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# Method For Determining Runoff

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METHOD FOR DETERMINING RUNOFF FOR MINOR HYDRAULIC STRUCTURES

The most common means for determining runoff for minor hydraulic structures is the rational formula:

Q=CIA

where
Q= peak discharge, ft3/s (m3/s)

C =runoff coefficient percentage of rain that appears as direct runoff

I= rainfall intensity, in/h (mm/h)

A= drainage area, acres (m2)

COMPUTING RAINFALL INTENSITY.

Chow lists 24 rainfall-intensity formulas of the form:

I=KFnl/(t +b)n

where
I= rainfall intensity, in/h (mm/h)
K, b, n, and n1 coefficient, factor, and exponents, respectively, depending on conditions that affect rainfall intensity

F =frequency of occurrence of rainfall, years
t= duration of storm, min =time of concentration

Perhaps the most useful of these formulas is the Steel formula:

I=K/(t +b)

where
K and b are dependent on the storm frequency and region.
The Steel formula gives the average maximum precipitation rates for durations up to 2 h.

Coefficients for steel formula

This table shows the values of K and
b according to regions 1 to 7 in United States.

 Frequency and symbols 2 K b 20630 14021 10617 7013 7016 6814 3211 4 K b 24729 19025 13119 9716 8113 7512 4812 10 K b 30036 23029 17023 11116 11117 12223 6013 25 K b 32733 26032 23030 17027 13017 15526 6710

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