Practice Question: Estimating Runoff from a Watershed using NRCS Curve Number

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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:

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

 

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

 

Intensity-Duration-Frequency curves for a 50-year storm, duration = 60 minutes, result in an intensity (I) of 3 inches per hour.

\[P=precipitation (in)=rainfall intensity×\ duration\]

\[P=3×1=3\ in\]

Maximum basin retention (in.) is given by:

\[S=\frac{1,000}{\overline{CN}}-10\]

\[S=\frac{1,000}{60.8}-10=6.45\]

Runoff depth is given by:

\[Q=\frac{{\left(P-0.2S\right)}^2}{P+0.8S}\]

\[Q=\frac{{\left(3-0.2\times 6.45\right)}^2}{(3+0.8\times 6.45)}=0.36\ in\]
 

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