TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
lications of the U.S. Weather Bureau or other refer-
carefully before being used in the design of a major de-
ence data. Maps showing amounts of rainfall that can
watering or drainage system.
be expected once every 2, 5, and 10 years in 10-, 30-,
and 60-minute duration of rainfall are shown in figure
capacity of power at a site should be investigated prior
3-6. The coefficient of runoff c within the excavation
to selecting or designing the pumping units for a dewa-
will depend on the character of soils present or the
tering system. Types of power used for dewatering sys-
treatment, if any, of the slopes. Except for excavations
tems include electric, natural gas, butane, diesel, and
in clean sands, the coefficient of runoff c is generally
gasoline engines, Electric motors and diesel engines
from 0.8 to 1.0. The rate of runoff can be determined
are most commonly used to power dewatering equip-
as follows:
ment.
Q = ciA
(3-3)
3-6. Surface water. Investigations for the con-
where
trol of surface water at a site should include a study of
rate of runoff, cubic feet per second
precipitation data for the locality of the project and de-
Q=
c=
coefficient of runoff
termination of runoff conditions that will exist within
i=
intensity of rainfall, inches per hour
the excavation. Precipitation data for various localities
A=
drainage area, acres
and the frequency of occurrence are available in pub
(U. S. Department of Agriculture Miscellaneous Publication No. 204)
Figure 3-6. Inches of rainfall during 10- and 30-minute and 1-hour periods.
3-8
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