NCRS Method

7/25/2019 NCRS Method

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NRCS (SCS) Curve Number Method.

Rational Method.

Estimation of Peak Flow


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Steps in using the NCRS Method

Calculate the composite curve number

Calculate the retention, S, using Equation 3

Calculate the depth of direct runoff using

Determine Ia/P from Table

Determine coefficients from Table

Determine tc

Calculate unit peak flow using.

Calculate peak flow using.


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SCS Curve Number Method for

Effective Rainfall (Runoff)

The model name originatesfrom the fact that, in its

application a watershed ischaracterised by a single

parameter called the curvenumber CN.

Many years of informationdeveloped by SCS based

analyses of gauged

watersheds facilitate CNestimates.

The method is widely used

due to its simplicity and theavailability of empiricalinformation on which to base

estimates of the curvenumber CN.

The method was developedfor computing abstractions

from storm rainfall


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Effective rainfall (Runoff)

For the storm as a whole,the depth of excess

precipitations or directrunoff

P

e is always less

than or equal to the depthof precipitationP

After runoff begins, theadditional depth of waterretained in the watershed,

F

a, is less than or equal to

some potential maximumretentionS.

There is some amount of

rainfall

I

a (initialabstraction before

ponding begins) for whichno runoff will occur, thuspotential runoff is

P

I

a.


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The basic equationfor computing the

depth of effective ordirect runoff from a

storm.

SIP

IPP

a

a

e

2)(


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By study of results frommany small experimentalwatersheds, an empiricalrelation was developed

between

I

a and

Sas

Thus

SIa

2.0

SP

SPPe

8.0)2.0(

2


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The curve number CN

andSare related by



Plotting the data forP andPefrom many watersheds, the SCScurves are obtained.

To standardize these curves, adimensionless curve number CN isdefined such that

0 CN 100.

For impervious and watersurfaces CN = 100.

For natural surfaces CN < 100

)(101000

)(25425400

)(4.252540

inCN

S

mmCN

S

cmCN

S


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The curve numbers used in the above equation apply

for normal antecedent moisture conditions (AMC II).

For dry conditions (AMC I) or wet conditions (AMC III),equivalent curve numbers can be computed by

condition)(wet)(13.010

)(23)(

condition)dry()(058.010

)(2.4

)(

IICN

IICNIIICN

IICN

IICNICN


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The range of antecedent moisture conditions

for each class

Total 5-day antecedent rainfall (cm)

AMC group Dormant season Growing season

I Less than 1.3 Less than 3.6

II 1.3 to 2.8 3.6 to 5.4

III Over 2.8 Over 5.4

Adapted from Chow et al. (1988)


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Curve numbers have been tabulated by the Soil Conservation

Service on the basis of soil type and land use and presented in

Chow et al., (1988)

Four

soil

groups

defined

Group A: Deep sand,deep loess,

aggregated silts

Group B: Shallowloess, sandy loam

Group C: Clay loam,shallow sandy loam,soils low in organiccontent, and soils

usually high in clay

Group D: Soils thatswell significantlywhen wet, heavyplastic clays, and

certain saline soils


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Runoff curve numbers for selected agricultural, suburban, and

urban land uses (antecedent moisture condition II, Ia = 0.2S)

Land Use Description Hydrologic Soil Group

A B C D

Cultivated land: Without conservation treatment

With conservation treatment

72 81 88 91

62 71 78 81Pasture or range land: Poor condition

Good condition

68 79 86 89

39 61 74 80

Meadow: good condition 30 58 71 78

Wood or forest land: Thin stand, poor cover, no mulch

Good cover

45 66 77 83

25 55 70 77

Open spaces, lawns, parks, golf courses, cemeteries, etc.

Good condition: grass cover on 75% or more or the area

Fair condition: grass cover on 50% to 75% of the area

39 61 74 80

49 69 79 84

Commercial and business areas (85% impervious) 89 92 94 95

Industrial districts (72% impervious) 81 88 91 93

Residential

Average lot size Average % impervious

1/8 acre or less 65 77 85 90 92

acre 38 61 75 83 871/3 acre 30 57 72 81 86

acre 25 54 70 80 85

1 acre 20 51 68 79 84

Paved parking lots, roofs, driveways, etc. 98 98 98 98

Streets and roads:

Paved with curbs and storm sewers 98 98 98 98

Gravel 76 85 89 91Dirt 72 82 87 89


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Adjusted equation for

ponding in drainage

basin

SCS Peak Flow Estimation

Having obtained the effective

rainfallPe, the peak flow is

estimated by the equation

Unit peak runoff rate is

estimated as

euAPqQ

K

fu Cq 10.2

1021010 )(loglog cc tCtCCK

QFQ pa


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Water Movers through a Watershed as:

Sheet flow

Shallow concentrated flow

Open channel flow, or

A combination of these.

V

LTt

3600

where:

Tt = travel time (hr)

L = flow length (ft)

V = average velocity (ft/s)

321 TTTTT

tc


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Sheet Flow

Shallow flow depth (< 0.1 ft) over plane surfaces

Only for flows up to 300 feet

4.05.0

2

8.0

)()(0913.0

sP

nLTt

where:

Tt = travel time (hr)

n = mannings roughness coefficient (table 3-1)L = flow length (m)

P2 = 2-year, 24-hour rainfall (mm)

s = slope of hydraulic grade line (land slope, m/m)


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Symbols Defined

Q = peak flow (m3/s)

qu = unit peak runoff rate

(m3/s/km2/mm)

A = catchment area (km2

) Pe= depth of effective

rainfall (mm)

tc= the time of concentration

(hr)

C0, C1, and C2 are coefficients

read from tables based on

Coefficients, listed in Tables, these

are a function of the 24 hourrainfall distribution type and Ia/P.

Qa = adjusted peak flow

(m3/s)

Fp = adjustment factor

Cf= conversion factor =

0.0043 for SI units

Ia = Initial abstraction (mm)with Ia= 0.2S


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Example

Find: The 10-year peak flow using the SCS peak flow method.

Given: The following physical and hydrologic conditions.

3.3 sq km of fair condition open space and 2.8 sq km oflarge lot residential

Negligible pond and swamp land

Hydrologic soil type C

Average antecedent moisture conditions Time of concentration is 0.8 hr

24-hour, type II rainfall distribution, 10-year rainfall of 150mm


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Step 1: Calculate the composite curve number using Table and

Equation

CN = (CNx Ax)/A = [3.3(79) + 2.8(77)]/(3.3 + 2.8) = 78

Step 2: Calculate the retention, S, using EquationS = 25.4(1000/CN - 10) = 25.4 [(1000/78) - 10] = 72 mm

Step 3: Calculate the depth of direct runoff using Equation

Pe = (P-0.2S )2 / (P+0.8S ) = [150 - 0.2(72)]2/[[150

+ 0.8(72)] = 89 mm

Step 4: Determine Ia/P from Table

Ia/P = 0.10 (Ia= 0.2S)


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Step 5: Determine coefficients from Table

C0 = 2.55323 ,C1 = -0.61512 C2 = -0.16403

Step 6: Calculate unit peak flow using Equation

qu = (0.000431) (10C0+C1log tc + C2 (log tc )2 )

qu=(0.000431)(10[ 2.55323+(0.61512) log (0.8)+(0.16403) [log (0.8)] 2])

qu = 0.176 m3/s/km2/mm

Step 7: Calculate peak flow using Equation qp = qu Ak Pe = (0.176)(3.3 + 2.8)(89) = 96 m

3/s


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NCRS Method

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