PE Civil Practice: Estimating Composite NRCS Curve Number for a Watershed

NCEES Civil PE Specification Hydraulics and Hydrology: Storm characteristics and runoff analysis (SCS/NRCS methods)

Problem

A watershed, covering an area of 285 acres, is divided into four distinct land use classifications, as outlined in the table below. What is the nearest value for the composite NRCS curve number for this watershed?

Select an Answer:

A) 50
B) 60
C) 70
D) 80

% Students Correctly Answered:

85.6%

Solution

The following values of curve number are obtained for average watershed condition:

(Ref: Civil PE Reference Handbook V2.0, Section: 6.5.2.2 NRCS (SCS) Rainfall Runoff Method)

Region	Area (acres)	Land Use	Soil Type	Curve Number
1	120	Residential homes on 1 ac lot	A	51
2	85	Lawn in good condition	B	61
3	15	Streets and roads	D	98
4	65	Woods in good condition	C	70

Composite curve number is given by:

\[\overline{CN}=\frac{A_1{CN}_1+A_2{CN}_2+\ .\ \ .\ \ .\ A_n{CN}_n}{A_1+A_2+.\ .\ .\ A_n}\]

\[\overline{CN}=\frac{120\times 51+85\times 61+15\times 98+65\times 70}{120+85+15+65}=60.8\]

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Practice Question: Estimating Composite NRCS Curve Number for a Watershed

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