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=KF^nl/(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.

If you have a query, you can ask a question here.