Page 406 - Civil Engineering Formulas

HYDRAULICS AND WATERWORKS FORMULAS 333

e actual vapor pressure, in (mm), of mercury, in air based on monthly
a
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
w monthly 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 the rational formula:
Q CIA (12.131)
3
3
where Q peak discharge, ft /s (m /s)
C runoff coefficient percentage of rain that appears as direct runoff
I rainfall intensity, in/h (mm/h)
2
A drainage area, acres (m )

COMPUTING RAINFALL INTENSITY

Chow lists 24 rainfall-intensity formulas of the form:

KF n1
I (12.132)
(t b) n
where I rainfall intensity, in/h (mm/h)
K, b, n, and n coefficient, factor, and exponents, respectively, depend-
1
ing 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:

K
I (12.133)
t b