encloses the divide may

be referred to as the drainage ~ watershed, or catchment of the stream.

d. Runoff hydrography. The runoff from the watershed that occurs over time at the watershed outlet is a

runoff hydrography. Figure 2-3 shows a runoff hydrography

from a watershed with ordinates of discharge versus time.

The runoff hydrographyenters the main stream channel, is

added to the base flow (flow existing without the rainfall

exeess occurring) in the channel, and is combined with

other runoff hydrography as it is translated through the

main stream system. The translation of the combined

hydrography through the stream system is also called

flood routing.

Precipitation - Losses = Precipitation excess or runoff

Figure 2-1. The hydrologic cycle

2-1
EP 1110-2-10

31 Jul 94

Figure 2-2. Drainage basin

e. Organization. The primary interests are the peal(

hydrographydischarge, the corresponding water level

reached in the channel and overbank, and the frequency

with which specific stages are reached. Paragraph 2-2

concentrates on developing the hydrographyand peak discharge, paragraphs 2-3 and 2-4 on water level determination, and paragraph 2-5 on frequency analysis.

2-2. Precipitation Runoff Relatlonshlp

a. Precipitation. Precipitation is derived from atmospheric moisture, resulting primarily from eva~ration

from the ocean. The predominate forms of precipitation

are rain and snow, with hail, fog, drizzle, sleet, etc. being

less important. The form of precipitation at the earth’s

surface is influenced by other climatic factors such as

wind, temperature, atmospheric pressure, and humidity.

Geographic factors such as latitude, altitude, topography,

and location of land and water surfaces dso influence the

nature and amount of precipitation. The primary form of

prwipitation that causes runoff and flooding is rainfall,

with melting snow also a contributor in some regions.

Spatial extent, time variations, and intensity are important

factors contributing to the runoff process. Areal distribution of precipitation is important and is highly correlated

to the time history of runoff.

(1) Rainfall measurement. Rainfall intensities are

measured by rain gages, which are either manually read

or mechanically recorded. Manual gages are relatively

inexpensive to install and rmt however, rainfall information is normally available only in 24-hr increments. For

most watersheds, rainfall intervals of less than 24 hr are

necess~ to adequately define the rainfall effects on the

runoff hydrography. Automated recorders are considerably

more expensive, but can give rainfall intensities for increments as small as 5 minutes, necessary for small urban

catchments. Figure 2-4 shows an automated precipitation

recording gage. The Nationat Weather Service (NWS)

maintains a network of both types of gages throughout the

United States; however, this network often has only

limited data for a specific watershed. Small, urban watersheds may require the installation of one or more rainfall

recorders to give site-specific information for a study

arm.

Flow

at

Out tIet

I et

Figure 2-3. Runoff hydrography

2-2
EP 1110-2-10

31 Jul 94

‘“’’””’*

2 ft. 10 In,

(0,86 m)

Top section

M Drain

pipe

10 F*,

(3<05) m)

Lower section

Figure 2-4. Automated rain gage

(2) Snowpack measurement.

(a) Snowfall is that part of precipitation which occurs

as ice crystals. The aerial extent, water equivalent, depth

of the snowpack, and how fast it melts contribute to the

runoff process. The influence of snowfall on flooding is

more important in northern and mountainous climates than

in other sections of the United States.

(b) The snowpack depth is measured either manually,

or by automated m~ns. Manu,al m~surement usually

involves catching the snowfall in a cylinder, or cutting a

sample from the snowpack, and then melting the collected

snow for quivalent water content. Automated means can

be used in remote areas and often consist of a “pillow”

which ruords the increasing weight of the pack with

time. Air temperature at the snowpack’s surface is also

necess~ to predict the rate of melting, and the corresponding water excess to the stre.arns.

b. Losses. hsses to precipiution falling on the earth

and runoff into stream channels include evapotranspiration

(evaporation from the ground surface and through

foliage), depression storage (surface irregularities or “puddles”), interception (rainfall coating foliage), and infiltration (movement or transmission of surface water into the

soil). Losses from evapotranspiration for flood events are

generally considered negligible. Interception and depression storage losses depend on the surface topography and

foliage of the system, but remain somewhat constant from

event to event. Infiltration is the dominant source of

losses during a flood event.

(1) Infiltration.

(a) Infiltration is a complex process involving the

conceptual sequences of surface entry, transmission or

percolation through the soil, and depletion of storage

capacity of the soil. The infiltration rate deereases as the

soil becomes saturated, thus resulting in greater runoff.

Infiltration capacity can also significantly change over

time due to development effects on the land surface.

(b) The major factors affecting infiltration are antecedent moisture conditions, land cover, and soil type.

Soils and land use cover vary spatially over the watershed, whereas antecedent moisture conditions vary from

event to event. bnd use cover may also vary seasonally

(vegetal cover) or over a period of time (urbanization).

Information on soil type and land use is collected to aid

in the estimate of infiltration losses.

(c) The effects of antecedent moisture conditions,

soil type, and land use cover are conceptually depicted by

Figure 2-5. The buckets represent the storage capacity of

the soil, which becomes smaller when saturated, as shown

by Figure 2-5a. In Figure 2-5b, the soil characteristics

were changed to demonstrate the transmission variability

of different soils. In Figure 2-5c, the surface entry of the

soil has been reduced because of urbanization (imperviousness) of the land surface (USACE 1981).

(2) Infiltration measurement. Although many

attempts have been made to measure losses directly, only

limited success has been achieved. hsses on a specific

watershed are usually inferred by measuring basin average

rainfafl using one or more gages (input to the basin),

measuring the runoff hydrographyat a stream gage (output

from the basin, or rainfall excess), and determining 10SWS

by subtracting rainfall excess from the rainfall. These

losses could be distributed over the time of the storm,

determining an average loss per time period to use for

other rainfatl events for which no discharge data are

available.

c. Disckrge hydrography. Discharge hydrography

are generally considered to have two parts, direct runoff

and base flow. Direet runoff is the rainfall excess

received from recent storm runoff, while base, or groundwater, flow occurs regardless of the storm runoff. Base

2-3
EP 1110-2-10

31 Jul 94

b, SOIL

Illlllli’1

TYP

1111/1/111

~ .Wv-

-. s

w~. w~s

EXCESS

INFILTRATION I~;\T::T~Ol

RATE GREAT

c. LAND USE COVER

(IMPERVIOUSNESS>

m,., /#Ggs

INFILTRATION INFILTRATION

RATE GREAT RATE SMALL

Figure 2-5. Infiltration losses

flow is a relatively small part of the overalf hydrography

and is important primarily on large watersheds. Base

flow and direct runoff are shown on Figure 2-6. Hydrographycharacteristics such as peak dischmge, time to peak,

and volume of runoff are based on the shape of the

hydrography. In turn, the shape is dependent on precipitation patterns, losses, and basin characteristics.

(1) Intensity patterns. Time-intensity patterns of rainfall excess can have a significant effect on the ~,

shape, and duration of the hydrography. Figure 2-7 shows

examples of the effects of various intensity patterns.

Changes in storm intensity must last for hours or days to

cause distinguishable effects on the hydrographyfor a large

watershed. For small basins, clearly defined peaks in the

hydrography may be caused by a few minutes of intense

rainfall excess.

(2) Characteristics affecting hydrography. Precipitation affects runoff directly only if the physical characteristics of the watershed are relatively constant. However,

these characteristics are often not uniform within a

— Pw

RISING

A

LIMB — RECESSION

‘LIMB

DIRECT

PRECEDING FLOOD

\

TIME

Figure 2-6. Dischsrge hydrography features

a. AREAL DISTRIBUTION OF PRECIPITATION

&

Rainfall

@

+Omfou

b. RAINFALL DURATIONS

,K,.t, excess ,R

T T

c, TIME INTENSITY PATTERNS

‘E=XC”SS” T T

Figure 2-7. Rainfall characteristic effects on runoff

hydrography

2-4
EP 1110-2-10

31 Jul 94

watershed or between watersheds.

affwting the hydrographyinclude:

Basin Characteristics

●

●

●

●

●

●

●

Six of the watershed.

Shape of the watershed.

Length of the main channel,

Land and channel slopes.

Roughness of land and channels.

Mnage density.

Valley storage.

(3) Effects of physical characteristics. The effwts of

the physical characteristics of a watershed on the runoff

hydrographyare conceptualized in Figures 2-8 and 2-9. As

the runoff enters the main channels,-the volume, shape,

peak flow, and timing all affat the magnitude of flow.

Th& characteristics are defined primarily through field

reconnaissance and map analysis.

ROUGHNESS IIF LAND AND CHANNELS

h

(ROuGH)

Q

T

h

(SMOOTH)

Q

T

P

DRAINAGE DENSITY

Q

Q k

T

VALLEY STORAGE

\ /

Q

h

(LARGE)

T

v

Q

(SMALL)

L

T

Figure 2-8. Effects of basin characteristics on runoff

hydrography

d. Hydrographymeasurements.

(1) StrW gages. Runoff hydrography and direct

runoff from a storm may be determined directly by measurement at a stream gage. A swam gage could be as

T

Qk

T

Figure 2-9. Additional effects of basin characteristic

on runoff hydrography

simple as a graduated board md once a day, or as

sophisticated as an automated gage reeording in 5-minute

intervals and reporting by satellite teleme~. me cost of

these installations varies significantly.

(2) Recorders. A manual recorder is quite inexpensive, but gives only stage, or water level readings, usually

once per day. These gage records produce a stage hydrography,but no information on discharge. A continuous

recorder measures water level at predetermined intervals,

providing a continuous trace of water level changes.

Water levels are converted to discharge by periodic physical measurements of the swam cross-sectional area and

river velocity, made with current meters. These discharge

mmurements are made once a week to once a month for

normal flows, and as often as possible during floods.

Over time, these measurements can define a relationship

between water level and discharge, allowing one to estimate discharge based on the water level.

(3) Gage installation. Bwause of the expense,

stra gages are not as numerous as one might wish.

When no gages exist in the study watershed, it may be

necessary to install one or more for a limited data

2-5
EP 1110-2-10

31 Jul 94

collection program. This activity must be accomplished

well in advance of the hydrologic analysis and other generat study activities. These data are supplemented with

additional information derived by methods discussed in

later chapters.

2-3. Channel Characteristics

a. General.

(1) Channel systems. Most streams flow within a

channel system bordered on one or both sides by a relatively flat area called a valley or floodplain. For

in-channel flow, the velocity is less nearer the bottom and

sides than it is nearer the center and surface due to boundary friction. For straight channel reaches with relatively

constant dimensions, the flow approaches one-dimensional

flow in the downstream direction. Channel bed deposition

and scour occur depending on channel slope, bed material,

and velocity of flow.

(2) Flow patterns. Channels, however, are seldom

straight for long reaches, and channel bends and curves

have an important effect on the flow. As the stream

enters the bend, the flow near the surface tends to move

towards the concave bank and the flow near the bottom

moves toward the convex bank, as shown in Figure 2-10.

This flow pattern results in erosion on the concave (outside) side of the bend and deposition on the convex

(inside) side of the bend. The flow pattern is somewhat

spiral-shaped, or three-dimensional, in its movement.

(3) Meandering. The smarn is thus constantly moving laterally or meandering in its natural state, with deposition occurring on one side and erosion on the other.

Meandering occurs slowly during normal flows, with the

rate increasing considerably during floods. This process,

plus overbank deposition of sediments during floods,

creates the floodplain shown in Figure 2-11.

(4) Alteration of flow patterns. In addition to bends,

other alterations to flow patterns are caused by changes in

flow, in the cross-sectional geometry of the channel area,

and in the boundary roughness of the channel area. These

alterations can cause eddies, backwaters, drawdowns, and

jumps. Changes in cross-sectional areas may result from

expansions, contractions, and obstructions to the flow

area. For flow within banks, these changes may occur

naturatl y, or from obstructions, such as boulders and

debris. The changes may also result from man’s channel

construction, bridges, pipeline crossings, and numerous

other modifications.

M exaggerated sketih of the pth ~

of rI partIcte of water In Its

screw+e path arcund

abendmarrfer h

Concavebank f

Clrcdotory current in water f(ou~g

around a river bed

Figure 2-10. Channel flow patterns

k Pr,esent lake

position now

/1

500 years ago In grandfather’s time

Materl

Figure 2-11. Floodplain development concepts

2-6
(5) Channel capacity. All of these physicti effects

result in a specific capacity for the channel, which can

vary somewhat along the rach of the stream. Channel

capacity is an important variable. Typically, @age

occurs when the channel flow capacity is exceeded.

b. Field measurements.

(1) General. Measurements of stage and discharge

have been previously discussed in paragraph 2-2c.

Changes in flow patterns are largely determined from

field surveys of the channel and overbank geometry

throughout the study reach. The location of survey data is

based on examination of the stream reach and determining

where significant changes in channel and overbank

geometry occur. Bridge obstructions are particularly

significant.

(2) Sedimentation. Where sedimentation is important, measurements of sediment flow as well as water

discharge are needed. Data collection and analysis at

sediment sampling sites are expensive, but necessary to

address the existing sediment regime and how various

flood damage reduction projects may affect it. Suspended

sediment samples are collected at a discharge site at similar intervals as discharge measurements. Over time, these

measurements produce a relationship of water discharge to

sediment discharge, so that knowledge of the stage can

allow estimates of both water and suspended sediment

discharge. An estimate of sediment moving along the bed

of the strain (bed load, or unmeasured load) is also necessary for complete definition of the sediment load for a

given discharge.

2-4. Flood Characteristics

a. General. Ffooding is a natural characteristic of a

stream system and can be considered an overbank flow.

It occurs when water in the stream system exceeds the

channel capacity, causing an overftow onto the vatley or

floodplain. Flood damage is the destruction or loss of

property caused by water that cannot be carried within the

normal channel. Flooding is usually the result of rainfall

excess or snowmelt, but occasionally can be from failure

of engineered structures.

b. Flow characteristics. As the flood hydrography

moves through the stream system, effects on flow characteristics can be dramatic. High velocities in the channel

may cause bank erosion and scour to the bed, increasing

the sediment load, which is subsequently deposited in

areas of slower velocities, such as the floodplain. Severe

floods have produced

bank characteristics.

EP 1110-2-10

31 Jul 94

major changes in channel and overC. Movement offlood hydrography. The movement

of the flood hydrographythrough the stream system affects

the hydrographysh~p-due to &e travel time of the flood

and to the natural storage in the floodplain, or to manmade storage such as in reservoirs. As the flood waters

increase in height and flow into the overbank areas

occurs, the storage in the overbanks tends to delay and

reduce the peak as shown in Figore 2-12.

Lag

I HTlrne

Hydrography

At A

aJ +ttenuatlon

?

2

u

m

ti

Tine

Figure 2-12. Effects of flood hydrography translation

d. Analysis requirements. Analysis of flood movement or routing may include the determination of the peak

stage or elevation at all key points. Usually peak stages

are determined separately through river hydradic studies.

Hydrology, therefore, normally encompasses the development of surface water runoff, hydrographycombination,

and routing to determine peak discharges at all key locations. Hydraulic analysis for flood damage reduction

studies utilizes these discharges to determine the peak

water surface elevation. How often the flood occurs

(frequency) must then be determined.

2-7
EP 1110-2-10

31 Jul 94

2-5. Frequency Analysls

a. General. Frequency forms the third pfiary analysis requirement along with water level and discharge.

The determination of wonomic benefits of a project

requti a knowledge of how often flooding occurs at

various flood levels. This requirement is met thugh the

analysis of shge- or discharge-frequency curvm for wnditions of interest.

b. Methods oj analysis. This analysis may be developed by statistical methods if a long-term hydrologic

record exists at a stra gage in the study reach. Typically, however, long-record data are scarce for most

hydrologic analyses. Even if such a record is present,

other locations, having limited da~ also must be evaluated for frequency. Therefore, frequent y determinations

usually consist of the application of hypothetical storms of

specified frequency (10-, 2-, and l-pent annual chance

exc-ce, etc.) to a hydrologic model of the watershed

to determine discharge-frequency relationships at all

desired locations.

c. Hydrologic models. Hydrologic models are often

calibrated so that observed rainfall frequency approximately co-rids to discharge@uency. Loss rates are

usually adjuste~ based on judgement, to reflect the severity of the h~thetical floods.

2-8
Chapter 3

Flood Studies

3-1. Overview

a. Development of work plan. The hydrologic enginwr must develop a work plan appropriate to the flood

problem being s~died and the type of flood damage

reduction alternatives under investigation. The flood

hazard must be defiied to determine the tangible damage

resulting, both for present conditions in the watershed and

for a future time peri~ if significant changes in the

watershed may occur. A level of detail commensurate

with the type of analysis must be determined, a method of

formulation and evaluation of the proposed flood damage

reduction alternatives adopted, and the consequences (both

positive and negative) addressed.

b. Hydrologic engineering. This chapter describes, in

general terms, the hydrologic work nmessary for a flood

study. Interested readers may review ER 1110-2-1460 for

additional information on hydrologic engineering management and for hydrologic engineering required for feasibility studies, respectively. Additional information on

hydrologic engineering and other engineering disciplines

in the feasibility and PED phases is referenced in

ER 1110-2-1150.

3-2. Definition of the Flood Hazard

The study process and how the plan formu~tion and

evaluation evolve must be defined to provide the hydrologic data needed by other disciplines. The method of

analysis, level of detail, time requirements, and format of

the hydrologic information must be commensurate with

the nwds of the study tm, including the rest-sharing

partner.

a. General. Most flood studies require the definition

of a stage-frequency relationship at key locations in the

watershed, and how these relationships change in time,

both with and without various flood damage reduction

projects. Flood frequencies ranging from a 50-pement

chance exceedance event through the Standard Project

Flood or a 0.2-percent chance exceedance frequency

eventl are selected to define a full range of frequency

1 Exceedance frequency is percent chance an event may

occur in any given year. An event with a 0.2-percent

chance excwdance frequency will occur once every

500 years, on the average, but can occur in any given

year.

EP 1110-2-10

31 Jul 94

events. Existing, or base (start-of-project operation),

conditions form one “snapshot” of land use conditions to

evaluate. At least one additional time period, usually

20-50 years in the future, based on available land use

planning information, is chosen to form a second time

period. If land use differences between the two times are

great, additional periods may be interpolated. Similarly,

additional

.ueriods could be extrapolated. The same two

times are also used to evaluate the changes in the watershed’s flood hydrology caused by the various flood reduction measures evaluated. Figure 3-1 illustrates frequency

curves at a location undergoing continuous urbanization,

resulting in changes to the relationship.

Figure 3-1. Frequency curves developed for an

urbanizing area

b. Hydrologic information. The hydrologic engineering information needed could include the following:

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Discharge hydrography.

Peak discharge frequency.

Runoff volume frequency.

Water surface profiles.

Flood inundation boundaries.

Flow velocities.

Warning times.

3-1
EP 1110-2-10

31 Jul 94

(8) Duration of flooding.

(9) Sediment deposition and erosion quantities.

(10) Operational performance of projects (amount of

flood reduction).

Of the above information, accurate flood inundation determinations have the most impact on project evaluation.

The extent and depth of flooding for with- and withoutproject conditions result in the estimate of flood damage

reduction benefits, which are the basis for determining the

economic feasibility of flood reduction projects. For

agricultural flood studies, duration and time of year of

flooding are important. The quality of the hydrologic

work, as well as the survey and mapping information, can

significant.1y affect the determination of project feasibilityy.

c. Field presence. The hydrologic engineer must

spend time in the field throughout all phases of the anaty -

sis, from the reconnaissance through the actual construction. A field presence is required to gather data needed

for the various phases of the study and to maintain continuous contact with local interests involved with the proposed project. Credibility is quickly lost when the

engineers involved in the project rwommendations have

spent little or no time in the study area. Field visits

should in many cases include other members of the study

team and the local sponsor. The hydrologic engineer’s

field presence is needed fon

(1) Determination of topogmphic and survey needs.

(2) Determination of high-water marks and times of

flocd peaks by interviewing local residents and researching newspaper files.

(3) Obtaining the characteristics of flood events,

including debris and sediment problems, flood dates,

velocity patterns, and changes to the stream and watershed

since historic floods have occurred.

(4) Estimation of friction values (Manning’s n) for

the channel and floodplain.

(5) Identification of obstructions to flood flow

(bridges, dams, logjams, levees, road embankments, etc.),

and historical floods which caused overtopping of these

obstructions, including how often they occurred and

whether debris or ice was a factor.

(6) Operation procedures for existing structures

(dams, pumping plants, drains, frequency of channel

dredging, etc.).

(7) Identification of the type and location of potential flood loss reduction measures, and any constraints on

these measures (relocations, environmental damage, etc.).

d. Level of detail. Most of the hydrologic engineering effort is concentrated in the reconnaissance-phase and

feasibility-phase studies.

e. Reconnaissance-phase study. The effort in the

reconnaissance phase emphasizes the use of existing data,

primarily due to the short duration of the study. Much

information is obtained from local residents and officials,

existing studies of the watershed, from measured and

readily available data, and from field reviews and engineering judgement. If time and funding are available, it is

desirable to establish the existing condition hydrology and

flood profiles to avoid major changes in the fewibility

phme. The reconnaissance-phase study evaluates several

potential alternatives to determine if at least one is economically justified. If wonomic justification exists and

there is a local sponsor willing to cost share, the study

continues into the feasibility phase.

f. Feasibility-phase study. The majority of hydrologic engineering work is performed in this phase. The

anaIysis must be sufficiently rigorous so that the project

recommended in the feasibility report is essentially what

is constructed, after detailed PED is completed. The

hydrologic engineer, working closely with the study manager, economist, cost engineer, and other members of the

Interdisciplinary Planning Team (PT), completes the

with- and without-project evaluations so a plan that maximizes net benefits is identified at completion of feasibility

planning. This end result requires a continuous exchange

of technical information among the various disciplines as

follows:

(1) Technical data to the hydrologic engineer:

(a) Survey and mapping information.

(b) Maps showing land use, soil types, vegetation,

storm sewer layouts, bridge plans, and other information

from local agencies.

(c) Rainfall information from the NWS or other

agencies.

(d) Stage, discharge, and sediment information from

the U.S. Geological Survey (USGS) or other agencies.

3-2
EP 1110-2-10

31 Jul 94

(e) Potential altemativ~ to analyze, in conjunction

with the study team and the local sponsor.

(2) Hydrologic data to the study team:

(a) Specification of topographic data needed by

surveyors.

(b) Stage-frequency retations for without-project

(base conditions), future without-project, and with-project

conditions.

(c) Effects (reduced flooding) of each flood mitigation component to the study manager.

(d) Component capacities or dimensions to the

designer and cost engineer.

(e) Velocity, sediment, duration, depth, and other

information to the environmental specialist and permit

specialist.

(f) Flood inundation boundaries with and without a

project to mapping and to the d estate specialist.

(3) me information is furnished via an iterative process, leading to the refinement of the recommended

project.

g. Design memoranda.

(1) Detailed hydraulic design. The emphasis in this

phase is on the detailed hydraulic design aspects, because

no additional plan formulation, economics, etc. should be

necessary. The hydrologic engineer is more involved in

refining the detailed design of the project components.

The overall component capacities, general design, etc. are

held relatively constant from that recommended in the

feasibility report. For instance, the feasibility report may

have recommended 5 miles of channel modifications

having spectiled channel dimensions. The design memoranda would refine these dimensions to fit the channel

through existing building and bridge constraints: perform

detailed hydrautic design of tibutary junctions, bridge

transitions, drop structures, and channel protwtion; and

conduct detailed sediment transport studies to identify

operation and maintenance requirements, and other

hydraulic design as~ts.

(2) Additionat activities. Additional topogmphic site

surveys and subsurface information are normally obtained

in this phase so that structural design, geotcchnical analysis, cost engineering, and other activities can be finalized.

The hydrautic design is often updated to reflect changes

in analysis parametem prior to completion of detailed

design. Additional information on the design memoranda

phase is referenced in ER 1110-2-1150.

h. Construction and operation. Unforeseen problems during construction frequently involve further modification and adaptation of the hydraulic design for onsite

conditions, Similarly, most projects require detailed operation and maintenance manuals and hydrologic engineering information can be a critical part of these manuals.

The operation of reservoirs, pumping stations, and other

flood mitigation measures can require considerable hydrologic operation studies to determine the best operating

procedures. Post-construction studies are necessary for

most projects. These studies monitor sediment deposition

and scour caused by the project, ensure that adequate

hydrologic design capacity is maintained, monitor the

correctness of the data used in anatyzing the proj~t, and

estimate the remaining useful life of the project.

3-3. Formulation and Evaluation of Solutions

a. Methods of solution. Appropriate application

methods of solution are required to provide the necessary

information to the study team and to evatuate both positive and negative aspects of the project. Simple versus

complex methods and a frequency approach versus a

period-of-record approach are considered, depending on

the phase of the study and the type of flood damage

reduction measures being evaluated. A simple regression

equation giving peak discharge, knowing drainage area

and slope, may be acceptable for a reconnaissance-phase

effort to size a channel, but is inadequate for determining

adverse effects downstr~m.

b. Investigation sites. Locations where hydrologic

information is nwded must be identified. This process

must include the study team’s requirements as envisioned

at the time of the determination. In general, these points

include:

(1) Locations of tributary junctions with the main

channel, to evaluate significant flood flow changes.

(2) Stream gages, to calibrate model output to actual

data.

(3) Points along rmches where flood damage reduction measures are to be evaluated.

(4) I.ocations in the watershed where land use, soil

type, etc. change significantly.

3-3
EP 1110-2-10

31 Jul 94

(5) Damage centers, to compute damage with and

without a proj~t.

Figure 3-2 illustrat~ how a watershed could be subdivided to determine where hydrologic information is

necessary.

3-4. Impsct Assessment

Analysis resdts are given by the impact of the flood

damage reduction measm on flooding. These impacts

are quantified through evaluation of with- and withoutproject comparisons of flooding represented by flooded

area, depth, and frequency curves.

a. Project formulation. Evaluations are conducted to

determine the performance characteristics of multiple sizes

to determine attributable flood damage reduction, costs,

and benefits. The analysis will determine the proj~t that

maximizes the net benefits or NED plan. The project

ident~led as the NED plan is that typically recommended

for project &sign and construction. The risk of excwdan~ is also tisplayed for the selwted design event to

better illustrate the likelihood of design exceedance.

b. Design exceedance. The proj~t design is

exceeded when top of protection is exceeded or structural

failure occurs. Every project can and eventually will be

exceeded if it remains in place over a long period of time.

When a flood overtops the Protwtion, significant damage,

possibly more than the darnage prior to the project, could

be experienced. Part of the analysis is to evaluate the

consequences of design exc~dance, and to dwign the

project to minimize damage to both the projwt and the

and configurations of measures. A range of flood events protected area.

is anatyzed for each measure. Each measure is analyzed

*

POTENTIAL RESERVOIR (AND ROUTING)

POTENTIAL LEVEE

DAMAGE INDEX STATIDN

POTENTIAL CHANNEL MODIFICATION

HYDROGRAPHY COMPUTATION POINT

ROUTING REACH

SUBAREA BOUNDARY

DAMAGE CENTER

Figure 3-2. Subarea delineation

3-4
EP 1110-2-10

31 Jul 94

c. Positive/negative effects. Quantification of the

positive effects of the project associated with its reduced

flood damage is the basis for economic justification.

However, the hydrologic analysis must address negative

consequences as well. The consequences of the major

types of flood damage reduction projects include but we

not limited to the following:

(1) Resemoirs.

(a) Positive impacts.

● Flood flow and stage reduction at downstream

locations.

● Downs&m damage reduction.

● Source of water for multiple uses during low flow.

● Recration usage.

● Hydropower generation.

(b) Possible negative impacts.

● Permanent inundation of reservoir lands.

● Eventual filling of reservoir with sediment.

● Changes to downstream sediment regime, usually

erosion.

● Conveyance encroachment from lack of large

floods.

● Changes in water quality.

● Increased losses due to evaporation.

● Elimination/mduction in fish spawning.

(3) Channelization.

(a) Positive impacts.

. Hood stage reductions even for events exceeding

the design capacity.

● Local damage reduction through project reaeh.

(b) Possible negative impacts.

. Potential effect on fish spawning and wildlife

habitat.

● Changed sediment transport characteristics.

. Increased channel maintenance requirements.

● Induced flooding downstream if extensive loss in

valley storage.

(4) Diversions.

(a) Positive impacts.

● Flood stage reductions even for events exceeding

the dmign capacity.

9 Local damage reduction through projmt reaeh.

(b) Possible negative impacts.

● Changed sediment transpoti characteristics caused

by uneven diversion of sediments.

● Induced flooding downstream of diversion rwntry

point.

(2) Levees.

(5) Nonstructural.

(a) Positive impacts.

(a) Positive impacts.

s No flooding from exterior until design is exceeded.

● Protmtion to properties behind levee. ● Individual structures protected.

● Essentially no change to environment.

(b) Possible negative impacts.

(b) Possible negative impacts.

● Induced flooding upstream and downstream of

levee if extensive loss in valley storage.

● Potential for sudden large losses when levee design

is exceeded.

. Interior flooding.

● Closures of openings may be required.

● High residual damage - infrastructure not protwted.

. Emergency response required on event-by-event

basis.

3-5
EP 1110-2-10

31 Jul 94

Chapter 4

Determining Flood Flows by Frequency

Methods

4-1. Overview

a. Measures of flood severity. The majority of flood

damage reduction studies rquire the evaluation of peak

discharge, often used as the main measure of flood severity. Other variables, such as the totaf runoff volume, may

also be critical for certain studies. Flood studies require

frequency estimates in order to judge the performa~ce of

proposed flood damage reduction projects. The development of pe& discharge-frequency relationships for a

catchment is an important part of flood evaluation for

Corps studies. This relationship is linked with elevationdischarge data and with elevation-damage data using riskbased analysis procedu~s to arrive at estimates of

expected annual damage for with- and without-project

conditions.

b. Disckrge-frequency estimates. Some degree of

uncertainty exists in all discharge-frequency estimates.

This uncertain y results from insufficient information.

The more data available, the better the estimate of discharge-frequency. In a typical flood damage reduction

study, a certain amount of known (gaged) data will exist,

but some of the study area may have no gaged data.

Consequently, a combination of gaged and ungaged techniques are often used for the hydrologic analysis.

4-2. Analysis for Gaged Areas

The development of discharge-frequency relationships at

gaged locations is a well-documented process involving

statistical anatysis of annuat peak discharges. Figure 4-1

shows the results of a statistical analysis of recorded data.

The analysis requires an adequate length of smam gaged

record, with the data being both homogeneousand of good

quality. References (Water Resources Council 1982,

EM 1110-2-1415) give the complete technical detail necessary for statistical anatysis of stream-gaged rwords.

a. Record length.

(1) Although opinions vary as to a minimum record

length, at least ten years of data is generally recommended. As one might suspect, ten years of data would

seem a very limited amount to estimate, say, the 1-percent

chance exceedance frequency peak discharge. The

absence of significant peak discharges, such as during an

extensive drought, or the occurrence of several floods

during this short period would result in a poor estimate of

the flood-frequency relationship. A “rule of thumb” suggests that the rarest flood that can be predicted with a

reasonable level of confidence is about double the period

of record. A 5-percent chance exceedance frquenc y

(20-year) flood would be the targest for 10 yas of data.

(2) Major changes in the estimates of return periods

of rare floods are not unusual as one acquires mom data.

Obviously, the longer the period of gaged data, the more

confidence one could have in the final result. Thirty or

more years of data is generally desired for “good” statistical frequency estimates. Even if one has a lengthy

record, comparison and modification of the relationship

derived by statistical means is often made. This effort

may involve regional studies considering nearby gages,

and hypothetical floods developed with hydrologic

models.

b. Record honwgeneitylquality. As the record

becomes lengthy, one becomes more concerned with

changes in the catchment upstream of the gage, potentially

resulting in a non-homogenous data record. Typicat

examples of non-homogenous records often cited are the

urbanization of the land upstream of the gage, or the

installation of a major reservoir. These man-induced

changes result in different runoff volumes, hydrography

shapes, and peak discharges for similar storms. If significant changes occur during a period of recorded data,

adjustments to or separation of the record is necessary.

Quality of the data should also be considered, as stream

gaging techniques can only estimate the total discharge

during flood events. The USGS, the source for most gage

data, evaluates the quality of its data at each of its gaged

sites. A description of “Excellent” means that 95 percent

of the daily discharges are within 5 percent of the true

value, “Good’’--within 10 percent, “Fair’’--within 15 percent, and “Poor’’--less than “Fair.” Accuracy and confidence level are much lower for a statistical analysis of

gaged data with a poor or fair rating than data with a

good or excellent rating.

c. Need for unguged techniques. When statistical

analyses of gaged data are performed for a long-record

station, the resulting estimate of discharge-frequency is

considered the most accurate of any technique available.

However, this relationship is only valid at the gage, and

not at points significantly removed from the site. Thus,

ungaged methods are almost always required along with

statistical methods. Besides giving a precise estimate of

discharge-frequent y, gaged data allow one to compare the

4-1
EP 1110-2-10

31 Jul 94

100000

100

I

o OBSERVM mu Ws

— COMPUTEDFRE~CY WRVE

— WITHEXPECTEDPROBABIUTY~4S~

— .0S MD ,95 CIINFIDENELIMITCURVES

I

1 1 t

[ 1 I 1[ i1 I <

I

1 t 1

I Y

Figure 4-1. Flood frequency analysis by statistical methods

results of ungaged techniques and calibrate and/or verify

the hydrologic methods used to estimate dischargefrequency relationship for ungaged areas.

4-3. Simplified Analysis for Ungaged Areas

Ungaged areas are those that have insufficient rwords to

perform a statistical frequency analysis of@ discharge.

This usually means no gages at all, but could also include

sit~ that have only a few years of gaged data available.

A wide variety of different techniques exist to determine

discharge-frequency for ungaged areas. The following

descriptions range from the simplest to the most complex.

a. SimpliJed equations. ~ese methods involve the

application of empirical relationships or simple envelope

curves to estimate a peak discharge. They are usually

applicable for only a certain size of catchment or for a

specific type of discharge. Examples include the rationat

formula (Q = CIA, for very small areas) and the Myers

Formula where discharge is function of area, giving the

potential maximum possible discharge (McCuen 1989).

These methods are msy to apply, but the results are of

dubious quality. These techniques are applicable for

certain prelimin~ level studies. Figure 4-2 illustrates the

most widely used simplified equation: the rational

.
EP 1110-2-10

31 Jul 94

@

(1-Q I = WML nAm m cJNrRmJ-rING

mwn~

Figure 4-2. Example of simplified equations

formula It is stitl the main method used to determine

design discharges for sizing storm sewers.

b. Transfer methods. This technique is also rather

simple to apply, but the results are of appreciably better

quality. It consists of a simple transfer of measwd data

from a gaged to an ungaged site, with the data being

modified, as necessary, to reflect the conditions at the

ungaged site. The modification could be a simple ratio of

drainage area of the gaged versus the ungaged site, or be

considerably more sophisticated. While discharge, sediment, and other gaged data are transferred to an ungaged

site, precipitation data are most commonly transferred.

Unless the region is mountainous, precipitation can be

readily transferred a moderate distance without adjustment. The transferred data are assumed to be as likely to

have occurred on the ungaged portion of the study watershed as on the gaged portion. Figure 4-3 illustrates the

use of transfer techniques, which could be valid in any

phase of the overall process.

1, BASINS MUST BE METERULOGICALLY AND

HYDROLOGICALLY SIMILAR,

2. WHAT DATA IS Ttl BE TRANSFERRED ?

a, ENTIRE HISTORIC RECORD

C, PORTION OF DATA

[b”

#TDc

Figure 4-3. Example of transfer techniques

c. Regression analysis.

(1) This method is a more detailed and sophisticated

subset of transfer twhniques and im development involves

considerable work effort. Fortunately, regression analyses

for peak discharges have been perfomed for most portions of the United States, usually by the USGS from

gaged data (USGS 1983). Figure 4-4 illustrates the use of

regression analysis. This technique develops the desired

information (usually peak discharge for given frequencies)

from a statistical analysis of long-term gaged records. A

regression analysis is then performed linking the calculated peak discharge for each frequency to m~surable

parameters, like area, slope, stream length, etc. A prediction equation results which allows one to calculate a value

for, say, the p~ discharge knowing the drainage area

and slope of the ungaged watershed. Differences betwmn

the discharge calculated with the regression equation and

that found with a statistical analysis are called “residuals.”

These residuals may be mapped and used to adjust the

discharge calculated for ungaged catchments. The regression analysis also allows one to estimate the accuracy of

the prediction equation results.

4-3
EP 1110-2-10

31 Jul 94

F~..~

e SELECT CUMPDNENTS SELECT DEFINABLE

❑F INTEREST BASIN CWRACTERISTICS

● MEAN ANNUAL FLOODS ● DRAINffiE AREA

● FLFOLOODW:REQUENCY c SLDPE

● ETC.

● ETC,

- DERIVE

t

PREDICTION REGIONALLY MAP

EQUATIONS RESIDUALS

Qm=K.D~,S

- am

Figure 4-4. Example of regression analysis

(2) Regression techniques areapplicable in all phases

of a hydrologic study and are valuable in evaluating the

reasonableness of peak discharges determined with a

hydrologic model. The main drawback to the technique is

that only a peak discharge is available and there is no

way to estimate how the peak discharge will change if a

flood damage reduction structure is placed in the system.

This tahnique is often used where only a peak discharge

is needed to estimate flood severity, with flood insurance

studies being a typical example. Regression analysis is

considered by many to be less accurate in estimating a

peak discharge than statistical analysis of gaged data at a

site, but more accurate than hydrologic modeling.

4-4. Detailed Analysis for Ungaged Locations

a. The preceding simplified methods can be applied

with minimal effort, but all have the same deficiency--

how does the fld hydrographychange as it moves

through the watershed system and how does the application of flood darnage reduction measures affect the flood

discharge? The only way in which these questions can be

answered lies in detailed hydrologic modeling of the

watershed. Figure 4-5 shows a schematic diagram of a

typical hydrologic simulation using a model. A

P

MSIC HYDRffiRMH

y MMYSIS PROCEDURE

me= 1. DETERMINE HYDR~C

SUBBASIN DATA

a. AVERME RAINFALL

b, LOSS RATE CRITERIA

G BASEFLOW

ma 24 DETERMINE M

WDRE~8HMSmmMSIN

3, CONSTRWT RUWF

HYDRffiRMHS AT EACH

SUBBMIN mTLET

v~~

4. CONBK HY=AW

FROM SUBBMINS 1 b 2

6.

7,

RCUTC C~BINED

HYDROWAPH (1 & 2) TO

OUTLET ~SUBMIN 3

COMPUTE R OFF HYDROWAPH SUB2 SIN 3 OUTLET

C~BIW TNE ROUTED

‘ WDRMRAPH VITH

SUBMSIN 3’s HYDROGRAPHY

FOR TK TUTA VATERSHED’S RWffF WDRIIGRAPH

,k&,,*@~

TIME

(STEP O <STEP 7)

Figure 4-5. Example of hydrologic modeling

hydrologic model is a computer program that simulates

the response of a hydrologic system based on meteorologic and physicat watershed characteristics. The successful application of a hydrologic model in no easy task and

requires knowledge and experience to prepare and operate

the model and evaluate the validity of the results.

b. In addition, calibration of the model to some

known data is important to gain confidence when applying the model to estimate unknown or rare events. Operation of the model for historical conditions (for calibration

and/or verification), and for existing and future conditions

(for establishing the severity of the flood problem and the

effects of various flood reduction alternatives) is the basis

for the overall flood reduction analysis.

c. There are many hydrologic models available to

de@rrnine runoff hydrography from a watershed. The

procedures by which these models operate vary widely

and not all models are applicable to a specific study area.

The use of a single- event model versus a continuous

simulation model (illustmted in Figure 4-6), actual versus

hypothetical (frequency) rainfall, various loss rate functions, modeling of subsurface flow and losses, unit hydrographyversus kenematic wave methods, hydraulic versus

4-4
EP 1110-2-10

31 Jul 94

EVENT MODELING

1 8

T (HOURS, DAYS)

CONTINUOUS SIMULATIIIN

P

r , t, 11’’” ‘

, 1 19’” 1,

IF’ p Ilu“

Q~

T (MONTHS, YEARS)

Figure 4-6. Slope event versus a continuous

simulation model

kinematic wave methods, hytiulic versus hydrologic

routing, ek. & features of the various models. Some

models are considerably more detailed and sophisticated

than others, requiring a higher level of expertise. The

rainfall-runoff process, which these progrms model, is

presented in Chapter 5.

4-5
EP 1110-2-10

31 Jul 94

Chapter 5

Determining Flood Flows by

Precipitation-Runoff Analysis Methods

5-1. Introduction

Detailed hydrologic modeling is usually required for flood

damage reduction studies. This ties of hydrologic engineering, along with river hydraulics, nomally takes the

butk of time and money in a study. This effort requires

determination of how to subdivide the watershed to give

required hydrologic information at points of interest, to

develop the precipitation, loss, runoff, discharge, and

routing information, and to calibrate and verify the model.

Detailed modeling usuatly takes place during the feasibility phase. ~is chapter describes the various components

of the hydrologic modeling performed.

5-2. Watershed/Subbasin Delineation

Delineation of the watershed into subareas to determine

discharge information was discussed in paragraph 3-3.

The study team must also participate by defining their

information needs during this process. Location of damage raches, potential flood damage reduction masures,

politicat boundaries, and other items may cause further

modification submas to provide the necessary hydrologic

data.

5-3. Analysis Approaches

a. General. The two main methods for determining

flood runoff can be described as single-event analysis or

continuous simulation, as illustrated in Figure 4-6. The

former refers to determining the runoff from a single

storm-flood event (the flood of 1986 or the 2-percent

chance hypothetical flood). The main problem with this

technique is a lack of knowledge of the antecedent soil

moisture, especially for hypothetical floods. Assumptions

as to wet or dry soil conditions may have a significant

effect on the corresponding runoff.

b. Continuous simulation. me continuous simulation

technique overcomes this problem as all periods of

str~mflow (droughts, floods, and all events in between)

we simulated. This process is much more satisfactory in

that more of the streamftow process is analyzed, but continuous simulation computer models are generally more

data intensive and time-consuming to operate than

models. A lack of knowledge of other hydrologic

ables needed for continuous models (evaporation,

event

variinterception, subsurface and groundwater flow, etc.) may

cause the resultsto be no more and perhaps less accurate

than those of the single-event model. Continuous simulation models are often used where agricultural flood damage is extensive, because the time of year in which the

flood occurs is important for darnage catcutations. Also,

agricultural flood damage analysis may be required for

relatively frequent events, such as the once- or twice-peryear flood. A flood this frequent is not usualty suitable

for event modeling.

c. Single-event analysis. Single-event models are

typically used in urban flood damage analyses, since time

of year is generally not important and the project design

is for a rarer frequency, like the l-percent chance flood.

This publication will address only the hydrologic analysis

related to a single- event model.

5-4. Precipitation/Runoff

Each subarea contributes a discharge hydrographyto the

water moving throughout the overall watershed. Runoff

from the severat subareas is combined to yield the total

discharge hydrographyat the outlet. Subbasin characteristics used to compute runoff includti rainfall, losses,

transforms. and base flow.

a. Precipitation. Precipitation is atmospheric water

in all its many forms. Flood reduction studies are primarily concerned with rainfatl, with snowfall/snowmelt also

of concern in certain regions of the United States. Rainfalt is also further defined as being historical (recorded) or

hypothetical.

(1) Historical rainfall. The enginmr requires historical or actuat rainfall for one or more storm events that

produced flooding in the study watershed. The purpose of

this historical rainfall is to catibrate the overall hydrologic

model, ensuring that the model’s output is representative

of the basin. The actuat rainfall that occurred over the

study watershed produced a flood that was measured at

one or more gages, or that reached heights that were

remembered by local residents and then surveyed to determine high-water mark elevations. Rainfall input is used

by the hydrologic model to produce flood hydrography

output at a gage site or a water surface elevation at a

point of a known high-water mark. If the model’s output

is reasonably close to known discharges or water surface

elevations, the model is considered to be calibrated and

ready for use in developing discharge-frequency relationships. Historical rainfalt for several actual storm-flood

events would be desired, with the rainfall time sequence

also being necessary. Depending on the size of the

5-1
EP 1110-2-10

31 Jul 94

watershed, incremental rainfallvalues ranging from

5-minute intervals to 24-hr increments woutd be nmessary. Figure 5-1 shows an example of historic rainfall for

application to a hydrologic model,

(2) Frequency rainfall.

(a) Hypothetic rainfall is required to determine discharge hydrography for specific flood frequencies. Hypothetical rainfall is taken from past studies of the NWS,

with Technical Publication ~) 40 (NWS 1961), TP 49

(NWS 1964), and National Oceanic and Atmospheric

Adminis~tion (NOAA) HYDRO-35 (NWS 1977) being

the sources of these data for the 35 statw east of the

Rocky Mountains. The other 13 states in the continental

United States have individual state atlasw @OAA 1973)

to give the detailed information r~uired in mountainous

terrain. Alaska wS 1963, 1965a) and Hawaii (NWS

1962, 1965b) also have guidance specific to those states.

Figure 5-2 gives an example of the type of information in

Nws TP 40.

(b) Rainfall information is extracted at the location of

the study watershed for each duration for a given frequency. The rainfall is incremented to determine depth in

each time period, adjusted to reftmt storm occurrence

over an m rather than a point, and arranged in an

appropriate pattern. An example of the adopted storm

pattern for a given frequency and watershed is shown in

Figure 5-3. Each frequency d~ired, from 50- tiugh

0.2-percent chance exceedance storms, is developed in a

similar fashion. Six or sevm separate frequency storms

are often required to give sufficient points to determine

the resulting discharge-frequency curve with hydrologic

modeling.

(3) Standard project stem.

(a) The hypothetical Standard Proj&t Storm (SPS)

is generated using a standard procedm (USACE 1965)

for areas east of 105 deg longitude. For western areas,

SPS’S are normally generatd by adjusting and transposing rare obsemed events to the study area from hydrologically and me~rologically similar areas. An example of

an SPS, arranged for appreciation, is shown in Figure 5-4.

(b) The SPS is used to develop the Standard Projwt

Ftood (SPF). The SPF is the flood that can be expected

from the most severe combination of meteorologic and

hydrologic conditions that are considered reasonably characteristic of the region. The primary application of the

PM AM PM

APRIL10,1979 APRIL11,1979

Figure 5-1. Example of historic rainfall

5-2
EP 1110-2-10

31 Jul 94

Figure 5-2. 100-year, 24-hr-duration rainfall map

I SPF is to evaluate the performance of projects for an

“0~ 1 ‘X”eme ‘vnt” Although a specific frequency cannot be

0,9

0!8 1

2-Y~R RETURNPERIOII

24-HOURtuMTIcN

1- INTIRVM

dla 24

assigned to the SPF, a return period of a few hundred to a

few thousand years is commonly associated with the

event.

(4) Probable maximum storm. This hypothetical

event is normally required when dams and reservoirs are

under consideration. Failure of a dam by overtopping

could be a catastrophe for which no risk of failure would

be allowed. Consequently, the Probable Maximum Storm,

or PMS, (NWS 1982) is used for dam and spillway

design to ensure that there is essentially no risk of design

exceedance. Figure 5-5 shows a PMS arranged for use in

a hydrologic model. The PMS is based on meteorologic

studies of potential water in the atmosphere under the

most extreme conditions.

TIME IN HOURS

I

(5) Snowfall/snowmelt.

I

(a) Snowfall is important in mountainous regions

Figure 5-3. Typical time distribution for a hypothetiml and in the northern portions of the United States. Unlike

storm

53
EP 1110-2-10

31 Jul 94

m

0.s

o

Figure 5-4. SPS arrangement

rainfall events, snowfall w accumulate throughout the

6H

1-

a

+

z

z

w

IY

n

t I II II II Ill

6 12 18243036 424854606672

TIME (HOURS)

winter as a snowpack, which melts when warmer wather

occurs. Therefore, the important variables for snow are:

depth of snowpack and comsponding water content, air

temperature, and topographic elevation. The last variable

is important because the air temperature deer- with

increasing elevation, and most air temperature monitoring

gages are Ioeated in lower elevations. Depth of snowpack

is monitored by physical measurements or by remote

telemetry, with the corresponding water content

determined.

(b) Snowpack information is critical for reservoir

operation or structures receiving meltwater runoff, which

includes most of the reservoirs in the western United

States. Ftood studies involving snowmelt are based on

recorded data when available. When snow data are not

available, it may be estimated by knowing rainfall and air

temperatures, and converting to an estimated snowfall.

No hypothetical basis is available for determining a synthetic snowmelt event.

Figure 5-5. Probable maximum storm arrangement

5-4
EP 1110-2-10

31 Jul 94

b. Losses.

(1) General. Many methods are available for determining losses during a rainfall-runoff event, ranging from

quite simple to very complex. For an event-type analysis,

loss rates have been estimated using the uniform and

initial method, the U.S. Soil Conservation Service (SCS)

Curve Number method, the Horton technique, the GreenAmpt procedure, the exponential method, etc. @SACE

1990a). For a continuous simulation analysis, loss rate

estimates could range from a simple runoff coefficient to

a complete soil moisture accounting system.

(2) Adjustment of loss rates. The appropriate method

is largely up to the judgement of the hydrologic engineer.

Since the loss rates during a runoff event are not known,

loss rates may be adjusted during the calibration analysis

to atlow a better reproduction of the known hydrographyor

high-water marks by the model. Loss rates may also be

adjusted depending on the storm severity, since the same

loss rate would not be expected for a 50-percent chance

(2-year) storm as for a l-percent chance (loO-year) storm.

A rare storm is typically one in a series of events, which

tend to increase the soil’s antecedent moisture level and

the corresponding runoff. Consequently, loss ra~s during

a rare event would be expected to be less than a more

common storm event. bss rate adjustment is one way

which the argument in favor of continuous simulation

models may be partially addressed. Figure 5-6 gives

examples of simple loss rate accounting procedures.

c. Runoff tran~ormations. After precipitation and

in

loss rate analyses are complete, the engineer is left with

an estimated runoff from the watershed expressed in

inches per time period for the storm. Runoff in cubic feet

per second, rather than (for instance) inches per hour, is

needed for hydrography analysis. Consequently, a transformation is required to obtain runoff quantities in the

desired format. Most hydrologic modeling makes this

transform using the unit hydrographytechnique. Occasionally in highly urbanized catchments, the kinematic wave

technique is used. The selection of which technique to

use is normally up to the hydrologic engineer.

(1) Unit hydrographymethod.

(a) This technique was first develoWd in the 1930’s

and is still the predominate tmhnique used in the Corps

for a runoff transformation. Many unit hydrography

(UHG) methods me available, with the main ones being

the Snyder, Clark and SCS techniques (USACE 1990a).

The unit hydrography technique involves the development

of a “pattern” hydrography,representing the runoff of 1 in.

----m

la

!

[

u

i“

9

m—

LOSS RATE ACCOUNTING

<INITIAL AND UNIFORM LOSSES>

INFILTRATION CAPACITY CURVE

i

i

TLnc

Figure 5-6. Examples of simple loss rate accounting

(or unit) of rainfall excess, occurring uniformly during a

specified duration (1 hour, 1 day, etc.) over a specified

watershed. The assumption is that any other rainfall

excess (more or less than 1 in.) during the same duration

produces a similar hydrograph with the discharge ordinates proportionally higher or lower than those of the unit

hydrography. Figure 5-7 illustrates this concept.

(b) Preferably, the UHG is derived from rworded

rainfall-runoff events recorded at stream gages. These

“known” unit hydrography may be related to measurable

basin pmeters through regression analyses to determine

unit hydrographyparameters at ungaged sites throughout

the watershed. This procedure is the same w described in

paragraph 4-3. Where no gage data are available, generalized techniques, such as the SCS methods, are

appropriate.

(c) The advantages of the unit hydrographymethod

include: extensive experience with usage, welldocumented theory, and applicability to the development

and use of regional parameters. The disadvantage is that

rainfall excess over the basin is transformed to a discharge hydrograph at the mouth, without specific

5-5
EP 1110-2-10

2oMay94

TIME

/

A

HYDR~#H FROM

IIISCNNGE 3, Re STORM

<CFS) AT VAT~SKD

MOUTH

.~

1234 S678 9 10

TIME

Figure S7. Unit hydrogreph concept

regional parameters. W disadvantage is that rainfall

excess over the basin is transformed to a discharge hydrography at the mouti without specific acwunting for the

movement of nmoff over land surf-. Unit hydrography

may differ somewhat as storm intensities increw therefore, using the same unit hydrom for a 2-in. storm and

for a lo-in. storm is getiy not advisable.

(2) Kinematic wave method.

(a) This technique was developed in the 1950’s and

attempts to trace the movement of nmoff through the

watershed to b basin outlet. The main assumption of

this technique is that water moves “cinematically,” or at

the slope of the land surface or channel bottom. This

movement is modeled by use of “typical” lengths and

slop for overliuri flow, ~llector channels, and the tributary or main channel. Friction values must *O be assigned to each element. Fi~ 5-8 and 5-9 skw cow

ceptually the watershed modeling and individual elements

used in applying the kinematic wave pr-dure,

respectively.

/1 FLLIVSFROM ❑VERL&:DKFNLTOsU

/-”

G+

%9

/

~h

SliBMSIN XNFLtlW / /

Figure 5-8. Watershed modellng using the kinematic wave method

5-6
EP 1110-2-10

31 Jul 94

-,91

Figure 5-9. Elements used in kinematic wave

eatoulatkns

(b) Application of this procedure requh considerable judgment in selection of ap~opriate variables for

each flow strip and to evaluate the discharge hytigraph

output for reasonableness. The advantage of this technique is that it is more physi~ly based and conceptually

complete in terms of the physics of runoff. The main

disadvantages are difficulty in determining average strip

lengths, slopes, and roughnesses, and reduced applicability

for low-slope land surfaces and charmels.

d. Base jlow and recession f70w. The pmeding

discussionfocused on rainfall ex~ss and the resulting

direct runoff. me restiting discharge hy&ograph dnot include the streamflow that would have occ~ed

without any rainfall excess, or the water that enters the

stream from groundwater flow well after tit runoff has

ended. The former inflow is called base flow and the

latter is termed recession flow. Figm 5-10 illustrates

base and recession flow segments of the total discharge

hydrography. Base and recession flow are relatively small

portions of the runoff hydrographyfor small watersheds

that are sometimes ignored, especially for small urban

catchments. These parameters become important as the

t t

Figure 5-10. Base/recession flow hydrography

components

basin arm increases and certainly cannot be ignored for

large watersheds.

5-5. Routing Concepts

After the foregoing analysis is complete, a discharge

hydrographyhas been computed at the outlet of a subarea

This hydrographymoves downstream, combines with

otherhydrographs, and moves through the channel and

floodplain towards the mouth of the main river. Means of

accounting for hydrographymovement is by routing, Routing is simply a method of translating the hydrographyin

time and accounting for the hydrograph’s change in shape

as it moves through the stream system. Hydrologic routing accounts for changes in the time distribution of volume and employs a relatively straightforward computation

procedure. Figure 2-12 illustrates the basic concept of

hydrologic routing. Hydraulic routing, or unsteady flow

computation, is much more difficult to apply and can

include the effects of pressure and momentum changw.

The application of hydraulic routing requires an engineer

with special experience and is further addressed in

Chapter 6.
EP 1110-2-10

31 Jul 94

a. Hydrologic routing computations.

(1) Routing techniques. Many techniques are available for hydrologic routing, ranging from simple graphicaf

methods to more complex techniques. These methods

include: lag-average, Tatum, Muskingum, MuskingumCunge, modified Puls routing and others (USACE 1990a).

All methods attempt to account for translation time

through the reach and for reach storage. The selection of

an appropriate routing procedure depends on the judgment

of the engineer, the availability of information to determine routing puameters, and the type of flood damage

reduction project under investigation.

(2) Reservoir and Puls routing. me most conceptually complete methods are reservoir (flat pool) and Puls

routing. The procedures for both are similar and directly

account for the storage available in the routing reach.

Figure 5-11 shows the results of a typical reservoir and

channel operation. Figure 4-5 shows the routing operation

as part of the overall modeling process.

(3) Routing example. Possibly the easiest way to

visualize a routing operation is with a reservoir example.

A dam constricts the outflow to whatever opening is

designed through the dam structure (conduit and spillway). Consequently the inflow hydrographyis largely

stored behind the dam and released at a lower rate

through the outlet, over a much longer time period. The

storage berrind the dam and the characteristics of the

outlet structu~ must be known to determine the outflow

hydrographyfrom the dam. A hydrologic analysis of the

latter two features will result in a storage versus outflow

relationship. This relationship plus the inflow hydrograph

can be used to route the inflow hydrographythrough storage, determi,,ing the outflow hydrographyand the maximum pool stage. This operation is important to determine

the adequacy of the spillway discharge capacity and to

ensure that the dam is higher than the design pool

elevation.

(4) Routing reaches. The subdivision of a total

watershed into subareas determines the routing reaches

I

CHANNEL ROUTING

RESERVOIR ROUTING

A

B

B

Q

TIME

TIME

Figure 5-11. Examples of reservoir and channel routing

S8
required. Travel times and storage within these reaches

are determined so that routing operations may be carried

out and the total hydrographymay be translated downstream. Figure 5-11 shows that reservoir routing greatly

affects both timing and shape of the outftow (routed)

hydrography, while channel routing mainly affects the

timing of the outflow (routed) hydrography.

(5) Flood reduction components. Routing studies are

important to evaluate the effects of flood reduction components throughout the watershed. Reservoir routings are

carried well downstream to evaluate reduced flooding

attribumble to the structure. heal protection projects

(levees and channel modifications) may affect nearby

aras adversely by removing or reducing storage available. The magnitude of these changes can only be

addressed by routing studies with and without the flood

reduction component.

5-6. Calibration of the Model

a. General. All of the foregoing components are

incorporated into the overall hydrologic model to simulate

discharge hydrography and determine discharge-frequency

relationships throughout the watershed. However, prior to

developing this information, the model must be operated

for storm-flood events having known input and output to

ensure that the model is reproducing actual floods. This

process is died “calibration” and is a key part of the

total hydrologic modeling process.

b. Calibration process. Historic rainfall from one or

more storms is used as input to the total model, which

consists of a number of subareas and routing reaches.

The model determines losses and rainfall excess, transforms excess to discharge hydrography, and routes and

combines the hydrography through the watershed. Calculated hydrography are compared with recorded hydrography at gage locations in the watershed. When the

model reasonably reproduces known hydrography at the

gages, the model is considered to be calculated. If the

reproduction of an actual event is pmr, one could consider adjusting loss rates, runoff transform coefficients,

routing coefficients, etc. (within reasonable limits) to

obtain an improved simulation.

c. With calibration, the modeler can have increased

confidence that the application of hypothetical (frequency)

rainfalls to the model should result in representative runoff hydrography of that frequency event. Calibration is

completed when discharge hydrography, measured versus

EP 1110-2-10

31 Jul 94

calculated, may be compared. Figure 5-12 shows a successful calibration of model output compared to recorded

discharge information at a stream gage. In the absence of

extensive gaged data, comparison of a calculated peak

discharge against that calculated by the regression analyses of paragraph 4-3c, or against high-water marks (after

calculating a water surface profiie with the hydrologic

model’s output for peak discharge) may be used to calibrate the model.

Figure 5-12. Example of a successful calibration

5-7. Verification of the Model

Verification is the final process in hydrologic modeling,

after satisfactory calibration has been achieved. Model

verification is the process of utilizing additional known

data not used in the calibration process to verify that the

calibrated model will give good results for unknown

storm-flood events. The calibrated model is used with

additional historic rainfall to give discharge hydrography

for comparison with gage data. No adjustments of the

calibrated model are made in the verification process. The

highest level of confidence in model output is achieved

when the calibrated model successful y ~produces the

known hydrography with this additional historic data.

However, verification is not always possible, as sufficient

known storm-flood events may not be available for both

calibration and verification.

5-9
Chapter 6

Determination of Flood Elevations

6-1. River Hydraulics

Chapter 5 presented methods for determining a peak

discharge or volume of runoff from a flood event. However, much of flood analysis and design requires the

severity of a flood to be measured in terms of a depth,

water surface elevation, or area flooded, rather than peak

discharge. This chapter describes the general methods

used to determine water surface elevation, given flow.

a. Simple versus complex. Many methods exist for

making the conversion from peak dischargeto flood elevation, ranging from a simple rating curve to multidimensional analysis. Each requires increased increments

of time, money, and engineering experience to be successfully applied. Paragraphs 6-2 through 6-5 describe the

most common methods and give a basis for proper

method selection.

b. Steady versus unsteady analysis. Ftood elevation

analyses may be subdivided into those based on steady

flow (discharge is constant with time) and those based on

unsteady flow (discharge varies with time). The latter is

closer to the real-world situation; however, the great

majority of analyses of river hydradics can be made

assuming steady flow. Unsteady flow evaluations are

considerably more complex. Paragraphs 6-3 and 6-4

describe these two types of analyses.

c. Rigid versus mbile boundary. Alluvial strms

experience modifications to their geometry with time, due

to sediment transport. Erosion and deposition cause

increases or decreases in a stream’s flow capacity, which

can be reflected by changed flood elevations. However,

most flood elevation determinations may be satisfactorily

made by assuming that the stream boundary is rigid,

greatly simplifying the river hydraulics anatysis. Pmgraph 6-6 discusses mobile boundary hydraulics and its

application.

6-2. Development and Use of Rating

Relationships

a. Gage sites. me conversion of discharge to river

stage, or water surface elevation, is most accurate (and

easiest) when performed at a gage. Continuous measurements of stage, along with periodic measurements of flow,

serve to give a direct relationship for discharge, when the

stage is known. Figure 6-1 gives an example of a

EP 1110-2-10

31 JuI 94

stage-to-discharge relationship at a river gage. This relationship is developed by many years of data accumdation

at the gage site. As seen, many points are available for

discharges within banks or that slightly exceed bank-full

stages. Higher discharges occur infrequently, only during

floods, and only a few points in this portion of the rating

curve may be available. me fewer actual data points, the

more uncetin the relationship.

0

INBANKS OIJTw

EAMS

#

DISCHARGE

Q 500

Figure 6-1. Rating curva developed from gaged data

b. Rating curves. Changes in land use, channel

cotilgumtion, and boundary conditions serve to cause

differences in water surface elevations for the same discharge. As mentioned earlier in this report, discharge

measurements are not absolute and an error of 5 percent

or so compared to the “true” discharge is not unusual.

Consequently, a rating curve is usuatly a best-fit relationship drawn through the accumulated data points. Similar

recurrences of past discharges may result in stages somewhat higher or lower than the past stages recorded.

c. Usefulness of rating relationship. As one moves

upstream or downstream from a gage site, the rating relationship provides less useful information. Synthesizing a

rating relationship at ungaged locations normally requires

computations of water surface profiles using a computer

program. Consequently, a measured rating relationship is

most useful at the gaged site for calibrating a river

hydraulics model to reproduce known stages for measured

discharges.

6-1
EP 1110-2-10

31 Jul 94

6-3. Stssdy-Flow River Hydraulics

The use of available gaged data alone is seldom sufficient

for a flood study. A flood study is normally performed

for a l~gth of stream, with flood information nmssary

throughout the reach, not just at a gage site. This requires

the calculation of water surfs elevations at many locations along the reach. This establishes a water surface

elevation profile for a given flood discharge, and is usually accomplished by using a computer program. These

programsassumesteady,gradually varied flow with a

rigid boundary. A steady flow assumption postuhtes that

the discharge changes so slowly with time that it can be

assumed to be constant for the computation period. A

gradually varied flow assumption states that depth and

velocity for a Pflc discharge change in very small

increments with distance as calculations proceed along a

reach of river. For the vast majority of all water surface

profde computations, these two key assumptions are quite

acceptable and form the basis for steady-flow river

hydraulics analysis (USACE 1990b).

a. Basic principles,

(1) Given the above two assumptions, steady-flow

river hydraulics analysis utilizes the conservation of mass

(continuity) and energy principles (Chow 1959). Figure 6-2 shows the basic equations for computing steady,

gradually varied profiles.

(2) The conservation of energy equation states that

energy cannot be created or destroyed. Changes in energy levels from one point to another in the stream system

occur when flowing water loses elevation in overcoming

friction effects between the two points. These energy

losses are primarily from boundary friction, with some

additionrd 10SW due to cross-wtion geometry fluctuations. Changes in area and velocity at each point are

calculated by the continuity equation. Velocity at each

point is found by use of Manning’s equation.

(3) Methods and procedures for steady, gradually

varied river hydraulics analysis are well-founded and

understood. However, application of the technique

requires the acquisition of considerable input data.

b. Geometric data.

(1) Introduction.

(a) The geometry of the stream reach under investigation must be defined. This requires surveying and

mapping work. Aerial contour mapping gives the most

information on the overbank areas, with supplemental

channel cross sections taken in the field. Crossing

obstructions must also be described. Although acquisition

of this survey data is expensive, the data have a variety of

uses be,sides hydradic modeling, including elevation of

structures for economic analysis and topographic information for structural flood control measures.

(b) Cross-sectional locations coincide with the calculation steps of the finite difference profile anatysis process. They are commonly located for the physical and

hydraulic reasons listed below.

●

●

●

✎

✎

Where distinct changes in stream bed slope mar.

Immediately upstream and downstream of locations

where changes in discharge occur.

Where variations in geometry, including abrupt

expansions and contractions in flow geometry,

occur.

Where variations in channel and overbank resistance occur.

At bends in the stream to ensure that channel and

overbank reach lengths are correctly defined.

(c) Interpolated cross sections may be required to

provide sufficient computation points to accurately compute the energy loss (USACE 1986).

(2) Friction loss coefficient data. hss coefficients

are determined by the hydraulic engineer from field

inspection of the study reach, comparison with published

references, and by engineering judgement. Friction loss

coefficients (Manning’s n) are often used as the main

adjustment parameter to improve the calibration of the

hydraulic model.

(3) Discharge data. Discharge is md from

discharge-frequency relationships that are determined by

hydrologic modeling or statistical analyses, as described in

previous chapters.

(4) Other data. Other needed information

(expansion-contraction losses, flow regime, boundary

conditions, etc.) usually require minimal time and effort to

develop.

(5) Calibration dam. Models using gradually varied,

steady-flow assumptions are calibrated to reproduce

known water surface elevations with known discharges at

gage sites. The main calibration technique is the adjustment of “n” values, the hydraulic parameter which contains

.

6-2
EP 1110-2-10

31 Jul 94

CRDSS SECTICIN 2

CROSS SECTION 1

Continuity fi uation

Q= AI XVI= ~2XV2 . .. . .. .. . .. .. .. . .. .. ... . . .. . .. . . . .. Equationl

Manning’s Equation

V=(l.486/n)XR0”67XS~-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...Eq~tiOn2

EnerE~ Euuation

22 +Y2 +a2(v’22/2g) ‘zl +yl +a1(V12/2g) +he . . . . . . . . . . . .. Equation3

where

Q=

A=

v=

n =

R=

Sf =

z =

Y =

g

=

Z+y =

he =

discharge, cubic feet per second

cross-s~tional area, square feet

average velocity, feet per second

Manning’s coefficient of friction, dimensionless

hydraulic radius (ar@wetted perimeter), feet

friction slope, feet/foot

elevation of channel invert

channel depth, feet

gravitational constant, feet/see/see

water surface elevation, feet above a datum (usually mean sea level)

energy loss between sections, feet

a = velocity distribution coefficient, dimensionless

a(V2/2g) = velocity had, feet

Figure 6-2. Gradually varied, steady-fiow equations

6-3
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31 Jul 94

contins the most uncefinty. Without sufficient gaged

data, the hydrologic engineer attempts calibration by

reproducing high-water marks from one or more actual

floods, as obtained from interviews with local residents.

c. Applications. Gradually varied stmdy-flow techniques m the primary method of determining flood elevations for most hydrologic analy~ and have a wide

applicability for Corps hydrologic studies. Common

applications include

(1) Development of flood profdes for land use planning for flood insurance/floodplain studies.

(2) Development of flood profiles for urban and rural

flood damage evaluations.

(3) Determination of changes in flood elevations due

to s~chual flood control improvements.

d. Li?rdtutions. Gradually varied, steady-flow analysis

can be considered applicable as long as (1) the discharge

is steady with time and gradually varied with distanm,

(2) the discharge can be considered one-dimensional (a

single elevation for one cross section), (3) the river slope

is small (less than one in ten, so a hydrostatic pressure

assumption is correct), and (4) ach cross swtion is rigid

(no significant scour or deposition). When any of these

assumptions is not acceptable, other techniques must be

used.

e. Need for advanced analysis. The complexity of

determining flood elevations increases significantly when

more advanced analysis techniqu~ are required. These

techniques are usually nmessary when the discharge

changes rapidly with time, thereby causing flow momentum to become signtilcant. The kinds of situations requiring more detailed computational analysis include

(1) Dam break analysis.

(2) Flood elevation predictions at multiple points and

times for very mild slopes.

(3) Where downs- boundary effects are changing, such as those caused by tidal fluctuations.

(4) Where flow is mpidly varying, such as during

hydropower operations, during locking operations, sudden

opening or closing of gates, abrupt start and stopping of

pumping plants, and flash floods on small streams.

6-4. Unsteady-Flow River Hydraulics

The next higher level in river hydraulics computational

difficulty is the application of one-dimensional unsteady,

varied flow analysis. One-dimensional means that one

elevation is still characteristic of each computational

point, or cross mtion; however, now the computations

= tilng performed at all time periods as well as all

points along the center line of the river. Changes along

the channel length can also k gradually varied with this

technique. Figure 6-3 illustrates the difference betwmn

the results of steady versus unsteady analysis. Differences betw=n steady and unstmdy flow analysis can rdso

be visualized by imagining one is standing on a riverbank

and observing the moving water. Steady-flow analysis is

adequate when the water surface appears to rise and fall

uniformly, without any observation of curving stralines.

Unsteady flow analysis is necessary if one would observe

an advancing wave front moving downstream, with obvious curvature to the streamlines. Figure 64 further illustrate this concep~

a. Hydrologic versus hydraulic routing. Unsteady

flow analysis is often referred to as hydraulic muting,

because elevations, velocity, and discharge information

are being calculated at all time periods and for each

desired location. Unsteady flow analysis can be broken

into two groupings hydrologic or hydraufic routing.

Hydrologic routing is discussed in paragraph 5-5a.

Hydraulic routing includes both continuity and momentum

conservation and yields information on velocity, discharge, water stiace elevation, travel times, etc. at each

computational point. This section will be concernedonly

with hydraulic routing, or graduatly varied unsteady flow.

b. Basic principles. Unstwdy flow analysis is

required when the inertial effects of flow, resulting in

unbalanced momentum, are large enough that they can no

longer be ignored. The listing of unsteady flow situations

in paragraph 6-3e represents many of these cases. The

basic equations for one-dimensional unsteady flow analysis are given in Figure 6-5. As seen, the difference

between stady and unsteady flow analysis is the inclusion of the local acceleration term in Equation 2, along

with the more rigorous presentation of the continuity

equation in Equation 1. Solution of the unsteady flow

equation is difficult and requires significant computational

operations, necessitating a high-speed computer. A number of unsteady flow analysis programs are available, e.g.,

Fread (1978).

6-4
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31 Jul 94

PUiNT 1

Figure 6-3. Steady- versus unsteady-flow analysis

Figure 6-4. Visualization of unsteady and steady flow
EP 1110-2-10

31 Jul 94

Equations for a Wide Channel with No Lateral Inflow

CRCISS SECTION 2

CROSS SECTION 1

Continuity Eu uation

y (aV/ax)+ qav/ax)

Momentum EUuation

Sf = so - (ay/ax) -

where

Y =

v =

x =

t

=

g

=

so =

Sf =

a =

2

x

1

x

+(~Y/~~=0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equation4

~(avnq - ~(av/at) .........................Eq~tion5

depth, feet

velocity, feet per second

distance, feet

time, minutes

gravitational constant, feetisecond/swond

channel invert slope, feet/foot

friction slope, feetifoot

partial derivative, “change with respt to”; i.e., av/aX is the change in velocity with

respect to distance

Figure 6-5. Unsteady, gradually varied flow equations

6-6
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c. Data requirements. Requirements are similar to

those of steady, gradually varied methods, with the exception of boundary conditions and calibration data. For

hydraulic routing, the boundary conditions must be completely deseribed as a stage or flow hydrography. It is

generally preferable for the stream gmmetry to be better

defined (more cross sections) than for steady-flow methods. Calibration data for unsteady flow are also more

extensive, requiring stages and/or discharges at a number

of different time periods. This information is usually

more costly to obtain than calibration data for stady-flow

applications.

d. Applications. The common applications of onedimensional unsteady flow anatysis have been previously

stated in para~aph 6-3e. A number of unsteady flow

models have been developed in recent years and utilization of these types of models has been made easier. A

higher level of engineering expertise is still necessary,

however, to use these techniques. As most situations for

calculating flood elevations use steady, gradually varied

flow, fewer individuals are sufficiently knowledgeable and

experienced to properly apply and interpret the results of

unsteady-flow models.

e. Liw”(ations. Since this method accounts for more

of the physical processes that are occurring than stady -

flow analysis does, there are fewer limitations. As the

next level of analysis is still more complex, situations

where one-dimensionat unsteady flow solutions are computationally inadequate are fortunately few. Situations

requiring a higher level of computational analysis include:

(1) Analysis of flow patterns in bays and estuaries,

where velocities and elevations may vary in the horizontal

and vertical directions.

(2) Cases in which a one-dimensional assumption

cannot model the elevations with sufficient accuracy; i.e.,

multiple bridge openings across a wide floodplain, major

river junctions, etc.

(3) Analysis of flow patterns around dike fields,

hydropower plants, and cofferd.ams.

6-5. Multi-Dimensional River Hydraulics

Although nearly all flood elevation determination requirements can be satisfied with either one-dimensional steady

or unsteady flow models, certain specialized problems

occasionally require a yet more sophisticated and complex

modeling approach. Use of multi-dimensional river

hydraulics is necess~ when one can no longer assume

that a single elevation at each computational point (cross

section) is appropriate. This problem requires the use of

a two-dimensional (2D) model, where hydraulic properties

vary across the section as well as along the length of

stream, or of a three-dimensional (3D) model, which

would include changes of hydraulic properties in the

vertical direction. Three-dimensional computer models

are currently under development and testing and are not

yet fully available. Thrm-dimensional efforts have

largely been through the application of physical models,

the subjwt of paragraph 6-7. Only 2D modeling is

addressed further in this section (EM 1110-2-1415).

a. Principles. Multidimensional models are usually

applied to evaluate a short mch of river, where average

depth is small compared to the average str~m width.

Because of the relative shallowness compared to length

and width dimensions, differences in the vertical for

hydraulic properties are often averaged to obtain a 2D

solution. This grmtty simplifies the work effort. me

basic equations to solve 2D unsteady-flow problems are

lengthy and are not included here. Assumptions inherent

in the application of this technique include gradually

varied flow, constant water density, and a rigid boundary

(or one that is changing insignificantly).

b. Data.

(1) General. Data requirements are considerably

grater than for previous methods. It is normatly insufficient to utilize a data set developed for steady or onedimensiond unsteady flow in a multi-dimensional model.

(2) Geometry. Geometry is usually derived from

map dam. Close interval contour mapping is most desirable, with 0.5-foot intervals often used. Since most applications of 2D models are for detailed analysis of a short

reach of stream, this type of topographic information is

usually feasible.

(3) Turbulent exchange coefficients. Turbulent

exchange coefficients, used for modeling eddy losses, are

required in addition to other coefficients such as

Manning’s n.

(4) Velocity. Velocity and velocity direction measurements are needed. As vertical velocities in a 2D

model are depth averaged, these prototype measurements

also must be depth averaged. Depth, water surface elevation, and veloeity data at many points in the distance-time

grid must be obtained.

6-7
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(5) Acquisition. Data required for 2D models are

site-s~ific and usually developed through a data collection program. Data acquisition is a considerable cost for

2D modeling.

c. Applications. Often, use of a multi-dimensional

model requires contracting with a Corps Lab or a private

consultant to develop the input data and operate the

model. Considerable start-up expense and time are

required to educate a new user of a multi-dimensional

model, afthough if additional applications in the near

future are foreseen, in-house capability should be further

investigated. Figure 6-6 shows a typical application of

2D modeling. Other examples were indicated in paragraph 6-4e with additional applications, including the

following:

(1) Channel deepening. Investigating the effects of

deepening a ship channel on velocity patterns and

shoaling.

(2) Encroachment. Investigating the effects of major

encroachment into a river channel on flow patterns and

water surface elevations.

(3) Velocity and flow patterns. Investigating the

velocity and flow patterns of water entering and leaving a

wide floodplain from the river channel.

d. Limitations.

(1) PracticaJ limitations. The practical limitations of

2D models are in their application and in the user skills

required. Because of input data needs and computational

requirements, applications are normally for a short (1 mile

or less) reach of river. Qualified personnel skilled in

utilizing 2D models are often more difficult to obtain.

(2) Technical limitations. Technical limitations

include the necessary assumptions of gradually varied

flow and of insignificant changes caused by sedimentation

4 ‘“

-V~NG WATERSURFACE

Figure 6-6. 2D flow representation in Cache Creek settling basin

6-8
EP 1110-2-10

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or erosion. If the vertical depth components cannot be

averaged and a 3D simulation is nmmsary, physical models must be employed.

6-6. Mobile Boundary Hydraulics

Mobile boundary anatysis is n~essary when the assumptions of a rigid boundary are no longer valid. For most

streams, a rigid boundary assumption is acceptable for a

design flood, as channel and overbank geometry are typically slow to change in response to the sediment transport

characteristics causing scour and deposition. Over time,

however, the stream does respond to changes in its sediment ~gime by adjusting its cross-sectiomd gametry,

stream slope, bed material composition, sediment load,

etc. Mobile boundary analysis is thus generally concerned

with the longer-term trends over the tife of a flood reduction or navigation projwt. The complexity level of a

mobile boundary analysis is similar to that of a onedimensional unsteady flow analysis (USACE 1991a).

a. Basic principles.

(1) Assumptions. The most common mobile boundary analysis incorporates a number of important assumptions, including:

(a) The analysis is one-dimensional (single water surface elevation at each point).

(b) me channel slope is small.

(c) Sediment-water density is constant.

(d) Manning’s n value applies.

(e) Gradually varied flow occurs along the stream

channel.

(2) Models. These assumptions may be incorporated

into mobile boundary models. me models commonly

combine a gradually varied steady flow analysis with

sediment transport calculations at the end of each flow

period. Changes in channel geometry are calculated

before starting the next computation period. Computations would normally take place over severat years of

di~harge data to identify long-term trends occuting in

channel gmmetry and water surface elevations.

(3) Rmults. The model may be calibrated and operated to give information like channel invert and water

surface profiles. The most valuable information is the

ident~lcation of trends and the comparison of the effecs

of a flood mitigation component on the sediment regime.

Model re,sdts can be used to evaluate and compare withand without-project conditions. A typical application

would determine how fast a channel improvement, or a

reservoir, loses capacity due to sediment deposition.

b. Basic data. A mobile boundary analysis typically

requires the most data of any of the methods of analysis

described in this chapter, necessitating hydrologic, geometric, and Sedment information.

(1) Hydrologic data. Discharge data are nmded for

all flow periods, from flood to hught. The time duration associated with each of the actual discharges is atso

necessary. The discharge and time data are often converted from a continuous, smooth hydrographyto a histogram, or bar graph, averaging smaller flows over long

time periods. The water temperature is also important, as

it has a significant effxt on how fast small particles settle

in the water column.

(2) Geometric data. Channel cross sections and

reach lengths are required, similar to the information

necessary for a gradually varied steady flow model.

Geometric data are normally less extensive than for a

water surface profile analysis, however. Longer distances

between sections are tolerable, and bridge sections are not

normally included. Manning’s n values are used for

boundary friction estimates.

(3) Sediment data. The sediment composition of the

channel section at each point is needed, with this data

coming from botings and/or “grab samples by the engineer in the field. The amount and composition of sediment flowing in the water column for a wide range of

discharges must be determined for the main channel and

any significant tributaries. T’hisinformation is best

obtained from actual mmsurements of sediment load at

gage locations, but may be derived in the absence of any

real data. The unmeasured, or bed load (that moving

within a few inches of the channel surface) must be estimated and included. Geometric and channel sediment

composition data require measurement at two or more

widely separated time periods to provide calibration information for the sediment transport model.

c. Appli~ations. The primary application of sediment transport models is to evaluate with-project against

without-project conditions to determine long-term trends

affecting project design and operation and maintenance of

the project. Typical applications include:

6-9
EP 1110-2-10

31 Jul 94

(1) Determining sediment rate in reservoirs and

length of useful life.

(2) Determining rate and location of deposition in

channel mtilcations to estimate frequency of dredging

and sediment removal, thereby maintaining design channel

capacity.

(3) Determining deposition along a levee over time

and the comsponding eff~ts of this deposition on

increasing flood heights, thereby decreasing the levee

protition.

(4) Maintaining adequate depth at dl times at locations where this is important such as for navigation

channels.

(5) Monitoring locations where great changes in

channel geome~ occur during a flood, such as flow

across an alluviat fan.

d. Li~”tarions. Limitations for one-dimensional

sediment transport analysis are the same as for onedimensional unsteady-flow problems. Sediment scour and

deposition that cannot be assumedreasonably uniform at a

channel wtion require multi-dimensional or physical

model mting. Scour evaluations around cofferdams,

navigation locks, or similar structures usually require a

higher level of analysis.

6-7. Use of Physical Models

by the component or problem under study. Threedimensional analysis most often results in physical model

@ting. These models are normally expensive m build

and operate, and require particular engineetig expertise

to utilize. Typical applications of physical modeling

include:

a. Analysis of river navigation improvements on

channel geometry and sediment characteristics.

b. Vetilcation/modification of hydrautic design of

flood reduction components to minimize operational problems and optimize performance under all adverse

conditions.

c. Simulation of navigation through potential hadous river reaches.

d. Water quality simulations, dispersal of pollutants,

and temperature stratification in reservoirs.

6-8. Comparison of Flood Elevation Determination Methods

Although comparisons between the various methods have

been made throughout this chapter, additional comparisons

are provided in the following tables. Table 6-1 illustrates

when the various methods are usually appropriate for

different reporting levels, while Table 6-2 gives a mther

subjective appraisal of the differences in experience level,

time, money, data needs, and computer requirements for

the various techniques.

Physical models are employed when mathematical models

cannot adequately simuhte the fu~ range of effects caused

6-1o
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Table 6-1

Modal Ueage for Hydrologic Engineering Studies

Study Existing Data

Stage & Criteria(’) GVSF MB GVUSF Multi-D Physical

Reconnaissance x x

Feasibility x x

X(2) 7(3)

Reevaluation x x

X(2) 7(3) ~

DM X(4) X(5)

(1) Existing data and criteria = available reports, U.S. Army Engineer Watemays Experiment Station (WES) criteria, regional relationships

for depth frequency, nmal depth rating relationships, etc.; GVSF = gradually varied steady flow; MB = mobile boundary analysis; GVUSF =

gradually varied unsteady flow, multi-dimensional analysis, Physical = physical models (by WES or similar agency).

(2) Use is possible, but unlikely, on most flood control studies

‘3) ? Possible, but very unusual--very dependent on problem being analyzed.

‘4) Typically employed to evaluate design petiormanca for a shofi reach of river, or in the immediate vicinity of a specific project component, or refine the hydraulic design of a project mmponent.

‘5) Typically performed to evaluate 3D or other specific conditions where mathematical modeling results are considered inaccurate.

6-11
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Table 6-2

Qualitative Comparison of Different Analyais Technique Requirements(’)

Hydraulic Speoiaf Tachni~l

Analysis Engineer’s Expertise Computer Data Study

Technique Time Requirements Requirement Requirement cost

Existing or 1 None(2) 0,1 1 1

simplified

criteria

Gradually varied, 10 None(2) 10

steady flow

Gradually varied, 30 ~me(3) 20

unsteady flow

10

20

50

100

Mobile boundary 30 *me(3) 40 30 150

analysis

Multi-dimensional 40 Many(4) 100 50 200

anafysis

Physical 100 Severe(5) ... 100 500

modeling

(1) Comparisons among techniques would be as follows: multi-dimensional analysis would require four times the amount of engineer time

and five times the amount of data compared to the gradually vatied, steady-flow technique.

(2) “Average” hydraulic engineer can adequately handle this technique.

(3) “Average” hydraulic engineer has limited experience in these techniques.

(4) “Average’ hydraulic engineer has no experience in this te~nique, specialized training/assistance by ~nsultants may be necessary.

(5) Would require the use of WES or similar consultant

6-12
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Chapter 7

Hydrologic Engineering Requirements

Flood Damage Reduction Measures

7-1. Ovewlew

This chapter provides an overview of the hydrologic engineering analyses necessary for the major structural and

nonstructural measures of flood damage reduction studies.

The types of analyses and hydrologic methods dwribed

in the previous chapters are used to show the analysis

requirements for different @s of mmures.

7-2. Without-Project Conditions

a. Flood damage analysis. Corps of Engineers flood

damage reduction analyses for different projects, both

structural and nonstructural, are similar in method. The

first step is the analysis of the discharge or stage versus

frequency of flooding relationships at key points in the

stream system for the existing or base project conditions.

This step is repeated for at last one time in the future,

assuming future land use conditions will result in changing discharge/stage versus frequency relationships. The

development of existing and future, without-project hydr~

logic and hydraulic relationships is criticat to establish the

magnitude of the flood problem so that flood damage

anatyses may be performed. The flood damage analysis

provides insight as to the location and the amount of

existing and future expected damage, and therefore the

amount of project costs that one could spend to mitigate

the flood darnage.

b. Hydrologic engineering studies. Hydrologic engineering studies nomatly require considerable time

establishing the existing and future without-project relationships by performing rainfall-runoff, frequency, river

hydraulics and reservoir operation studies. Specific methods used during the analysis of each flood damage reduction measure are based on the amount of data available,

the complexity of the study area, and the needs of the

Interdisciplinary Planning Team (IPT).

7-3. Screening of Alternatives

a. Structural measures. Following development of

without-projwt conditions, analysis of different structural

and nonstructural flood damage reduction measures is

performed. Not all mmsures presented in this chapter

would likely be evaluated in a specific study. Rather, the

IFT, including representatives of various Corps disciplines

and the local cost-share partner, wodd identify one or

more likely feasible measures and plans to evaluate for

the study area. Reservoirs are practical because they

reduce flooding at downstream locations; however, they

are often the most costly alternative and the most difficult

to economically justify. If flood darnage reduction is for

a single site along a stream, a local protection project

(channel modification, levee, or diversion) would be

examined. These projects m normally lws costly than a

reservoir and provide site-spec~lc protection to a single

area. However, local protection projects can have adverse

effects on flooding elsewhere.

b. Nonstructural measures. Nonstructural measures

are required to be analyzed as a means of reducing flood

damage (Section 73 of Public Law 93-251). Nonstructural alternatives may be examined with structural solutions, or by themselves. ~ese solutions are typically the

least expensive, but often provide the least flood damage

reduction to the area If the existing/future withoutproject damages are small, nonstructural solutions may be

the only ones feasible.

7-4. Reservoirs

The intent of flood control reservoirs is to store and gradually release upstra flood runoff after downstream

flooding is over. Reservoirs are practical flood damage

reduction solutions &ause they reduce flooding throughout the downstr~m river system, although the effects of

the reservoir decrease as the distance from the reservoir

increases. A flood control reservoir is anatyzed to accomplish flood damage reduction and to ensure safety of the

structure in extreme floods.

a. Flood control.

(1) Flood control analysis determines the storage

volume in the reservoir that should be reserved to control

flooding. The hydrologic modeling effort requires varying

magnitudes of floods to be routed through the reservoir

and to downstream damage centers. The anatysis yields

with- and without- reservoir discharge-frequency relationships. Figure 7-1 illustrates this analysis.

(2) Historic data for the routings are prefemed and

are usually available for sites in larger rural areas. Urban

reservoirs usually have little or no data and synthetic

rainfall-runoff modeling is normally employed. Discharge

is converted to stage at downstream locations to determine

project damage. The difference between with- and without-project damage is the flood inundation reduction benefits attributed to the projwt.

7-1
EP 1110-2-10

31 Jul 94

f

Q

A

B

c

Figure 7-1. Effects of a reservoir

b. Safety. A safe~ analysis specifically determines

the height of dam and size of spillway necessary to ensure

that essentially no risk of dam overtopping exists. For

high hazard dams, where overtopping would cause a

downstiam catastrophe, a very high safety design standard - typically the Probable Maximum Flood should be

selected.

7-5. Local Protection Projects

Channels, levees, and diversions are considered local

protection projecs. Protection of a specific damage center is accomplished with each, although channeli=tion,

levee systems, and major diversions have been constructed to protmt a series of damage centers. Each project reduces the seventy and frequency of flooding to the

protected area. They may, in unusual circumstances, also

increase flooding immediately adjacent to the protection

area.

a. Channels.

(1) New channels or modifications to existing channels attempt to d~mase flood stage by increasing charnel

efficiency. The effeet of a channel project is illustrated in

Figure 7-2.

(2) Channelintion is a typical mmsure for urban

flooding situations. An improved channel can provide a

smoother flow path (less boundary friction), increase the

cross-sectional area of the channel, improve the efficiency

of the channel, or combinations of these changes. If an

extensive reach of channelization is to be constructed, the

effects of these changes will be to increase the severity of

downstiam flooding by accelerating the flood hydrography

through the reach, causing higher peak discharges downstream. The hydrologic analysis must address this problem, as well as the beneficial effects of channelization.

River hydraulics dominate channelization studies, with

storage routing becoming more important in determining

adverse effwts as the channel reach becomes longer.

b. Levees andj700dwalls.

(1) Levees and floodwalls prevent floodwaters from

entering the protwted area until the design event is

exceeded and the levee or floodwall is overtopped. Figure 7-3 illustrates the usual effect of a levee or floodwall

for the area protected and for unprotected areas upstream.

(2) River hydraulics is the major analysis component

for evaluating levee grade and alignment, as well as

7-2
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A

A

I I ~

CHANNEL ENLARGEMENT

~AFTER CHANNEL

Figure 7-2. Effects of a channel

certain adverse effects. Upward shifts in river stage for

the same discharge may occur when the levee or floodwall confines the flood to arws outside the protected area.

This effeet may extend upstream from the lev~ unit,

inducing additional flooding to unprotected mas.

(3) An extensive levee projector a system of levee

projects can remove significant floodplain storage. ~is

lost storage can result in increases in pA discharge

downstream of the levee(s). Hytiulic routing is required

to satisfactorily evaluate storage effects on flood

magnitude.

(4) hvees have the potential in unusual circumstances for inducing flooding, both upstream and downstream of the protected area. Thus, the hydrologic design

should minimize these adverse effects as much as

practicaJ.

(5) Levees and floodwalls greatly reduce the dirwt

threat of flooding by the main river or lake. However,

the nature of this solution may introduce a secondary

flood problem, which is the remaining or residual interior

area flooding. This flooding results from interior pending

by rainfall on the leveed interior, blockage of existing

flow paths, and seepage water through the levee during

high interior stages. During lengthy high exterior stages,

interior flooding caused by intenor ponded water could

negate much of the damage prevented by the levee or

floodwall. ~erefore, an interior flood control analysis is

an integral part of any levee or floodwall projwt. Rainfall-runoff analysis and storage operations m the dominate fwtures of interior flood control analyses. These

analyses are complex because they must adtiss the joint

probability of high river stages and of sign~lcant interior

runoff, Period-of-record analysis is preferred, but gaged

data are seldom available for an accurate application.

Hypothetical events are often used. Interior flood control

studies are among the most difficult hydrologic engineering analysis. EM 1110-2-1413 provides additional information on these complex studies.

c. Diversions. These components remove water

from the main channel upstream of the area to be protected, and usualty reintroduce the diverted water downstream of the area. Figure 74 illustrates the impact of a

diversion. River hydraulics is the dominate means of

analysis. The potential exists for adverse effeets on flood

heights downstream of the diversion reentrance. This

problem would be analyzed through storage operations.

7-3
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BEFORE AFTER LEVE{

Figure 7-3. Effects of a levee/floodwall

7-6. Nonstructural Measures

Structural solutions modify the watershed’s hydrology/

hydraulics to reduce flood damage to the protected

arw(s). Nonstructural m~sures operate in a reverse fashion, by reducing the damage potential in the flood-prone

area without changing the hydrology and hydraulics of the

watershed, Rainfall-runoff, frequency, river hydraulics,

and storage operations may be utilized in development of

existing hydrologic/hydraulic conditions. Nonstructural

measures include: floodplain management and flood

insurance, floodproofmg, relocations, and flood wamingpreparedness planning. A reference (Hydrology SubCommittee 1985) further describes nonstructural analyses.

a. Floodproofing. This aftemative minimizes damage

by raising the elevation where floodwaters first enter a

structure. Usual means of floodproofing are the installation of waterproof shields to doorways and basement

windows. Two feet is the practicaf maxim urn depth for

floodproofing before the pressure of water on exterior

walls could result in structural failure. Floodproofing

applications are most suitable when first-floor flooding is

more frequent than a 5-percent chance event, and the

difference between frequent and infrequent flood elevations is 1 to 2 feet.

b. Relocation. This alternative refers to permanently

moving flood-damageable items to a higher elevation

(second floor, etc.) or moving the entire structure to

higher ground. Moving the structure is most feasible

when flooding of the first floor is more frequent than a

5-percent chance (20-yem) event and the structure has

sufficient value for relocation to be wonomicafly justified.

c. Flood warning-preparedness planning. This alternative refers to a formaf system and plan for ascertaining

that a flood threat is imminent and ensuring that

7-4
A—

DIVERS1ON

A

0

BEFORE

DIVERSION

DIVERSION w

Figure 7-4. Effects of a diversion

appropriate actions are taken to minimize the threat to

human life and decrease flood damage. The system usually includes rainfall and river gages upstram of the

damageable area, a communication network to get the

measured information to the appropriate personnel, a

forwast model or other indicator to estimate flood

severity and a detailed response plan to address all necessary actions. In addition to the need to identify flood

EP 1110-2-10

31 Jul 94

stagesat key locations for different flood events, adequate

warning time is needed to take the appropriate actions.

The measured information and the forecasting model must

be accurate to minimize the threat of false alarms and

resulting loss of confidence in the system. The complexity of the system must be commensurate with the

ability of the sponsor to operate and maintain the system.

7-7. Floodplain Management Studies (FPMS)

Thesestudi~ include floodplain management reports,

flood hazard reports, and flood insurance studies. The

FPMS program is intended to provide flood information

for wise land use planning by local mmmuniti~. Knowledge u)nceming future flood levels is instrurnenti in

preventing development of flood-prone land. The hytilogy and hydraulics performed for flood insurance studies

also provide the technical basis for the purchase of flood

insurance by individuals almdy occupying the floodplain.

Flood insurance studies also require additional river

hydraulics studies to establish a floodway, normally for

the l-percent chance event. The floodway specifies the

portion of the floodplain that can be encroached without

adversely affecting upstream flood heights more than a

s~ified amount, normally 1 foot.

7-8. Hydrologic Analysis Requirements Summary

The type of technical studies required to analyze specific

flood darnage reduction mmsures are shown in Table 7-1.

The information presented in Table 7-1 should be considered typical and may vary depnding on specific study

conditions and requirements.

7-5
EP 1110-2-10

31 Jul 94

Tablo 7-1

Hydrologic Anafysis Needs for Ffood Damage Reduction Memures’

Riwr Storage

Measure Rain-Runoff(l) Frequency Hydraulics Operations

A(2) B C DEF GH IJK

Reservoirs

Flood Control Y(3) Y Y YYX

Safety x

Channels Y

Levees Y

interior x

Flood Control

Diversions Y

Floodplain Management x

Nonstructural x

Yx NNN

YY XYN

YY XYN

YY NYN

YY XYN

YN XYN

YN XYN

(1) Dominate analysis types but not necessarily done for every case. For instanm,

flood control studies if the data were available.

(2) (A) Reconstitute historic floods, (B) damlop hypothetical floods, (C) analyze the

Y

Y

Y

Y

x

Y

Y

Y

Y

N

x

x

N

x

N

N

YYX

YYN

XXN

XXN

YYX

XXN

XNN

XNN

historic frequency analysis would be done for interior

changed discharge/stage-frequency, (D) develop historic data, (E) develop from hypotheti~l events; (F) “volume-duration s“W&es, (G) elevation ~stage) con;ersio; from ~scharge; (H) sd”iment

transpotideposition analyses, (1) routing operations, (J) fatility sizing by routing, (K) sequential (period of re~rd) routing.

(3) —Y Usually done (major part of study), ~ Done less often (not a major part of study), —N not usually done.

‘ In raeneral, not a detailed specification.

7-6
EP 1110-2-10

31 Jul 94

Chapter 8

Hydrologic Engineering Studies

8-1. General

Hydrologic engin&ring studies area keypartof an overall Corps flood damage reduction analysis. These studies

form the technical basis to define existing, or base without-project conditions; future without- project conditions;

and the same conditions with project. The best technical

hydrologic engineering analysis cannot be done independent of input by others. Two-way communications with

members of the study team and all other concerned individuals and groups are important. This chapter briefly

describes some of the input of others necessary to perform

hydrologic engineering studies.

8-2. Study Design and Management

The hydrologic engineering study must be planned and

detailed to allow the effective and efficient management

of the technical work. Before any hydrologic modeling or

analytical calculations are undertaken, considerable planning efforts should be performed (ER 1110-2-1460).

a, Scope of study. The overall scope of the study

should be resolved early, ideally while preparing the

Initial Project Management Plan during the reconnaissance

study, through meetings with the entire IPT and the local

sponsor. The time and cost required are a direct function

of the study scope and amount of detail necessary to fully

evaluate the range of problems and potential solutions.

The hydrologic engineer should formalize these scoping

meetings and ideas on addressing the problems through

preparation of a Hydrologic Engineering Management

Plan (HEMP). This plan would be reviewed and

approved by the technical supervisor and furnished to the

study manager. Unusual problems or solutions would

make it wise to receive division review also. The HEMP

is especially important to develop immediately after the

reconnaissance-phme report has identified the problems

for further anat ysis in (and prior to initiating) the

feasibility-phase study.

b. Study team coordination. Every cost-shared feasibility study has an IPT, headed by a study manager. The

team consists of working-level members from the arms of

economics, hydraulics, geotmhnical, design, rwl estate,

environmental, and cost estimating. The local sponsor is

also a member, although the sponsor may not wish to

attend all In meetings. Frequent meetings of the IPT

should be held (once a week to once a month), depending

on the level of study activity and complexity. The advantage of frequent meetings lies in communication and the

exchange of ideas between team members. The most

successful studies are those having free and easy communication among team members.

c. Technical procedures. General technical procedures have b~n addressed throughout this document. me

hydrologic engineer should selmt those procedures which

adequately address the problem(s) under study. Choose

the simplest technical methods that will do this---usually

hydrologic modeling. Where more difficult methods

appear necessary; i.e., 2D unsteady flow analysis, etc.,

these methods should be presented in the reconnaissancephase report for higher level technical review and

concurrence.

d. Quality control and review. The assurance of

quality work and an adequate review come from both the

technical supervisor and the IPT. The development of the

HEMP and the supervisor’s concurrence in the methods

and procedures for study analysis give the hydrologic

engineer a road map for the entire study. Frequent

updates and consultations between the engineer and technical supervisor are important. With these steps followed,

technical quality should be acceptable for the final report.

Similarly, scoping of the problems and necessary hydrologic information supplied to other IPT members will be

accomplished through IPT meetings and discussions.

Unusual technical problems or policy issues may require

the review of higher level authority.

e. Relationship with cost-share par[ner. The costshare partner is a full member of the IPT and often provides twhnical assistance in many arms of the study.

The partner also has valuable insights on the study area

and its problems that may not be apparent to the study

tam. The cost-share partner should have as much (or as

little) input and access to the planning and technicat analysis as desired. All hydrologic engineering negotiations

with the cost-share partner must involve the hydrologic

engineer.

8-3. Level of Detail/Completeness

This subject was more fully addressed in earlier chapters

and is only summarized here. For feasibility reports,

hydrologic engineering must fully address the hydrology

of the study watershed and the level of flooding throughout. Feasible solutions are formulated and evaluated.

These requirements necessitate that hydrologic engineering be complete and final upon completion of the feasibility report. Refined hydraulic design should be the

8-1
EP 1110-2-10

31 Jul 94

primary effort for the hydrologic engineer after the feasibility phase.

8-4. Documentation and Reporting

The t~hnid analysis shodd be fully and completely

pmented in the portion of the feasibility report dealing

with hydrologic engineering. A separate appendix for the

hydrologic engineering effort is normally prepared. The

appendix should present a complete and accurate description of the hydrologic enginmring studies. A reviewer

should be able to follow the logic and thought processes

of the technical engineer and be able to reach the same

conclusions concerning the make-up of the recommended

plan. The appendix should describe the methods used,

input parameters, calibration and vetilcation Procwses,

assumptions made, sensitivity mts performed, alternatives

analyzed, plan selected, consequences of design exceedance of the recommended plan, and overall conclusions on

project effectiveness. The complete, recommended project must be presented including work required by other

Federal and non-Federal agencies nwessary to allow full

functional operation of the recommended plan. The

hydrologic engineer must also prepare the necessary technical studies outline and time and cost estimates for the

Project Management Plan, which must also accompany

the completed feasibility report.

8-5. Local Sponsor Coordination

Sponsor participation in the study process should be continuous. Study layout and scoping, ~ meetings and

decisions, alternative evaluation and project selection, and

report recommendations and review should all involve the

local cost-share partner.

8-6. Summary

The Corps of Engineers has moved into a new era of

feasibility planning, requiring a local partner to participate

financially in the study process. These fiscal requirements by the Corps on the partner must also allow more

participation of the partner in the study evaluation process. Further understanding of the hydrologic engineering

analysis requirements during the feasibility phase by the

local sponsor and others should allow for a better finat

product. This document is intended to provide an initial

step in this direction.

8-2
EP 1110-2-10

31 Jul 94

Appendix A

References

Public Law 93-251, Section 73

ER 1105-2-100

Guidance for Conducting Civil Works Planning Studies

ER 1110-2-1150

Enginwring and Design for Civil Works Projects

ER 1110-2-1460

Hydrologic Engineering Management

EP 1110-2-7

Hydrologic Risks

EP 1105-2-10

Six Steps to a Civil Works Project

EM 1110-2-1411

Standard Projwt Flood Determinations

EM 1110-2-1413

Hydrologic Anal ysis of Interior Areas

EM 1110-2-1415

Hydrologic Frequency Analysis

EM 1110-2-1416

River Hydraulics

Chow 1959

Chow, Ven Te. 1959. Open Channel Hydraulics,

McGraw-Hill, New York.

Fread 1978

Fread, D. L. 1978. “NationaJ Weather Service Operational Dynamic Wave Model, Verification of Mathematical and Physical Models in Hydraulic Engineering,”

Proceedings of 26th Annual Hydraulics Division Specialty

Conference, ASCE, College Park, MD.

Hydrology Sub-Committee 1985

Hydrology Sub-Committ&. 1985. “Guidelines on Community Local Flood Warning and Response Systems,”

Federal Inter-agency Advisory Committee on Water Da@

U.S. Department of the Interior, Geological Survey,

Office of Water Data Coordination, Reston, VA.

McCuen 1989

McCuen, Richard H. 1989. Hydrologic Analysis and

Design, Prentice-Halt, New Jersey.

National Oceanic and Atmospheric Administration

1973

National Oceanic and Atmospheric Administration. 1973.

Precipitation Frequency Atlas of the Western United

States, Department of Commerce, National Oceanic and

Atmospheric Administration, National Weather Semite,

Silver Spring, MD, Atlas 2, Vol 1-13.

National Weather Service 1961

National Weather Service. 1961. “Rainfall Atlas of the

United States, 30-minute to 24-hour Durations, 1 to

If)o-year Return Periods,” Technical Publication 40,

Department of Commerce, National Weather Service,

Washington, DC.

National Weather Service 1962

National Weather Service. 1962. “Rainfall Frequency

Atlas for the Hawaiian Islands, Durations to 24 Hours,

Return Periods to 100 Years,” Technical Publication 43,

Department of Commeme, National Weather Service,

Washington, DC.

National Weather Service 1963

Nationat Weather Service. 1963. “Probable Maximum

Precipitation and Rainfall Frequency Data for Alaska,

Durations to 24 Hours, Return Periods to 100 Years,”

Technical Publication 47, Department of Commerce,

National Weather Service, Washington, DC.

National Weather Service 1964

National Weather Service. 1964. “Two to Ten Day

Precipitation for Return Periods of 2 to 100 Years in the

Contiguous United States,” Technical Publication 49,

Department of Commerce, National Weather Service,

Washington, DC.

National Weather Service 1965a

National Weather Service. 1965a. “Two to Ten Day

Precipitation for Return Periods of 2 to 100 Years in

Alaska,” Technical Publication 52, Department of Commerce, Nationat Weather Service, Washington, DC.

National Weather Service 1965b

National Weather Service. 1965b. ‘“Two to Ten Day

Rainfall for Return Pericds 2 to 100 Years in the Hawaiian Islands,” Technical Publication 51, Department of

Commerce, National Weather Service, Washington, DC.

A-1
EP 1110-2-10

31 Jul 94

National Weather Service 1977

National Weather Service. 1977. “Five to Six~ Minute

Precipitation Frequency for the Eastern Central U.S.,”

Tahnical Memorandum NWS HYDRO-35, Office of

Hydrology, Department of Commerce, National Oceanic

and Atmospheric Administration, Nationat Weather Service, Silver Spring, MD.

National Weather Service 1982

National Weather Service. 1982. “Application of Probable Maximum Precipitation Estimates-United States East

of the 105th Mendian,” Hydrometeorological Report

No. 52, Office of Hydrology, Department of Commerce,

National Oceanic and Atmospheric Administration,

National Weather Service, Silver Spring, MD.

U.S. Army Corps of Engineers 1981

U.S. Army Corps of Engineers. 1981. “Hydrologic Engineering for Planners,” Training Document No. 14,

U.S. Army Corps of Engineers Hydrologic Engineering

Center.

U.S. Army Corps of Engineers 1986

U.S. Army Corps of Engineers. 1986. “Accuracy of

Computed Water Surface profiles; U.S. *y Corps of

Engin&rs Hydrologic Engineering Center.

U.S. Army Corps of Engineers 1990a

U.S. ~y Corps of Engineers. 1990a. “HEC- 1, Flood

Hydrogmph Package,” Hydrologic Engineering Center.

U.S. Army Corps of Engineers 1990b

U.S. Army Corps of Engineers. 1990b. “HEC-2, Water

Surface Profiles,” Hydrologic Engineering Center.

U.S. Army Corps of Engineers 1991a

U.S. Army Corps of Engineers. 1991a. “HEC-6, Scour

and Deposition in Rivers and Reservoirs,” Hydrologic

Engineering Center.

U.S. Army Corps of Engineers 1991b

U.S. Army Corps of Engineers. 199lb. “UNET, OneDimensional Unsteady Flow Through a Full Network of

Open Channels,” Hydrologic Engineering Center.

U.S. Department of the Interior 1982

U.S. Department of the Interior. 1982. “Guidelines for

Determining Ftood Flow Frequencies,” Bulletin 17B, Geological Survey, Inter-Agency Advisory Committee on

Water Data, Hydrology Sub-Committee, Office of Water

Data Coordination, Reston, VA.

U.S. Geological Survey 1983

U.S. Geological Survey. 1983. “Flood Characteristics of

Urban Watersheds in the United States; Water SUpply

Paper 2207.

Water Resources Council 1982

Water Resources Council. 1982. “Guidance for Determining Flood Flow Frequency,” Bulletin No. 17B, Geological

Survey, U.S. Department of the Interior.

A-2