urban hydrology for small watersheds usda

United StatesDepartment ofAgriculture

NaturalResourcesConservationService

ConservationEngineeringDivision

TechnicalRelease 55

June 1986

Urban Hydrologyfor SmallWatershedsTR-55

Chapter 2

2–1(210-VI-TR-55, Second Ed., June 1986)

Technical Release 55Urban Hydrology for Small Watersheds

Estimating RunoffChapter 2 Estimating Runoff

SCS runoff curve number method

The SCS Runoff Curve Number (CN) method is de-scribed in detail in NEH-4 (SCS 1985). The SCS runoffequation is

QP I

P I Sa

a

=−( )

−( ) +

2

[eq. 2-1]

where

Q = runoff (in)P = rainfall (in)S = potential maximum retention after runoff

begins (in) andIa = initial abstraction (in)

Initial abstraction (Ia) is all losses before runoffbegins. It includes water retained in surface depres-sions, water intercepted by vegetation, evaporation,and infiltration. Ia is highly variable but generally iscorrelated with soil and cover parameters. Throughstudies of many small agricultural watersheds, Ia wasfound to be approximated by the following empiricalequation:

I Sa = 0 2. [eq. 2-2]

By removing Ia as an independent parameter, thisapproximation allows use of a combination of S and Pto produce a unique runoff amount. Substitutingequation 2-2 into equation 2-1 gives:

QP S

P S= −( )

+( )0 2

0 8

2.

.[eq. 2-3]

S is related to the soil and cover conditions of thewatershed through the CN. CN has a range of 0 to 100,and S is related to CN by:

SCN

= −100010 [eq. 2-4]

Figure 2-1 and table 2-1 solve equations 2-3 and 2-4for a range of CN’s and rainfall.

Factors considered in determin-ing runoff curve numbers

The major factors that determine CN are the hydro-logic soil group (HSG), cover type, treatment, hydro-logic condition, and antecedent runoff condition(ARC). Another factor considered is whether impervi-ous areas outlet directly to the drainage system (con-nected) or whether the flow spreads over perviousareas before entering the drainage system (uncon-nected). Figure 2-2 is provided to aid in selecting theappropriate figure or table for determining curvenumbers.

CN’s in table 2-2 (a to d) represent average antecedentrunoff condition for urban, cultivated agricultural,other agricultural, and arid and semiarid rangelanduses. Table 2-2 assumes impervious areas are directlyconnected. The following sections explain how todetermine CN’s and how to modify them for urbanconditions.

Hydrologic soil groups

Infiltration rates of soils vary widely and are affectedby subsurface permeability as well as surface intakerates. Soils are classified into four HSG’s (A, B, C, andD) according to their minimum infiltration rate, whichis obtained for bare soil after prolonged wetting.Appendix A defines the four groups and provides a listof most of the soils in the United States and theirgroup classification. The soils in the area of interestmay be identified from a soil survey report, which canbe obtained from local SCS offices or soil and waterconservation district offices.

Most urban areas are only partially covered by imper-vious surfaces: the soil remains an important factor inrunoff estimates. Urbanization has a greater effect onrunoff in watersheds with soils having high infiltrationrates (sands and gravels) than in watersheds predomi-nantly of silts and clays, which generally have lowinfiltration rates.

Any disturbance of a soil profile can significantlychange its infiltration characteristics. With urbaniza-tion, native soil profiles may be mixed or removed orfill material from other areas may be introduced.Therefore, a method based on soil texture is given inappendix A for determining the HSG classification fordisturbed soils.


Estimating RunoffChapter 2

2–2 (210-VI-TR-55, Second Ed., June 1986)

Cover type

Table 2-2 addresses most cover types, such as vegeta-tion, bare soil, and impervious surfaces. There are anumber of methods for determining cover type. Themost common are field reconnaissance, aerial photo-graphs, and land use maps.

Treatment

Treatment is a cover type modifier (used only in table2-2b) to describe the management of cultivated agri-cultural lands. It includes mechanical practices, suchas contouring and terracing, and management prac-tices, such as crop rotations and reduced or no tillage.

Hydrologic condition

Hydrologic condition indicates the effects of covertype and treatment on infiltration and runoff and isgenerally estimated from density of plant and residuecover on sample areas. Good hydrologic conditionindicates that the soil usually has a low runoff poten-tial for that specific hydrologic soil group, cover type,and treatment. Some factors to consider in estimatingthe effect of cover on infiltration and runoff are (a)canopy or density of lawns, crops, or other vegetativeareas; (b) amount of year-round cover; (c) amount ofgrass or close-seeded legumes in rotations; (d) percentof residue cover; and (e) degree of surface roughness.

Curve

numbe

r = 10

0

95

90

85

80

75

70

65

60

55

50

45

40

121110987654321 00

1

2

3

4

5

6

7

8

Dire

ct r

unof

f (Q

), in

ches

Rainfall (P), inches

Curves on this sheet are for the case Ia = 0.2S, so that

Q =(P–0.2S)2

P + 0.8S

Figure 2-1 Solution of runoff equation.

Chapter 2



Estimating Runoff

Table 2-2a Runoff curve numbers for urban areas 1/

Curve numbers for------------------------------------------- Cover description ----------------------------------------- -----------hydrologic soil group -------------

Average percentCover type and hydrologic condition impervious area 2/ A B C D

Fully developed urban areas (vegetation established)

Open space (lawns, parks, golf courses, cemeteries, etc.) 3/:Poor condition (grass cover < 50%) .......................................... 68 79 86 89Fair condition (grass cover 50% to 75%) .................................. 49 69 79 84Good condition (grass cover > 75%) ......................................... 39 61 74 80

Impervious areas:Paved parking lots, roofs, driveways, etc.

(excluding right-of-way) ............................................................. 98 98 98 98Streets and roads:

Paved; curbs and storm sewers (excludingright-of-way) ................................................................................ 98 98 98 98Paved; open ditches (including right-of-way) .......................... 83 89 92 93Gravel (including right-of-way) ................................................. 76 85 89 91Dirt (including right-of-way) ...................................................... 72 82 87 89

Western desert urban areas:Natural desert landscaping (pervious areas only) 4/ ..................... 63 77 85 88Artificial desert landscaping (impervious weed barrier,

desert shrub with 1- to 2-inch sand or gravel mulchand basin borders) ...................................................................... 96 96 96 96

Urban districts:Commercial and business ................................................................. 85 89 92 94 95Industrial ............................................................................................. 72 81 88 91 93

Residential districts by average lot size:1/8 acre or less (town houses) .......................................................... 65 77 85 90 921/4 acre ................................................................................................ 38 61 75 83 871/3 acre ................................................................................................ 30 57 72 81 861/2 acre ................................................................................................ 25 54 70 80 851 acre ................................................................................................... 20 51 68 79 842 acres .................................................................................................. 12 46 65 77 82

Developing urban areas

Newly graded areas(pervious areas only, no vegetation) 5/ ................................................................ 77 86 91 94

Idle lands (CN’s are determined using cover typessimilar to those in table 2-2c).

1 Average runoff condition, and Ia = 0.2S.2 The average percent impervious area shown was used to develop the composite CN’s. Other assumptions are as follows: impervious areas are

directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space ingood hydrologic condition. CN’s for other combinations of conditions may be computed using figure 2-3 or 2-4.

3 CN’s shown are equivalent to those of pasture. Composite CN’s may be computed for other combinations of open spacecover type.

4 Composite CN’s for natural desert landscaping should be computed using figures 2-3 or 2-4 based on the impervious area percentage(CN = 98) and the pervious area CN. The pervious area CN’s are assumed equivalent to desert shrub in poor hydrologic condition.

5 Composite CN’s to use for the design of temporary measures during grading and construction should be computed using figure 2-3 or 2-4based on the degree of development (impervious area percentage) and the CN’s for the newly graded pervious areas.




Table 2-2b Runoff curve numbers for cultivated agricultural lands 1/

Curve numbers for------------------------------------------ Cover description --------------------------------------------- ------------- hydrologic soil group ----------------

HydrologicCover type Treatment 2/ condition 3/ A B C D

Fallow Bare soil — 77 86 91 94Crop residue cover (CR) Poor 76 85 90 93

Good 74 83 88 90

Row crops Straight row (SR) Poor 72 81 88 91Good 67 78 85 89

SR + CR Poor 71 80 87 90Good 64 75 82 85

Contoured (C) Poor 70 79 84 88Good 65 75 82 86

C + CR Poor 69 78 83 87Good 64 74 81 85

Contoured & terraced (C&T) Poor 66 74 80 82Good 62 71 78 81

C&T+ CR Poor 65 73 79 81Good 61 70 77 80

Small grain SR Poor 65 76 84 88Good 63 75 83 87

SR + CR Poor 64 75 83 86Good 60 72 80 84

C Poor 63 74 82 85Good 61 73 81 84

C + CR Poor 62 73 81 84Good 60 72 80 83

C&T Poor 61 72 79 82Good 59 70 78 81

C&T+ CR Poor 60 71 78 81Good 58 69 77 80

Close-seeded SR Poor 66 77 85 89or broadcast Good 58 72 81 85legumes or C Poor 64 75 83 85rotation Good 55 69 78 83meadow C&T Poor 63 73 80 83

Good 51 67 76 80

1 Average runoff condition, and Ia=0.2S2 Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.3 Hydraulic condition is based on combination factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas,

(b) amount of year-round cover, (c) amount of grass or close-seeded legumes, (d) percent of residue cover on the land surface (good ≥ 20%),and (e) degree of surface roughness.

Poor: Factors impair infiltration and tend to increase runoff.

Good: Factors encourage average and better than average infiltration and tend to decrease runoff.

Chapter 2



Estimating Runoff

Table 2-2c Runoff curve numbers for other agricultural lands 1/

Curve numbers for--------------------------------------- Cover description -------------------------------------- ------------ hydrologic soil group ---------------

HydrologicCover type condition A B C D

Pasture, grassland, or range—continuous Poor 68 79 86 89forage for grazing. 2/ Fair 49 69 79 84

Good 39 61 74 80

Meadow—continuous grass, protected from — 30 58 71 78grazing and generally mowed for hay.

Brush—brush-weed-grass mixture with brush Poor 48 67 77 83the major element. 3/ Fair 35 56 70 77

Good 30 4/ 48 65 73

Woods—grass combination (orchard Poor 57 73 82 86or tree farm). 5/ Fair 43 65 76 82

Good 32 58 72 79

Woods. 6/ Poor 45 66 77 83Fair 36 60 73 79

Good 30 4/ 55 70 77

Farmsteads—buildings, lanes, driveways, — 59 74 82 86and surrounding lots.

1 Average runoff condition, and Ia = 0.2S.2 Poor: <50%) ground cover or heavily grazed with no mulch.

Fair: 50 to 75% ground cover and not heavily grazed. Good: > 75% ground cover and lightly or only occasionally grazed.

3 Poor: <50% ground cover. Fair: 50 to 75% ground cover. Good: >75% ground cover.

4 Actual curve number is less than 30; use CN = 30 for runoff computations.5 CN’s shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed

from the CN’s for woods and pasture.6 Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.

Fair: Woods are grazed but not burned, and some forest litter covers the soil. Good: Woods are protected from grazing, and litter and brush adequately cover the soil.




Table 2-2d Runoff curve numbers for arid and semiarid rangelands 1/

Curve numbers for---------------------------------------- Cover description ----------------------------------------------- --------------- hydrologic soil group -------------

Hydrologic Cover type condition 2/ A 3/ B C D

Herbaceous—mixture of grass, weeds, and Poor 80 87 93low-growing brush, with brush the Fair 71 81 89minor element. Good 62 74 85

Oak-aspen—mountain brush mixture of oak brush, Poor 66 74 79aspen, mountain mahogany, bitter brush, maple, Fair 48 57 63and other brush. Good 30 41 48

Pinyon-juniper—pinyon, juniper, or both; Poor 75 85 89grass understory. Fair 58 73 80

Good 41 61 71

Sagebrush with grass understory. Poor 67 80 85Fair 51 63 70

Good 35 47 55

Desert shrub—major plants include saltbush, Poor 63 77 85 88greasewood, creosotebush, blackbrush, bursage, Fair 55 72 81 86

palo verde, mesquite, and cactus. Good 49 68 79 84

1 Average runoff condition, and Ia, = 0.2S. For range in humid regions, use table 2-2c.2 Poor: <30% ground cover (litter, grass, and brush overstory).

Fair: 30 to 70% ground cover.Good: > 70% ground cover.

3 Curve numbers for group A have been developed only for desert shrub.

Chapter 3



Time of Concentration and Travel TimeChapter 3 Time of Concentration and

Travel Time

Travel time ( Tt ) is the time it takes water to travelfrom one location to another in a watershed. Tt is acomponent of time of concentration ( Tc ), which isthe time for runoff to travel from the hydraulicallymost distant point of the watershed to a point ofinterest within the watershed. Tc is computed bysumming all the travel times for consecutive compo-nents of the drainage conveyance system.

Tc influences the shape and peak of the runoffhydrograph. Urbanization usually decreases Tc,thereby increasing the peak discharge. But Tc can beincreased as a result of (a) ponding behind small orinadequate drainage systems, including storm draininlets and road culverts, or (b) reduction of land slopethrough grading.

Factors affecting time of concen-tration and travel time

Surface roughness

One of the most significant effects of urban develop-ment on flow velocity is less retardance to flow. Thatis, undeveloped areas with very slow and shallowoverland flow through vegetation become modified byurban development: the flow is then delivered tostreets, gutters, and storm sewers that transport runoffdownstream more rapidly. Travel time through thewatershed is generally decreased.

Channel shape and flow patterns

In small non-urban watersheds, much of the traveltime results from overland flow in upstream areas.Typically, urbanization reduces overland flow lengthsby conveying storm runoff into a channel as soon aspossible. Since channel designs have efficient hydrau-lic characteristics, runoff flow velocity increases andtravel time decreases.

Slope

Slopes may be increased or decreased by urbanization,depending on the extent of site grading or the extentto which storm sewers and street ditches are used inthe design of the water management system. Slope willtend to increase when channels are straightened anddecrease when overland flow is directed throughstorm sewers, street gutters, and diversions.

Computation of travel time andtime of concentration

Water moves through a watershed as sheet flow,shallow concentrated flow, open channel flow, orsome combination of these. The type that occurs is afunction of the conveyance system and is best deter-mined by field inspection.

Travel time ( Tt ) is the ratio of flow length to flowvelocity:

TL

Vt =3600

[eq. 3-1]

where:

Tt = travel time (hr)L = flow length (ft)V = average velocity (ft/s)

3600 = conversion factor from seconds to hours.

Time of concentration ( Tc ) is the sum of Tt values forthe various consecutive flow segments:

T T T Tc t t tm= + +

1 2K [eq. 3-2]

where:

Tc = time of concentration (hr)m = number of flow segments


Time of Concentration and Travel TimeChapter 3


Figure 3-1 Average velocities for estimating travel time for shallow concentrated flow

10 20

Average velocity (ft/sec)

Wat

erco

urs

e sl

op

e (f

t/ft

)

.20

.50

Unp

aved

Pav

ed

.06

.04

.02

.10

.01

.0051 2 4 6

Chapter 3



Time of Concentration and Travel Time

Sheet flow

Sheet flow is flow over plane surfaces. It usuallyoccurs in the headwater of streams. With sheet flow,the friction value (Manning’s n) is an effective rough-ness coefficient that includes the effect of raindropimpact; drag over the plane surface; obstacles such aslitter, crop ridges, and rocks; and erosion and trans-portation of sediment. These n values are for veryshallow flow depths of about 0.1 foot or so. Table 3-1gives Manning’s n values for sheet flow for varioussurface conditions.

For sheet flow of less than 300 feet, use Manning’skinematic solution (Overtop and Meadows 1976) tocompute Tt:

TnL

P st =

( )( )

0 0070 8

20 5 0 4

..

. . [eq. 3-3]

where:

Tt = travel time (hr),n = Manning’s roughness coefficient (table 3-1)L = flow length (ft)P2 = 2-year, 24-hour rainfall (in) s = slope of hydraulic grade line

(land slope, ft/ft)

This simplified form of the Manning’s kinematic solu-tion is based on the following: (1) shallow steadyuniform flow, (2) constant intensity of rainfall excess(that part of a rain available for runoff), (3) rainfallduration of 24 hours, and (4) minor effect of infiltra-tion on travel time. Rainfall depth can be obtainedfrom appendix B.

Shallow concentrated flow

After a maximum of 300 feet, sheet flow usually be-comes shallow concentrated flow. The average veloc-ity for this flow can be determined from figure 3-1, inwhich average velocity is a function of watercourseslope and type of channel. For slopes less than 0.005ft/ft, use equations given in appendix F for figure 3-1.Tillage can affect the direction of shallow concen-trated flow. Flow may not always be directly down thewatershed slope if tillage runs across the slope.

After determining average velocity in figure 3-1, useequation 3-1 to estimate travel time for the shallowconcentrated flow segment.

Open channels

Open channels are assumed to begin where surveyedcross section information has been obtained, wherechannels are visible on aerial photographs, or whereblue lines (indicating streams) appear on United StatesGeological Survey (USGS) quadrangle sheets.Manning’s equation or water surface profile informa-tion can be used to estimate average flow velocity.Average flow velocity is usually determined for bank-full elevation.

Table 3-1 Roughness coefficients (Manning’s n) forsheet flow

Surface description n 1/

Smooth surfaces (concrete, asphalt,gravel, or bare soil) .......................................... 0.011

Fallow (no residue) .................................................. 0.05Cultivated soils:

Residue cover ≤20% ......................................... 0.06Residue cover >20% ......................................... 0.17

Grass:Short grass prairie ............................................ 0.15Dense grasses 2/ ................................................ 0.24Bermudagrass . ................................................. 0.41

Range (natural) ......................................................... 0.13Woods:3/

Light underbrush .............................................. 0.40

Dense underbrush ............................................ 0.80

1 The n values are a composite of information compiled by Engman

(1986).2 Includes species such as weeping lovegrass, bluegrass, buffalo

grass, blue grama grass, and native grass mixtures.3 When selecting n , consider cover to a height of about 0.1 ft. This

is the only part of the plant cover that will obstruct sheet flow.


Time of Concentration and Travel TimeChapter 3


Manning’s equation is:

Vr sn

= 1 492

3

1

2. [eq. 3-4]

where:

V = average velocity (ft/s)r = hydraulic radius (ft) and is equal to a/pw

a = cross sectional flow area (ft2)pw = wetted perimeter (ft)

s = slope of the hydraulic grade line (channelslope, ft/ft)

n = Manning’s roughness coefficient for openchannel flow.

Manning’s n values for open channel flow can beobtained from standard textbooks such as Chow(1959) or Linsley et al. (1982). After average velocity iscomputed using equation 3-4, Tt for the channel seg-ment can be estimated using equation 3-1.

Reservoirs or lakes

Sometimes it is necessary to estimate the velocity offlow through a reservoir or lake at the outlet of awatershed. This travel time is normally very small andcan be assumed as zero.

Limitations

• Manning’s kinematic solution should not be usedfor sheet flow longer than 300 feet. Equation 3-3was developed for use with the four standardrainfall intensity-duration relationships.

• In watersheds with storm sewers, carefully identifythe appropriate hydraulic flow path to estimate Tc.Storm sewers generally handle only a small portionof a large event. The rest of the peak flow travelsby streets, lawns, and so on, to the outlet. Consult astandard hydraulics textbook to determine averagevelocity in pipes for either pressure or nonpressureflow.

• The minimum Tc used in TR-55 is 0.1 hour.

• A culvert or bridge can act as a reservoir outlet ifthere is significant storage behind it. The proce-dures in TR-55 can be used to determine the peakflow upstream of the culvert. Detailed storagerouting procedures should be used to determinethe outflow through the culvert.

Example 3-1

The sketch below shows a watershed in Dyer County,northwestern Tennessee. The problem is to computeTc at the outlet of the watershed (point D). The 2-year24-hour rainfall depth is 3.6 inches. All three types offlow occur from the hydraulically most distant point(A) to the point of interest (D). To compute Tc, firstdetermine Tt for each segment from the followinginformation:

Segment AB: Sheet flow; dense grass; slope (s) = 0.01ft/ft; and length (L) = 100 ft. Segment BC: Shallowconcentrated flow; unpaved; s = 0.01 ft/ft; andL = 1,400 ft. Segment CD: Channel flow; Manning’sn = .05; flow area (a) = 27 ft2; wetted perimeter(pw) = 28.2 ft; s = 0.005 ft/ft; and L = 7,300 ft.

See figure 3-2 for the computations made onworksheet 3.

A B C D

7,300 ft1,400 ft100 ft

(Not to scale)

Chapter 4



Graphical Peak Dischage MethodChapter 4 Graphical Peak Discharge Method

This chapter presents the Graphical Peak Dischargemethod for computing peak discharge from rural andurban areas. The Graphical method was developedfrom hydrograph analyses using TR-20, “ComputerProgram for Project Formulation—Hydrology”(SCS 1983). The peak discharge equation used is:

qp = quAmQFp [eq. 4-1]

where:

qp = peak discharge (cfs)qu = unit peak discharge (csm/in)

Am = drainage area (mi2)Q = runoff (in)Fp= pond and swamp adjustment factor

The input requirements for the Graphical method areas follows: (1) Tc (hr), (2) drainage area (mi2), (3)appropriate rainfall distribution (I, IA, II, or III), (4)24-hour rainfall (in), and (5) CN. If pond and swampareas are spread throughout the watershed and are notconsidered in the Tc computation, an adjustment forpond and swamp areas is also needed.

Peak discharge computation

For a selected rainfall frequency, the 24-hour rainfall(P) is obtained from appendix B or more detailed localprecipitation maps. CN and total runoff (Q) for thewatershed are computed according to the methodsoutlined in chapter 2. The CN is used to determine theinitial abstraction (Ia) from table 4-1. Ia / P is thencomputed.

If the computed Ia / P ratio is outside the range inexhibit 4 (4-I, 4-IA, 4-II, and 4-III) for the rainfall distri-bution of interest, then the limiting value should beused. If the ratio falls between the limiting values, uselinear interpolation. Figure 4-1 illustrates the sensitiv-ity of Ia / P to CN and P.

Peak discharge per square mile per inch of runoff (qu)is obtained from exhibit 4-I, 4-IA, 4-II, or 4-III by usingTc (chapter 3), rainfall distribution type, and Ia / Pratio. The pond and swamp adjustment factor is ob-tained from table 4-2 (rounded to the nearest tablevalue). Use worksheet 4 in appendix D to aid in com-puting the peak discharge using the Graphical method.

Figure 4-1 Variation of Ia / P for P and CN

50

151311975310

0.2

0.4

0.6

0.8

1.0

I a/P

Rainfal l (P), inches

CN = 40

6070

8090

Curve Ianumber (in)

40 ...................... 3.00041 ...................... 2.87842 ...................... 2.76243 ...................... 2.65144 ...................... 2.54545 ...................... 2.44446 ...................... 2.34847 ...................... 2.25548 ...................... 2.16749 ...................... 2.08250 ...................... 2.00051 ...................... 1.92252 ...................... 1.84653 ...................... 1.77454 ...................... 1.70455 ...................... 1.63656 ...................... 1.57157 ...................... 1.50958 ...................... 1.44859 ...................... 1.39060 ...................... 1.33361 ...................... 1.27962 ...................... 1.22663 ...................... 1.17564 ...................... 1.12565 ...................... 1.07766 ...................... 1.03067 ...................... 0.98568 ...................... 0.94169 ...................... 0.899

Curve Ia

number (in)

70 ...................... 0.85771 ...................... 0.81772 ...................... 0.77873 ...................... 0.74074 ...................... 0.70375 ...................... 0.66776 ...................... 0.63277 ...................... 0.59778 ...................... 0.56479 ...................... 0.53280 ...................... 0.50081 ...................... 0.46982 ...................... 0.43983 ...................... 0.41084 ...................... 0.38185 ...................... 0.35386 ...................... 0.32687 ...................... 0.29988 ...................... 0.27389 ...................... 0.24790 ...................... 0.22291 ...................... 0.19892 ...................... 0.17493 ...................... 0.15194 ...................... 0.12895 ...................... 0.10596 ...................... 0.08397 ...................... 0.06298 ...................... 0.041

Table 4-1 Ia values for runoff curve numbers


Graphical Peak Dischage MethodChapter 4


Table 4-2 Adjustment factor (Fp) for pond and swampareas that are spread throughout thewatershed

Percentage of pond and swamp areas Fp

0 ..................................................... 1.000.2 .................................................. 0.971.0 .................................................. 0.873.0 .................................................. 0.75

5.0 .................................................. 0.72

Limitations

The Graphical method provides a determination ofpeak discharge only. If a hydrograph is needed orwatershed subdivision is required, use the TabularHydrograph method (chapter 5). Use TR-20 if thewatershed is very complex or a higher degree ofaccuracy is required.

• The watershed must be hydrologically homoge-neous, that is, describable by one CN. Land use,soils, and cover are distributed uniformly through-out the watershed.

• The watershed may have only one main stream or,if more than one, the branches must have nearlyequal TC' s.

• The method cannot perform valley or reservoirrouting.

• The Fp factor can be applied only for ponds orswamps that are not in the Tc flow path.

• Accuracy of peak discharge estimated by thismethod will be reduced if Ia / P values are used thatare outside the range given in exhibit 4. The limit-ing Ia / P values are recommended for use.

• This method should be used only if the weightedCN is greater than 40.

• When this method is used to develop estimates ofpeak discharge for both present and developedconditions of a watershed, use the same procedurefor estimating Tc.

• Tc values with this method may range from 0.1 to10 hours.

Example 4-1

Compute the 25-year peak discharge for the 250-acrewatershed described in examples 2-2 and 3-1. Figure 4-2 shows how worksheet 4 is used to compute qp as345 cfs.




500

300

200

100

80

60

40

Un

it p

ea

k d

isc

ha

rge

(q

u),

(c

sm

/in

)

Time of concentration (Tc), (hours)

Ia /P = 0.100.20

0.25

0.30

0.35

0.40

0.45

0.50

.1 .2 .4 .6 1 2.8 4 6 8 10

Exhibit 4-I Unit peak discharge (qu) for NRCS (SCS) type I rainfall distribution

Chapter 4



Graphical Peak Dischage Method

200

100

80

60

40

Un

it p

ea

k d

isc

ha

rge

(q

u),

(c

sm

/in

)


30

Ia /P = 0.10

0.20

0.25

0.30

0.50

.1 .2 .4 .6 1 2.8 4 6 8 10

Exhibit 4-IA Unit peak discharge (qu) for NRCS (SCS) type IA rainfall distribution




1000

400

300

200

100

60

50

Un

it p

ea

k d

isc

ha

rge

(q

u),

(c

sm

/in

)


80

500

600

800

Ia /P = 0.100.300.350.40

0.45

.1 .2 .4 .6 1 2.8 4 6 8 10

0.50

Exhibit 4-II Unit peal discharge (qu) for NRCS (SCS) type II rainfall distribution

Chapter 4



Graphical Peak Dischage Method

Ia /P = 0.100.300.350.400.45

.1 .2 .4 .6 1 2.8 4 6 8 10

0.50

Un

it p

ea

k d

isc

ha

rge

(q

u),

(c

sm

/in

)


400

300

200

100

60

40

80

500

600

700

Exhibit 4-III Unit peal discharge (qu) for NRCS (SCS) type III rainfall distribution


Storage Volume for Detention BasinsChapter 6


Example 6-3 Estimating qo

A development is being planned for a 10-acre water-shed (0.0156 mi2). A county ordinance requires thatthe developed-condition outflow from the watershedfor 24-hr, 100-year frequency storm does not exceedthe outflow for present conditions. The peak dischargefrom the watershed for present conditions, 35 cfs, iscalculated from procedures in chapter 4. For devel-oped conditions, runoff (Q) is 5.4 inches, peak dis-charge from the watershed is 42 cfs from proceduresin chapter 4, and rainfall distribution is type II.What will be the peak outflow discharge (qo) from adetention basin that is located at the outlet and hasmaximum allowable storage volume (Vs) of 35,000 ft3

and peak inflow discharge (qi) of 42 cfs? Figure 6-4shows how worksheet 6b is used to estimate qo as 33cfs, which is within the 35 cfs limit. An outflow devicewill be selected to discharge 33 cfs at a stage corre-sponding to a Vs of 35,000 ft3.

A–1(210-VI-TR-55, Second Ed., June 1986)

Exhibit A: Hydrologic Soil Groups for the United StatesAppendix B Synthetic Rainfall Distributions and

Rainfall Data Sources

The highest peak discharges from small watersheds in the United States are usually caused by intense, brief rain-falls that may occur as distinct events or as part of a longer storm. These intense rainstorms do not usually ex-tended over a large area and intensities vary greatly. One common practice in rainfall-runoff analysis is to developa synthetic rainfall distribution to use in lieu of actual storm events. This distribution includes maximum rainfallintensities for the selected design frequency arranged in a sequence that is critical for producing peak runoff.

Synthetic rainfall distributions

The length of the most intense rainfall period contributing to the peak runoff rate is related to the time of concen-tration (Tc) for the watershed. In a hydrograph created with NRCS procedures, th

Appendix A Hydrologic Soil Groups

Soils are classified into hydrologic soil groups (HSG’s)to indicate the minimum rate of infiltration obtained forbare soil after prolonged wetting. The HSG’s, which areA, B, C, and D, are one element used in determiningrunoff curve numbers (see chapter 2). For the conve-nience of TR-55 users, exhibit A-1 lists the HSG classifi-cation of United States soils.

The infiltration rate is the rate at which water enters thesoil at the soil surface. It is controlled by surface condi-tions. HSG also indicates the transmission rate—the rateat which the water moves within the soil. This rate iscontrolled by the soil profile. Approximate numericalranges for transmission rates shown in the HSG defini-tions were first published by Musgrave (USDA 1955).The four groups are defined by SCS soil scientists asfollows:

Group A soils have low runoff potential and high infil-tration rates even when thoroughly wetted. They consistchiefly of deep, well to excessively drained sand orgravel and have a high rate of water transmission(greater than 0.30 in/hr).

Group B soils have moderate infiltration rates whenthoroughly wetted and consist chiefly of moderatelydeep to deep, moderately well to well drained soils withmoderately fine to moderately coarse textures. Thesesoils have a moderate rate of water transmission (0.15-0.30 in/hr).

Group C soils have low infiltration rates when thor-oughly wetted and consist chiefly of soils with a layerthat impedes downward movement of water and soilswith moderately fine to fine texture. These soils have alow rate of water transmission (0.05-0.15 in/hr).

Group D soils have high runoff potential. They havevery low infiltration rates when thoroughly wetted andconsist chiefly of clay soils with a high swelling poten-tial, soils with a permanent high water table, soils with aclaypan or clay layer at or near the surface, and shallowsoils over nearly impervious material. These soils have avery low rate of water transmission (0-0.05 in/hr).

In exhibit A-1, some of the listed soils have an addedmodifier; for example, “Abrazo, gravelly.” This refers toa gravelly phase of the Abrazo series that is found inSCS soil map legends.

Disturbed soil profiles

As a result of urbanization, the soil profile may be con-siderably altered and the listed group classification mayno longer apply. In these circumstances, use the follow-ing to determine HSG according to the texture of thenew surface soil, provided that significant compactionhas not occurred (Brakensiek and Rawls 1983).

HSG Soil textures

A Sand, loamy sand, or sandy loam

B Silt loam or loam

C Sandy clay loam

D Clay loam, silty clay loam, sandy clay, siltyclay, or clay

Drainage and group D soils

Some soils in the list are in group D because of a highwater table that creates a drainage problem. Once thesesoils are effectively drained, they are placed in a differ-ent group. For example, Ackerman soil is classified asA/D. This indicates that the drained Ackerman soil is ingroup A and the undrained soil is in group D.

A–2 (210-VI-TR-55, Second Ed., June 1986)

Exhibit A: Hydrologic Soil Groups for the United StatesExhibit A: Hydrologic Soil Groups for the United States

AABAB ............................................ DAABERG ......................................... DAARON ........................................... CAARUP ........................................... DAASTAD ......................................... BAAZDAHL ....................................... BABAC .............................................. DABAJO ............................................ CABALAN ......................................... DABALOBADIAH .............................. BABARCA ......................................... BABBAYE ......................................... BABBEYLAKE .................................. AABBIE ............................................. BABBOTT ......................................... DABBOTTSPRING ........................... BABBOTTSTOWN ........................... CABEGG ........................................... BABELA ............................................ BABELL ............................................ BABENAKI ........................................ BABERDEEN ................................... CABERONE ...................................... BABERSITO ..................................... CABERT ............................................ BABES .............................................. DABGESE ......................................... BABILENE ........................................ CABIN ............................................... BABIQUA .......................................... BABIQUA, Flooded .......................... CABIQUIU ......................................... CABITA ............................................. CABO ................................................ CABOR ............................................. DABORIGINE ................................... DABOTEN ......................................... DABRA .............................................. BABRACON ...................................... BABRAM ........................................... DABRAZO, Gravelly ......................... CABRAZO ......................................... DABREU ........................................... BABRIGO .......................................... BABSAQUIL ..................................... BABSAROKEE ................................. CABSAROOK Cool .......................... BABSAROOK ................................... CABSAY ............................................ DABSCO ........................................... AABSCOTA ...................................... AABSHER ......................................... DABSTON ......................................... CACACIO .......................................... BACADEMY ...................................... CACADIA .......................................... DACADIANA ..................................... DACAMPO ........................................ CACANA ........................................... DACANOD ........................................ CACASCO ........................................ DACCELERATOR ............................ BACCOLA ......................................... BACEITUNAS ................................... BACEL .............................................. CACHELAKE .................................... BACHIMIN ........................................ CACKELTON .................................... BACKER ........................................... BACKERMAN ............................... A/DACKETT ......................................... DACKLEY ......................................... BACKMEN ........................................ BACKMEN, Wet ............................... CACKMORE, Poorly Drained .......... BACKNA ........................................... BACKWATER ................................... DACME ............................................. CACO ................................................ BACOMA .......................................... CACORD ........................................... CACOVE ........................................... CACREE ........................................... CACRELANE .................................... CACTEM ........................................... D

ACTI ................................................ DACTON ........................................... BACUFF ............................................ BACUNA ........................................... CACY ................................................ CADA ................................................ CADABOI .......................................... CADAIR ............................................. CADAMANT ...................................... BADAMS ........................................... AADAMSLAKE ................................. BADAMSON ..................................... BADAMSVILLE ................................. CADATON ......................................... DADAVEN ......................................... CADCO ............................................. DADDER ....................................... A/DADDERTON ................................... BADDICKS ....................................... DADDIELOU ..................................... BADE ................................................ AADEK .............................................. BADEL .............................................. BADEL, Wet ..................................... DADELAIDE ...................................... DADELANTO .................................... BADELINO ........................................ BADELINO, Saline-Alkali ................ CADELMANN ................................... CADELPHIA .................................. B/DADEN .............................................. CADENA ........................................... CADGER ........................................... DADIEUX .......................................... BADILIS ............................................ BADINOT .......................................... DADIOS ............................................ DADIRONDACK ............................... DADJIDAUMO .................................. DADJUNTAS .................................... CADKINS .......................................... BADLER ............................................ CADMAN ........................................... DADOBE ........................................... DADOLPH ..................................... B/DADOS ............................................. CADRIAN ...................................... A/DADVOKAY ...................................... DADWELL ......................................... CADY ................................................ BADYEVILLE .................................... CAECET ............................................ CAENEAS ......................................... BAETNA ............................................ CAFFEY ............................................ CAFLEY ............................................ BAFLEY, Extremely Stony .............. CAFTAD ............................................ BAFTADEN ....................................... DAFTON ....................................... C/DAGA ................................................ CAGAN ............................................. DAGAR ............................................. BAGASSIZ ........................................ DAGATE ........................................... DAGATHA ......................................... BAGAWAM ....................................... BAGEE .............................................. DAGENCY ........................................ BAGENCY, Stony ............................ CAGERDELLY .................................. DAGFAYAN ...................................... DAGNAL ........................................... DAGNESS ......................................... BAGNEW .......................................... CAGNOS ........................................... DAGON ............................................. CAGORT ........................................... CAGRA ............................................. DAGUA ............................................. BAGUA DULCE ................................ BAGUA FRIA .................................... CAGUADILLA ................................... AAGUALT ......................................... BAGUEDA ........................................ BAGUILARES ................................... B

AGUILITA ....................................... BAGUIRRE ....................................... DAGUSTIN ....................................... BAHART ........................................... BAHCHEW ........................................ DAHL ................................................. CAHLSTROM ................................... DAHMEEK ........................................ CAHOLT ............................................ DAHPAH ........................................... CAHREN ........................................... BAHRNKLIN ..................................... CAHRS .............................................. BAHSAHKA ...................................... CAHTANUM .................................. C/DAHWAHNEE ................................... BAIBONITO ...................................... DAIDO ............................................... DAIKEN ............................................. BAIKMAN .......................................... DAILEY ............................................. BAIMELIIK ........................................ BAINAKEA ........................................ BAINSLEY ........................................ BAINSWORTH ................................. BAIRMONT ....................................... CAIRPORT, Wet ............................... CAIRPORT ....................................... DAITS ................................................ BAJAX ............................................... DAJO ................................................. CAJOLITO ........................................ DAKAD ..............................................CAKAKA ............................................ AAKAN .......................................... B/DAKASKA ......................................... BAKBASH ......................................... BAKELA ............................................ DAKELEY ......................................... AAKERCAN ...................................... BAKERITE ........................................ BAKERS ........................................... BAKERUE ......................................... DAKHONI .......................................... DAKINA ............................................. BAKINVILLE ..................................... BAKLER ............................................ DAKSARBEN .................................... BALABASTER .................................. DALADDIN ........................................ BALADSHI ........................................ BALAE .............................................. AALAELOA ....................................... BALAGA ............................................ AALAKAI ........................................... DPARANAT ...................................... CALAMA ........................................... BALAMADITAS ................................ CALAMANCE ................................... BALAMBIQUE .................................. BALAMEDAWELL ............................ BALAMO ........................................... DALAMOGORDO ............................. BALAMUCHEE ................................. BALANGO ......................................... DALANOS ......................................... BALANOS, Cool ............................... DALAPAHA ................................... B/DALAPAI ........................................... AALAZAN ......................................... CALBAN ............................................ BALBANO ......................................... DALBANY ......................................... CALBATON ....................................... DALBEE ............................................ CALBEMARLE .................................. BALBERS ......................................... DALBERTI ........................................ CALBERTON .................................... BALBERTVILLE ............................... CALBICALIS ..................................... DALBINAS ........................................ BALBION .......................................... BALBRIGHTS ................................... CALBURZ ..................................... B/C

ALBUS ............................................ BALCALDE ....................................... DALCAN ............................................ DALCESTER .................................... BALCOA ........................................... BALCONA ......................................... BALCOT ............................................ AALCOVA ......................................... BALCOVY .........................................CALDA, Saline .............................. B/DALDA ..............................................CALDAPE .........................................DALDAX ............................................ DALDEN ............................................ DALDENLAKE .................................. BALDER ............................................ CALDERDALE .................................. CALDERFLATS ................................ DALDERMAND ................................. BALDERON ...................................... BALDERWOOD ................................ CALDI ................................................ DALDINE .......................................... DALDING .......................................... DALDINO .......................................... CALDO .............................................. AALDRICH ........................................ CALEDO ........................................... CALEGROS ...................................... CALEKNAGIK ................................... CALEMEDA ...................................... CALESNA .........................................CALEX .............................................. BALEX, Wet Substratum ................. CALEXANDER .................................CALEXANDRIA ................................ CALFLACK ....................................... CALFORD ......................................... BALGARROBO ................................ AALGIERS .................................... C/DALGOA ........................................... CALGOMA ........................................ DALIBATES ...................................... BALIBI ............................................... CALICEL ........................................... BALICIA ............................................ BALIDA ............................................. BALIKCHI ......................................... BALINE ............................................. AALIRE ............................................. BALIVAR ........................................... BALKABO .........................................CALKIRIDGE .................................... CALLAGASH .................................... BALLAMORE .................................... DALLANTON ................................ B/DALLARD ......................................... BALLDOWN ...................................... CALLEN ............................................ BALLENDALE .................................. CALLENS PARK .............................. BALLENS PARK, Stony ................... CALLENTINE .................................... DALLENWOOD ................................ BALLHANDS .................................... DALLINGHAM .................................. BALLIS ..............................................DALLIVAR ......................................... BALLKER .......................................... BALLOUEZ ....................................... BALLOWAY ...................................... BALLUVIAL LAND ........................... AALLWIT .......................................... DALMAC ........................................... BALMAVILLE .................................... DALMIRANTE ................................... BALMO ............................................. DALMOND ........................................ CALMONT .........................................DALMORA ........................................ BALNITE ........................................... DALNULT .......................................... BALOGIA .......................................... CALOHA ........................................... CALOMAX .........................................D

A–41(210-VI-TR-55, Second Ed., June 1986)

Exhibit A: Hydrologic Soil Groups for the United States

WINTON ......................................... CWINTONER .................................... BWINU .............................................. CWINWELL ....................................... CWISBEY .......................................... BWISBY ............................................ BWISCOW ........................................ DWISCOY ......................................... CWISE .............................................. CWISEMAN ...................................... AWISFLAT ........................................ DWISHARD ...................................... CWISHBONE .................................... BWISHEYLU .................................... CWISHKAH ................................... C/DWISKAN ......................................... CWISKISPRINGS ............................. DWISNER ..................................... B/DWISTER .......................................... CWISTONA ....................................... BWITCHER ....................................... BWITHAM ......................................... DWITHEE .......................................... CWITHERBEE .............................. A/DWITHERELL ................................... DWITHERS ....................................... CWITTEN .......................................... DWITTENBERG ............................... BWIVILLE ......................................... BWIX ................................................. CWIXOM ........................................... BWIZARD ......................................... CWOCKLEY ..................................... CWOCKUM ....................................... BWODA ............................................ DWODAVAR ..................................... DWODEN .......................................... BWODOMONT ................................. DWODSKOW ................................ B/CWOHLY .......................................... BWOLCO .......................................... CWOLDALE .................................. C/DWOLFCREEK ................................ BWOLFER ........................................ BWOLFESON ................................... CWOLFESON, Wet .......................... DWOLFEY ........................................ CWOLFPEAK ................................... BWOLFPEN ...................................... AWOLFTEVER ................................. CWOLFVAR ...................................... BWOLLARD ...................................... CWOLLENT ...................................... DWOLOT .......................................... BWOLVERTON ................................ BWOMACK ....................................... CWOODBECK .................................. BWOODBINE ................................... BWOODBURN .................................. CWOODBURY .................................. DWOODCANYON ............................ CWOODCHOPPER .......................... BWOODCUTTER ............................. DWOODCUTTERS, Stony ............... BWOODFORD .................................. DWOODHURST ............................... CWOODIN ........................................ CWOODINGTON .......................... B/DWOODINVILLE .......................... C/DWOODLAWN ................................. BWOODLEAF ................................... CWOODLY........................................ BWOODMANSIE .............................. BWOODMERE ................................. BWOODMONT ................................. CWOODPASS .................................. BWOODROCK ................................. CWOODS .......................................... DWOODSFIELD ............................... CWOODSIDE ................................... BWOODSLAKE ................................ DWOODSON .................................... DWOODSPOINT .............................. BWOODSTOCK ............................... DWOODTEX ..................................... D

WOODVILLE .................................. DWOODWARD ................................. BWOODWEST .................................DWOOFUS ....................................... CWOOLLY ........................................ BWOOLPER .....................................CWOOLSEY ..................................... BWOOLSTALF ................................. BWOOLSTED ................................... BWOOLWICH ................................... BWOONSOCKET ............................. BWORCESTER ................................CWORFSTONE ................................CWORKMAN .................................... CWORLAND .....................................CWORLEY ........................................ DWORMCREEK ............................... BWORMET ....................................... BWORMSER .................................... CWORSHAM .................................... DWORSWICK ................................... CWORTHENTON ............................. DWORTMAN .................................... AWORWOOD ................................... CWOVOKA ....................................... DWRANGELL ................................... DWRAYHA ........................................ CWRAYS .......................................... BWREDAH........................................ BWREFORD .....................................DWRENCOE .....................................DWRENGART .................................. CWRENMAN .................................... CWRENTHAM .................................. CWRIGHTMAN .................................CWRIGHTSBORO ........................... CWRIGHTSVILLE ............................DWRIGHTWOOD ............................. BWUKOKI ......................................... BWUKSI ............................................ AWULFERT ...................................... DWUNABUNA .............................. C/DWUNJEY ........................................ BWUPATKI ....................................... DWURTSMITH ................................. AWUTCHUMNA ............................... DWYALUSING .................................. DWYANDOTTE ................................DWYANT ...........................................CWYARD .......................................... BWYARNO ....................................... BWYATT ...........................................CWYCOLO........................................ CWYEAST ........................................ DWYECREEK ................................... BWYETH ........................................... BWYEVILLE .....................................CWYICK ............................................ DWYKOFF ........................................ BWYLO ............................................. DWYNHOFF, Moist .......................... BWYNHOFF .....................................CWYNN ............................................. BWYNNVILLE .................................. CWYNONA ....................................... CWYNOOSE .....................................DWYNTOON ..................................... BWYOMINGCREEK......................... CWYOTITE ....................................... BWYRICK ......................................... BWYSOCKING ............................. C/DWYVA ............................................. DXANA .............................................. BXANADU ......................................... BXANKEY ......................................... BXAVIER .......................................... BXERTA ............................................ CXERXES .........................................DXICA ...............................................CXINE ...............................................CXIPE................................................ DXMAN ............................................. DXOBOBO ........................................ BYABAMAR ...................................... DYACHATS ...................................... B

YAD ................................................ DYAGGY, Protected ........................ BYAGGY ........................................... CYAHANA ......................................... DYAHARA ......................................... CYAHMORE ..................................... BYAHNE ........................................... CYAHOO ........................................... DYAINAX .......................................... BYAKOBI .......................................... DYAKUS ........................................... DYAKUTAT ....................................... AYALELAKE ..................................... BYALESVILLE .................................. CYALLANI ......................................... BYAMHILL ........................................ CYAMSAY ......................................... DYANA .............................................. BYANCY ........................................... DYANKEE ......................................... DYANKEEFORK............................... BYANKTON ...................................... BYAP ................................................. BYAPOAH ......................................... AYAQUI ............................................ BYAQUICAN .................................... DYAQUINA ................................... C/DYATA .............................................. CYATAHONEY ................................. CYATAHONEY, Stony ..................... DYATAMA ......................................... BYATES ............................................ DYAUCO ........................................... CYAUHANNAH ................................. BYAUPON ........................................ DYAWHEE ........................................ BYAWKEY ........................................ BYAWKOLA ...................................... CYAXING .......................................... BYEAGER ......................................... AYEARIAN ........................................ DYEARY ........................................... CYEATON ......................................... CYECROSS ...................................... AYEDLICK ........................................ BYEGUAS ......................................... CYELLOW HORSE .......................... DYELLOWBANK .............................. DYELLOWBAY ................................. BYELLOWDOG ................................ AYELLOWHILLS .............................. BYELLOWLARK ............................... CYELLOWMULE .............................. CYELLOWRIVER ............................. BYELLOWROCK .............................. BYELLOWSTONE ............................ DYELLOWWASH ............................. DYELM .............................................. CYELTON ......................................... CYENNICK ....................................... BYENSUS ......................................... BYEOPIM .......................................... BYERBA ........................................... DYERINGTON .................................. AYESUM ........................................... BYIGO ............................................... BYIKES ............................................. AYLIG ................................................ CYNOT .............................................. BYOAKRAN ...................................... AYOCHUM ........................................ CYODAL ........................................... BYODER ........................................... BYODY ............................................. CYOGAVILLE ................................... DYOHN ............................................. BYOKAYO ........................................ DYOKUT ........................................... BYOLLABOLLY ................................ DYOLOGO ........................................ DYOMONT ........................................ BYONCALLA .................................... CYONNA ........................................... DYORBA ........................................... DYOREL ........................................... B

YORK ............................................. CYORKSHIRE .................................. CYORKTOWN .................................. DYORKTREE ................................... CYOSEMITE ..................................... BYOST .......................................... C/DYOTES ........................................... BYOUGA, Sandy Substratum ......... DYOUJAY ......................................... DYOUMAN ........................................ CYOUNGSTON, Wet ....................... CYOURAME ..................................... BYOUTLKUE .................................... BYOVIMPA ....................................... DYPSI ................................................ CYRIBARREN .................................. DYTURBIDE ..................................... AYTURRIA ........................................ AYUCCA ........................................... BYUKON ........................................... DYUNES ........................................... DYURM ............................................. DYUTAN ........................................... BYUTRUE ......................................... DYUZARRA ...................................... BZAAR ..............................................DZABA .............................................. BZABOROSKY ................................. BZACA ..............................................DZACHARY ...................................... CZADE ..............................................CZADOG ....................................... A/DZADVAR ......................................... DZAFOD ........................................... BZAGG ............................................. CZAIDY ............................................. CZAKME ........................................... DZALCO ............................................ AZALEA ............................................ BZALESKA ....................................... DZALLA ............................................. AZALVIDEA ...................................... BZAMORA ........................................ BZANBUR ......................................... BZANE .............................................. BZANGO ........................................... DZAPA ..............................................CZAPATA .......................................... CZAQUA ........................................... DZARK ..............................................CZASTER ......................................... CZAU ................................................. CZAVALA .......................................... BZAVCO ........................................... CZAYANTE ....................................... AZBART ............................................ DZEALE ............................................ BZEB ................................................. BZEE ................................................. BZEEBAR ......................................... BZEEGEE ......................................... DZEEKA ............................................ CZEELAND ....................................... CZEELNOT ....................................... BZEEMAL ......................................... DZEGRO ........................................... CZEIBRIGHT .................................... BZEKIAH .......................................... DZELA ............................................... DZELDA ............................................ DZENDA ........................................... CZENIFF ........................................... BZENITH ........................................... BZENOBIA ........................................ BZENORIA ....................................... CZEOMONT ..................................... AZEPH ..............................................DZEPHYR ......................................... DZEPOL ............................................ BZERKEL ......................................... BZERKER ......................................... BZEUGIRDOR .................................. BZIBATE ........................................... DZIBETOD ........................................ DZIEGENFUSS ................................ D

B–1(210-VI-TR-55, Second Ed., June 1986)

Appendix B Synthetic Rainfall Distributions andRainfall Data Sources

The highest peak discharges from small watersheds inthe United States are usually caused by intense, briefrainfalls that may occur as distinct events or as part ofa longer storm. These intense rainstorms do not usu-ally extended over a large area and intensities varygreatly. One common practice in rainfall-runoff analy-sis is to develop a synthetic rainfall distribution to usein lieu of actual storm events. This distribution in-cludes maximum rainfall intensities for the selecteddesign frequency arranged in a sequence that is criticalfor producing peak runoff.

Synthetic rainfall distributions

The length of the most intense rainfall period contrib-uting to the peak runoff rate is related to the time ofconcentration (Tc) for the watershed. In a hydrographcreated with NRCS procedures, the duration of rainfallthat directly contributes to the peak is about 170percent of the Tc. For example, the most intense 8.5-minute rainfall period would contribute to the peakdischarge for a watershed with a Tc of 5 minutes. Themost intense 8.5-hour period would contribute to thepeak for a watershed with a 5-hour Tc.

Different rainfall distributions can be developed foreach of these watersheds to emphasize the criticalrainfall duration for the peak discharges. However, toavoid the use of a different set of rainfall intensities foreach drainage area size, a set of synthetic rainfalldistributions having “nested” rainfall intensities wasdeveloped. The set “maximizes” the rainfall intensitiesby incorporating selected short duration intensitieswithin those needed for longer durations at the sameprobability level.

For the size of the drainage areas for which NRCSusually provides assistance, a storm period of 24 hourswas chosen the synthetic rainfall distributions. The 24-hour storm, while longer than that needed to deter-mine peaks for these drainage areas, is appropriate fordetermining runoff volumes. Therefore, a single stormduration and associated synthetic rainfall distributioncan be used to represent not only the peak dischargesbut also the runoff volumes for a range of drainagearea sizes.

The intensity of rainfall varies considerably during astorm as well as geographic regions. To representvarious regions of the United States, NRCS developedfour synthetic 24-hour rainfall distributions (I, IA, II,and III) from available National Weather Service(NWS) duration-frequency data (Hershfield 1061;Frederick et al., 1977) or local storm data. Type IA isthe least intense and type II the most intense shortduration rainfall. The four distributions are shown infigure B-1, and figure B-2 shows their approximategeographic boundaries.

Types I and IA represent the Pacific maritime climatewith wet winters and dry summers. Type III representsGulf of Mexico and Atlantic coastal areas where tropi-cal storms bring large 24-hour rainfall amounts. TypeII represents the rest of the country. For more precisedistribution boundaries in a state having more thanone type, contact the NRCS State Conservation Engi-neer.

0 3 6 9 12 15 18 21 24

1.0

0.5

0.0

IA

I

II

III

Time (hours)F

rac

tio

n o

f2

4-h

ou

r ra

infa

ll

Figure B-1 SCS 24-hour rainfall distributions

B–2 (210-VI-TR-55, Second Ed., June 1986)

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Type I

Type IA

Type II

Type IIIIII

III

III

I

IA

II

III

Rainfall

Distribution

Figure B-2 Approximate geographic boundaries for NRCS (SCS) rainfall distributions

Rainfall data sourcesThis section lists the most current 24-hour rainfall datapublished by the National Weather Service (NWS) forvarious parts of the country. Because NWS TechnicalPaper 40 (TP-40) is out of print, the 24-hour rainfallmaps for areas east of the 105th meridian are includedhere as figures B-3 through B-8. For the area generallywest of the 105th meridian, TP-40 has been supersededby NOAA Atlas 2, the Precipitation-Frequency Atlas ofthe Western United States, published by the NationalOcean and Atmospheric Administration.

East of 105th meridian

Hershfield, D.M. 1961. Rainfall frequency atlas of theUnited States for durations from 30 minutes to 24hours and return periods from 1 to 100 years. U.S.Dept. Commerce, Weather Bur. Tech. Pap. No. 40.Washington, DC. 155 p.

West of 105th meridian

Miller, J.F., R.H. Frederick, and R.J. Tracey. 1973.Precipitation-frequency atlas of the Western UnitedStates. Vol. I Montana; Vol. II, Wyoming; Vol III, Colo-rado; Vol. IV, New Mexico; Vol V, Idaho; Vol. VI, Utah;Vol. VII, Nevada; Vol. VIII, Arizona; Vol. IX, Washing-ton; Vol. X, Oregon; Vol. XI, California. U.S. Dept. of

Commerce, National Weather Service, NOAA Atlas 2.Silver Spring, MD.

Alaska

Miller, John F. 1963. Probable maximum precipitationand rainfall-frequency data for Alaska for areas to 400square miles, durations to 24 hours and return periodsfrom 1 to 100 years. U.S. Dept. of Commerce, WeatherBur. Tech. Pap. No. 47. Washington, DC. 69 p.

Hawaii

Weather Bureau. 1962. Rainfall-frequency atlas of theHawaiian Islands for areas to 200 square miles, dura-tions to 24 hours and return periods from 1 to 100years. U.S. Dept. Commerce, Weather Bur. Tech. Pap.No. 43. Washington, DC. 60 p.

Puerto Rico and Virgin Islands

Weather Bureau. 1961. Generalized estimates of prob-able maximum precipitation and rainfall-frequencydata for Puerto Rico and Virgin Islands for areas to 400square miles, durations to 24 hours, and return periodsfrom 1 to 100 years. U.S. Dept. Commerce, WeatherBur. Tech. Pap. No. 42. Washington, DC. 94 P.


Figure B-3 2-year, 24-hr rainfall

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U.S. DEPARTMENT OF AGRICULTURE NATURAL RESOURCES CONSERVATION SERVICE

0 200 600 Mi

Rainfall iso-line

Rainfall iso-line half unit

Rainfall iso-line (depression)

Albers Equal Area Projection

400

2-Year 24-Hour Rainfall (inches)

1.5

2.0

2.53.0

3.5

4.0 4.5

5.05.5 6.0

6.05.5

5.0

4.55.5

6.06.05.5

5.0

5.0

5.0

4.54.0

4.0

3.5

3.0

2.5

3.0

3.5

4.03.5

5.0

2.0

2.0

2.0

2.5

3.0

3.5

4.0

4.5

2.0USE NOAA ATLAS 2 MAPS FOR

WESTERN STATES

Figure B-4 5-year, 24-hour rainfall

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400

5–Year 24–Hour Rainfall (inches)

USE NOAA ATLAS 2 MAPS FOR

WESTERN STATES

7

6.5

6

6.57

7.58

6.5

65.5

5

4.5

4.55

4.55

4

4 3.5

3

2.5

2.5

2.5

3

2.5

3

3.5

4

4.5

3

5

5.5

6

6.57

2.5

56

33.5

44.5

5

5.56

6.5

77.5

8 8.5

8.5 87.5

7

B–4 (210-VI-TR-55, Second Ed., June 1986)


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25-Year 24- Hour Rainfall (inches)

7

8

67

66.5

6

6

8.5

9

10

11

11

1099

810

101112

11

10

98

7

65

4

3.5

43.5

3.5

4

3.5

3.55

6

7

8

9

7 89

4

5


WESTERN STATES


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3

3.5

45

6

7

8

910

109

87

8

8

9

9

6

7

5

4

3.5

5

6

6

67

3

3

3.5

3

3


WESTERN STATES

3.5

4

5

6

7

8




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400



WESTERN STATES

6

6

7

5

4.5

6

7

8

9

10

11

78 8

910

10

1112

11

10

10

10

11

12

13

1211

109

8

7654.5

43.5

5

4.54

4

4.5

5

4

7

89


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8

9

76

5

4

10

11

12 13

1415

15 1413

12

1111

13

14

1413

12

11

11

1110

9

8

7

6

5

7

8

89

89

10

5

6

5 7

8

9

10

11

12

54

13

4

4

400

10


WESTERN STATES

12

urban hydrology for small watersheds usda

Documents

approximate geographic boundaries

intensities vary greatly

national weather service

shallow concentrated flow

high infiltration rates

unit peak discharge

actual storm events

natural desert landscaping