chapter 2 procedures

8/8/2019 Chapter 2 Procedures

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Part 630

National Engineering Handbook

ProceduresChapter 2United StatesDepartment ofAgriculture

NaturalResourcesConservationService

Part 630 HydrologyNational Engineering Handbook

Chapter 2 Procedures

Rain clouds

Cloud formation

Precipitation

Tran

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ation

from

so

il

from

ocean

Transpiration

OceanGround water

Rock

Deep percolation

SoilPercolation

Infiltration

Surfacerunoff

Evap

oratio

nfrom

vegeta

tion

from

stre

ams

Evaporation


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ProceduresChapter 2

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Issued April 1999

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Part 630


ProceduresChapter 2 Acknowledgments

2i

Chapter 2, Procedures , was originally prepared by Victor Mockus (retired)

in 1964. It was reprinted with minor revisions in 1972. This version was

prepared by the Natural Resources Conservation Service under the guid-ance ofDonald E. Woo dward, national hydraulic engineer, Washington,

DC.


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Part 630


ProceduresChapter 2

Chapter 2 Procedures

Contents:

7i

630.0200 Introduction 21

630.0201 Work outline plan and schedule re lationship 21

(a) Data collec tion .............................................................................................. 21

(b) Computat ions ................................................................................................ 21

(c) Analyses ......................................................................................................... 21

630.0202 Hydrologic evaluation process 22

(a) Work sequence .............................................................................................. 22

(b) Analysis methods .......................................................................................... 24

630.0203 Design hydrology 26

Figures Figure 21 General process hydrology of watershed project 23

evaluation with streamflow and rainfall data available

Figure 22 General process hydrology of watershed project 25

evaluation with streamflow or rainfall data not available

Figure 23 Design hydrology for storage and spillways 27

in floodwater retarding structures

2iii


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Part 630


ProceduresChapter 2Chapter 2 Procedures

6 3 0 . 0 2 0 0 I n t r o d u c t i o n

Hydrology for the evaluation of watershed projects is a

major concern in pa rt 630 of the National Engineering

Handbook. The evaluation is a detailed investigation of

present (n o project) and future (with project) condi-

tions of a watershed to de termine wheth er given

objectives will be met. It is the bas is on whichrecom-

mendations for or against the project are founded. A

summary of the evaluation is included in a work plan,

which is the official document for ca rrying out, main-

taining, and operating the project. The hydrology is

not difficult, but it is complex. The proced ures de-

scribed in this chapter serve both as a guide to hydrol-

ogy studies and as a unifying introduct ion to succe ed-

ing chapters of part 630.

A project evaluation begins with a preliminary investi-

gation (PI), which is a brief study of a potential project

to es timate whe ther a de tailed investigation is justified

(see chapter 3). If it is, information from the PI is used

in writing a work outline that gives the desired scope,

intensity, and schedule of the planning study; its

estimated cost ; the person nel needed; and the comple-

tion date for a work plan.

An important part o f the planning study is the hydro-

logic evaluation, in which data collection, computa-

tion, and analysis are equally important divisions of

work . Availability governs the collection of data. Size

or cost of project influences t he choice of computa-

tional and analytical methods (see chapter 1). National

Resources Conservation Service (NRCS) policy deter -

mines the numb er and kind of analyses. Nevertheless,

the basic evaluation procedure does not vary. It is

flexible because some tasks can be done simulta-

neously or in a preferred sequence and near ly all tasks

can be done by a preferred method, but the general

plan is invariable. The work outline schedule follows

the plan in principle. The plan, schedule, and ch apters

in part 630 are re lated as shown in the following sec-

tions.

6 30 .02 01 Wo rk o u t l i n e

p l an a n d sc h e du l e re la t io n -s h i p

( a ) D a t a c o l l e c t i o n

Base maps, project area maps (chapter 3), rainfall data

(chapte r 4), and runoff data (chapte r 10) are collected

early in the study. Field surveys provide stream cross

sections and profiles (chapter 6) and dam site maps.

Interviews with local NRCS personne l provide data on

hydrologic soil-cover complexes (chapters 7, 8, and 9)

and runoff curve numbers (chapter 10).

( b ) Co m p u ta ti o n s

Storm runoffs (chapter 10), snowmelt runoffs (chapter

11), special effects of land use an d treatme nt (chapte r

12), and the re lations of stream stages to inundation

(chapte r 13) and discharge (chapter 14) are computed

early in this phase of the s tudy. Travel times and lags

(chapte r 15) are computed for use in hydrograph

constru ction (chapter 16) and flood routing (chapter

17). Runoff or pea k discharge frequencies (c hapter18), transmission losses (chapter 19), and watershed

yield (chapte r 20) are comp uted only if they are re-

quired in th e stu dy.

( c ) An a l y s e s

Four conditions of a watershed are studied in accor-

dance with NRCS policy. In order of study they are:

1. PresentCondition of the watershed at the time

of the survey; and the base to which the pro-

posed project is added.

2. Future with no projectExpected future condi-tion of the watershed with no project ac tion

taken.

3. With future land use and treatment measures

Proposed land use and treatment measures a re

added to the first condition. The measu res are

described in the National Watershed Manual.

4. With future land use and treatment measures and

structu resWatershed protect ion and flood

prevention structures are added to the third

condition. The structures are desc ribed in the

National Watershed Manual.


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ProceduresChapter 2

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This order makes the analysis fall into a na tural se-

quence in which measures tha t are first to affect

runoff are first to be evaluated. Flood routings for thepresen t condition give the discharges from which

presen t flood damages are computed in the economic

evaluation. The ro utings are mod ified (chapter 12) to

give discharges for de termining the effects of land use

and treatment. New routings or further modifications

(chapte r 17) are made for the t hird condition to give

discharges for determining the effects of structures .

Generally, it is the third condition that is st udied at

great length because an optimum numbe r and location

of structu res are desired. Final design of individual

structures is made late in the investigation or after the

work plan is approved. The hydrology and NRCS

hydrologic criteria for design are given in chapter 21,

TR60, and sec tion IV of the Field Office Technical

Guide (FOTG).

6 3 0 . 0 2 0 2 H yd r o l o g i c

eva l u a t io n pr o ce s s

In both the computational and analytical phases, use

of hydrologic and hydraulic computer models can

substanially reduce the work time. Such models can

estimate runoff hydrographs; route hydrographs

through rese rvoirs, lakes, channe ls, and flood plains;

combine hydrographs as necessary; and determine

stage/discharge/acres flooded relationships. Two

frequently used NRCS compute r mod els include Tech-

nical Release 20 (Project Formulation - Hydrology,

1983) and Part 630, chapter 31, Computer Program forWater Surface Profiles (1994). The Corps of Engineers

also have several hydrologic an d hydraulic models that

can be useful in project ana lyses.

( a ) Wo r k s e q u e n c e

The sequence of work in t he hydrologic evaluation is

shown in figure 21. The forms of maps, graphs, and

tables are simplified representa tionsof the various

standard forms used in the different States. The pre-

liminary investigation, which precedes the evaluation,

is described in chapter 3. The design hydrology comeslater, and de tails are given in chapter 21.

After evaluation for the present cond itions (the first

condition) is completed, the early steps of the evalua-

tion process do not always need to be repeated for the

remaining conditions. Evaluations for future condi-

tions should include one that considers the future with

no project measures and that accounts for expected

future land use cha nges without any project. Depend-

ing on the nature of these expec ted changes, the

hydrologic soil-cover complexes and co rresponding

runoff curve numbers would be altered, affecting therunoff hydrographs. The con dition with t he future land

use and treatment measures would start the evaluation

process at the hydrologic soil-cover complexes step.

At this step the soil-cover complexes would be modi-

fied to reflect different land use /treatment conditions,

which would ultimately again be reflected in the flow

hydrographs. Finally, the condition with future land

use and treatment measures plus structural measures

would start the evaluation process a t the unit hydro-

graph step by modifying the unit h ydrograph to reflect

the struc tures being in place.


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Part 630


ProceduresChapter 2

Figure 21 General process hydrology of watershe d project evaluation with strea mflow and rainfall data available

Subwatershed map

Cross section location map

Subwatershed 3

Subwatershed 2

Reach 3

Reach 2

Economicevaluation

Subwatershed timeof concentration

Flood routing (chapter 17)

Runoff frequency (chapter 10)

Runoff above reach(chapter 10)

Hydrologic soil-covercomplexes(chapter 9)

Watershed inventory(chapter 8)

Hydrologic soil group map

Historical storms(chapter 4)

(chapter 15)

Unit hydrograph

Unit hydrograph

Unit hydrograph

(chapter 16)

(chapter 7)

Stream reach location map

(chapter 6)

(chapter 3)

Flood damage mapBase map

Water-surface profiles(chapter 14)

Reach2 5

4

6

Reach1Reach

3

(chapter 14) (chapter 3)

Future-w/LU&T plus structures Reach 3


Future-w/LU&T plus structures Reach 1Future-with land use & treatment Reach 3

Future-with land use & treatment Reach 2


Reach 3

Reach 2

No project conditions



Reach 1

Present Reach 3

Present Reach 2

Present Reach 1Physical effects of watershed

programs summarized

Cross section 3

Cross section 2

Storm of 6/1/40

Storm of 7/3/36

Subwatershed 2

Subwatershed 3

Hydrauliccomputations

A

lternate

Alternate

Field surveyscross s ection-profiles

Stream flow datafor checking results

(chapter 5)

Subwatershed

Subwatershed 1 Reach 1

Present

Reach 3

Reach 2

Reach 1

Reach 1

Storm of 6/12/353"

3.5"

4"

2.5" 2"

Subwatershed 1

Reach 3

Reach 2

Reach 1

With structures

Reach 2

Reach 3

Time Runoff (depth)

Present With structures

Rate

Peakdischarge

Reach 3

Reach 2

Reach 1Cross section 1

Area flooded

Stage

Discharge

Stage

Frequency-years1.25 2 5 10 25 50 100

Runoffdepth

Time

2 3Reach 1

Rate

Discharge

Stage

Pre

sent

Withstructur

es

*For area not above structures

123

Time of concentration (Tc)

Present W/structure*

(hr)0.51.73.2

(hr)0.50.61.7

Present FutureStorm event

PresentSubwatershed

3.2"0.8"1.6"3.2"

3.0"0.5"1.5"3.0"

6/12/357/3/368/1/406/10/59

Future

86818379

83778274

1234

Present(acres)

B

C

D

Land use andtreatment

Future(acres)

5050

5050

CultivatedStraight rowTerraced

over 6'

2'-4'0-2'

4'-6'Total

Stormseries

100 yr50 yr

33 1/2 yr25 yr

Peakdischarge

Area flooded0'-1' 1'-2' 2'-3' etc


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Part 630


ProceduresChapter 2

24 (210-vi-NEH, April 1999)

Of the basic data needed in the evaluation, only the

historical rainfall and st reamflow data are likely to be

unavailable; the rest are obtainable from field surveys.Lacking rainfall and runoff data, the procedure goes as

shown in figure 22. The rainfall-frequency data shown

in the figure are from U.S. Weather Bureau, National

Weather Service, and NOAA publications (see part

630, chapter 4). Direct chec ks on runoff cannot be

made, but indirect checks can be made if nearby

waters heds are gaged (see table 52).

Some steps in the procedures of figures 21 and 22

are taken in an entirely different way in the metho ds

for regional analysis.

(b ) An a l ys i s me th o ds

(1) Regional analysis method

This method estimates the magnitudes and frequencies

of peak discharges or runoff volumes for ungaged

waters heds by using relationships from nearby gaged

waters heds. Some of the hydraulic work, construction

of hydrographs, and flood routing are reduced o r

eliminated from the evaluation, but not from the

design hydrology. The method in its simplest form is

as follows:

Step 1 Select nearby gaged watersheds t hat are

climatically and physically similar to the ungaged

watershed. These watersheds compose the region that

gives the meth od its name.

Step 2 Construct frequency lines (chapter 18) for

peak discharges or runoff volumes of the gaged water-

sheds.

Step 3 Plot peak discharges or run off volumes for

selected frequencies (only the 2- and 100-year frequen-

cies if the frequency lines are straight) of each gaged

waters hed against its d rainage area size. Use log-logpaper for the plotting, and make st raight-line r elation-

ships for each frequency. A simple regression between

log (drainage area) and log (discharge or runoff vol-

ume) aids in estimating this best fit str aight line

through the data.

Step 4 Construct th e frequency line for the ungaged

waters hed (or any of its subdivisions). To do th is,

enter the plot with drainage area, find the magnitudes

at each line of relationship, plot the magnitudes a t

their proper places on p robability paper, and draw the

frequency line through the points.

Step 5 Apply the frequency lines of step 4 in the

procedure for presen t conditions. Discharges or vol-

umes for with-project con ditions are obtained by use

of auxiliary relationships descr ibed in chapte rs 12 and

17

In practice the meth od is more complex, but generally

only in step 3. In this step variables in addition to

drainage area are r elated to the peaks or volumes. The

variables include one or mo re of the following, alone

or in combination, directly or by means of index

numbers:

type of climate

mean annual precipitation or rainfall or snowfall

mean seasonal precipitation or rainfall or snow-

fall

maximum or minimum average monthly rainfall

s torm pa tt ern

s to rm d irection

x-year frequency, y-hour duration rainfall

mean number of days with rainfall greater than x

inches

mean annual number of thunderstorm days

mean annual or seasonal or monthly temperature maximum or minimum average monthly tem-

perature

orographic effects

aspect

s tream density

s tr eam pa tt ern

length of watershed

length to center of gravity of watershed

length of main channel

average watershed width

altitude

watershed r ise

main channel slope land s lop e

depth or top width of main channel near outlet

for x-year frequency discharge

time of concentration

lag

t ime to peak

percentage of area in lakes or ponds

extent or depth of shallow soils

extent of major cover


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Part 630


ProceduresChapter 2

Figure 22 General process hydrology of watershe d project evaluation with strea mflow or rainfall data not available

Subwatershed map

Cross section location map

Subwatershed 3

Subwatershed 2

Reach 3

Reach 2







Reach 3

Reach 2

Economicevaluation




Reach 1

Present Reach 3

Present Reach 2

Present Reach 1Physical effects of watershed

programs summarized

Subwatershed timeof concentration

Flood routing (chapter 17)

Runoff frequency (chapter 10)

Runoff above reach(chapter 10)

Hydrologic soil-covercomplexes(chapter 9)


Hydrologic soil group map

(chapter 15)

(chapter 7)

Stream reach location map

(chapter 6)

(chapter 3)

Flood damage mapBase map

Water-surface profiles(chapter 14)

Reach

2 5

4

6

Reach1Reach

3

(chapter 14) (chapter 3)

Cross section 3

Cross section 2

Subwatershed 2

Subwatershed 3

Hydrauliccomputations

Distance

Field surveyscross s ection-profiles

Subwatershed

Subwatershed 1 Reach 1

Present

Reach 3

Reach 2

Reach 1

Reach 1

Rainfall from U.S. Weather Bureau TP-40

Subwatershed 1

Reach 3

Reach 2

Reach 1

With structures

Reach 2

Reach 3

Time Runoff (depth)

Present With structures

Rate

Peakdis

charge

Reach 3

Reach 2

Reach 1Cross section 1

Area flooded

Stage

Discharge

Stage

Frequency-years1.25 2 5 10 25 50 100

Runoffdepth

Time

2 3Reach 1

Rate

Discharge

Stage

Present

Withstructure

s

*For area not above structures

123

Time of concentration (Tc)

Present W/structure*

(hr)0.51.73.2

(hr)0.50.61.7

Present FutureStorm event

PresentSubwatershed

1.2"1.92.3

1.11.72

2 yr5 yr10 yr

Stormseries

100 yr50 yr

33 1/2 yr25 yr

Peakdischarge

Area flooded0'-1' 1'-2' 2'-3' etc

Future

86818379

83778274

1234

Present(acres)

B

C

D

Land use andtreatment

Future(acres)

5050

5050

CultivatedStraight rowTerraced

over 6'

2'-4'0-2'

4'-6'Tota

l

Unit hydrograph

Unit hydrograph

Unit hydrograph

(chapter 16)


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Part 630


ProceduresChapter 2

26 (210-vi-NEH, April 1999)

hydrologic soil-cover complex

geologic region

infiltration rate mean base flow

mean annual runoff

watershed shape

Combinations of these variables are u sed as single

variables in the analysis, one such combination being

the product of watershed length and length to center

of gravity divided by the square root of the main chan-

nel slope. Index numbers (chapter 18) are used for

variables, such as geologic region, not ordinarily

defined by numerical values.

Multiple regression methods ( chapter 18) must be

used if more than one variable appears in the relation-

ship. The only adequate measure of the accuracy of

the re lationship (therefore of the r egional analysis) is

the standard error of estimate in arithmetic units.

Computation of the er ror is illustrate d in chapter 18.

( 2) USGS regional regression equations

Another source for determining relative effects of

waters hed charac teristics on discharge is United

States Geological Survey (USGS) regional regression

equations. The USGS has performed multiple regres-

sion analyses on gaged watersheds for each stat e.They correlated such watershed characteristics as

drainage area, climatic region, watershed slope, water-

shed storage, and others to peak discharge. The re-

gression equations can be use ful for tran sferring data

from gaged watersheds to the waters hed of interest .

6 3 0 .0 2 0 3 De s i gn

h yd ro lo g y

The storage and spillway capacities of floodwater

retarding structures are determined as shown by the

flowchar t in figure 23. Chapter 21 gives deta ils of the

various steps and provides the NRCS criteria of the

design hydrology. That chapter also contains design

hydrology in outline form for channel improvement,

levees, and minor project or onfarm structures .


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Part 630


ProceduresChapter 2

Figure 23 Design hydrology for storage and spillways in floodwater retarding structures

Field surveysreservoir

sites 1, 2, 3, . . . Water supplystorage

(SCS-TR 19)

Sediment storage

(SCS-TR 12)

Drainage area mapPrecipitation

Time of concentration (TC)(chapter 15)

24 hr. pointAdjust. t/areaAdjust. t/duration

Auxiliaryspillway

hydrograph

Freeboardhydrograph

Rainfall-inchesStructure TC (hr)

123

9.24.63.5

109.510.5

2422.625.1

Hydrologic soilgroups

C

D

Structure 3

Structure 2


Design hydrographs(chapter 21)

Principal spillway (NEH-5)Storage capacity

Reservoir routing Auxiliary spillwayselection (chapter 17 & SCS TR 2)

Reservoir routingAuxiliary spillway

Auxiliary spillway

Hydrologic soil covercomplex numbers

(chapter 9)Structure 1

Present

(acres)

Land use and

treatment

Future

(acres)

6436

46

10218

46

Hydrologic soil group D:Positive (fair)Cultivated (St. R.)

Hydrologic soil group C:Cultivitated (ST. R.)

PresentAbove structure Future

756580

706580

123

Storage requirement(chapter 21 & SCS-TR 19)

Acre fast

5302,3003,100

SedimentWater supplyFloodwater

Runoff above structure(chapter 10)

(chapter 7)

Auxiliaryspillway

hydrograph(inches)

Str. no. Class. Freeboardhydrograph

(inches)

(c)

(b)(b)

6.7

5.34.7

20.7

9.89.5

1

23

Class (c)

Class (b)Class (a)Min. 24 hr. precip. forfreeboard hydrograph

Structure 3

Structure 2

Class (c)

Class (b)

Class (a)

Structure 3-Class (b)

Structure 2-Class (b)Structure 3

Structure 2

Structure 3

Structure 2

Structural design

Rainfall from U.S. Weather Bureau TP-40 &NOAA Atlas #2

Structure 1

Structure 3

Structure 2

Structure 1

Structure 1-Class (c)

Structure 1

Structure 3Structure 2

Structure 1

Structure 3Structure 2

Structure 1

Structure 3

Structure 2

Structure 1

Structure 3

Structure 2

Structure 1

Time

Discharge

Discharge

Trial spillway 321

Time

Auxiliary spillwayinflow hydrograph

Outflow trialspillway

23

Storage

Time

Rainfall Runoff

Storage

Principal spillway release

Freeboard hydrograph

Auxiliary spillwayhydrograph

Discharge

Stage

Stage

Discharge

Structure 3

Structure 2

Structure 1

Structure 1

Width

Trial spillwlay 3

Design vel. 2

1Criticalvelo

city

Structure 3

Structure 2

Structure 1

Time

Freeboard hydrograph

Selecteds

pillway

Outflow

Discharge

Structure 3

Structure 2

Structure 1

Discharge

Stage

Stage

Min. 24 hr. precip. forauxiliary spillway hydrograph

(chapter 21)ES-1020

102110221023

Structure 3

Structure 2

Structure 1

Accumulatedvol.

Floodwater storage


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