WORLD FOR CIVIL

Method For Determining Runoff

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.

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