PE Civil Practice: Runoff analysis

NCEES Civil PE Specification Hydraulics and Hydrology– Runoff analysis

Problem

You are responsible for the design of a detention basin system intended to collect stormwater from a 1.5-inch rainfall event.

Each basin has internal dimensions of 20 ft x 40 ft with a maximum depth of 6 ft.

Given the following information, use the SCS/NRCS method to determine the number of standard basins required to accommodate the rainfall event:

Total land \(A = 5 \ acre\).
The land is a fully developed urban area (vegetation established).
The land is composed of 60% fair condition grass cover and 40% commercial development.
The land infiltration rate is 0.2 in/hr.
Initial abstraction = 20% of maximum basin retention.

None

Select an Answer:

A) 1
B) 2
C) 3
D) 4

% Students Correctly Answered:

27.6%

Solution

Answer: B) 2 Basins

Refer to PE Civil Reference Handbook Section 6.5.2.2

\[Q= {\frac{(P-0.2\times S)^2}{P+0.8\times S}}\\\]
\[S= {\frac{1000}{CN}-10}\\\]
where:

\(Q= runoff \ depth \ (in)\)

\(P=Precipitation = 1.5 \ in\)

\(S= maximum \ basin \ retention \ (in)\)

\(CN = Curve \ Number\)

Refer to SCS hydrological soil group type classification, knowing that soil infiltration = 0.2 in/hr. --> Soil is under group B

Refer to runoff curve number page 381 in the PE Civil Reference Handbook.

Land portion 1 - 60% fair condition grass cover:
\[CN_1 = 69\]

\[S_{1} = 1000/69 -10 = 4.49\\\]\[Q_{1}= {\frac{(1.5-0.2\times4.49)^2}{1.5+0.8\times4.49}}=0.071\ inch\]\[V_{1} = 0.071 /12 (ft) \times 60\% \times 5\ acre \times 43560\ ft^2/acre = 773.2\ ft^3\]
Land portion 2 - 40 % commercial and business areas:
\[CN_2 = 92\]
\[S_{2}= 1000/92 -10 = 0.87\\\]
\[Q_{2}= {\frac{(1.5-0.2\times0.87)^2}{1.5+0.8\times0.87}}=0.8\ inch\]

\[V_{2} = 0.8 /12 (ft) \times 40\% \times 5\ acre \times 43560\ ft^2/acre = 5808\ ft^3\]

Total Volume:
\[773.2+5808= 6581.3ft^3\\\]

Volume of one standard basin:

\[20\times40\times6 = 4800 ft^3\\\]

Required number of basins:
\[6581.3/ 4800 = 1.4 \cong 2\ Basins\\\]

Leave us feedback (this site is actively updated for errors, discrepancies, and references):

Practice Question: Runoff analysis

👋 Finding this Page Helpful? Please leave us some feedback!