NCRS Method
Transcript of 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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SCS Curve Number Method for
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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SCS Curve Number Method for
Effective rainfall (Runoff)
The basic equationfor computing the
depth of effective ordirect runoff from a
storm.
SIP
IPP
a
a
e
2)(
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SCS Curve Number Method for
Effective rainfall (Runoff)
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
SCS Curve Number Method for
Effective rainfall (Runoff)
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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SCS Curve Number Method for
Effective rainfall (Runoff)
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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SCS Curve Number Method for
Effective rainfall (Runoff)
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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SCS Curve Number Method for
Effective rainfall (Runoff)
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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