CIVIL ENGINEERING FORMULAS

HYDRAULICS AND WATERWORKS FORMULAS 333

eaactual vapor pressure, in (mm), of mercury, in air based on monthly

mean air temperature and relative humidity at nearby stations for

small bodies of shallow water or based on information obtained

about 30 ft (9.14 m) above the water surface for large bodies of deep

water

wmonthly mean wind velocity, mi/h (km/h), at about 30 ft (9.14 m)

aboveground

#wind factor

As an example of the evaporation that may occur from a large reservoir, the
mean annual evaporation from Lake Mead is 6 ft (1.82 m).

METHOD FOR DETERMINING RUNOFF FOR MINOR
HYDRAULIC STRUCTURES

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

(12.131)

whereQpeak discharge, ft^3 /s (m^3 /s)
Crunoff coefficient percentage of rain that appears as direct runoff
Irainfall intensity, in/h (mm/h)
Adrainage area, acres (m^2 )

COMPUTING RAINFALL INTENSITY

Chow lists 24 rainfall-intensity formulas of the form:

(12.132)

where Irainfall intensity, in/h (mm/h)
K,b,n, and n 1 coefficient, factor, and exponents, respectively, depend-
ing on conditions that affect rainfall intensity
Ffrequency of occurrence of rainfall, years
tduration of storm, min
time of concentration

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

I (12.133)

K

tb

I

KFn^1

(tb)n

QCIA