TM 5820-4/AFM 88-5, Chap 4
dom exceed 0.2 inches per hour and could be sub-
Table 21. Typical Values of Infiltration Rates
stantially less than that rate.
Soil group
Infiltration,
f. In selecting the design storm and making other
Description
symbol
inches/hour
design decisions, particular attention must be given
0.8-1.0
GW, GP
Sand and gravel mixture
to the hazard to life and other disastrous conse-
SW, SP
quences resulting from the failure of protective
GM, SM
Silty gravels and silty sands to
0.30.6
ML, MH
works during a great flood. Potentially hazardous
inorganic silt, and well-devel-
oped loams
OL
situations must be brought to the attention of the
SC, CL
0.20.3
Silty clay sand to sandy clay
using service and others concerned so that ap-
0.10.2
Clays, inorganic and organic
CH, OH
propriate steps can be taken.
0.0-0.1
Bare rock, not highly fractured
--------
U.S. Army Corps of Engineers
23. Infiltration and other losses.
a. Principal factors affecting the computation of
ponds along the drainage route. The effects are
runoff from rainfall for the design of military
reflected in the C factor of the Rational Formula
drainage systems comprise initial losses, infiltra-
(given below) or in the shape of the unit hydro-
tion, transitory storage, and, in some areas, per-
graphy. Flow-routing techniques must be used to
colation into natural streambeds. If necessary data
predict major storage effects caused by natural
are available, an excellent indication of the mag-
topography or man-made developments in the area.
nitudes of these factors can be derived from thor-
ough analysis of past storms and recorded flows
d. Streambed percolation losses to direct runoff
by the unit-hydrograph approach. At the onset of
need to be considered only for sandy, alluvial wa-
a storm, some rainfall is effectively retained in
tercourses, such as those found in arid and semi-
"wetting down" vegetation and other surfaces, in
arid regions. Rates of streambed percolation com-
satisfying soil moisture deficiencies, and in filling
monly range from 0.15 to 0.5 cubic feet per second
surface depressions. Retention capacities vary
per acre of wetted area.
considerably according to surface, soil type, cover,
and antecedent moisture conditions. For high in-
tensity design storms of the convective, thunder-
2-4. Runoff computations.
storm type, a maximum initial loss of up to 1 inch
a. Design procedures for drainage facilities in-
may be assumed for the first hour of storm pre-
volve computations to convert rainfall intensities
cipitation, but the usual values are in the range
expected during the design storm into runoff rates
of 0.25 to 0.50 inches per hour. If the design rain-
which can be used to size the various elements of
fall intensity is expected to occur during a storm
the storm drainage system. There are two basic
of long duration, after substantial amounts of im-
approaches: first, direct estimates of the propor-
mediately prior rain, the retention capacity would
tion of average rainfall intensity that will appear
have been satisfied by the prior rain and no fur-
as the peak runoff rate; and, second, hydrography
ther assumption of loss should be made.
methods that depict the time-distribution of run-
b. Infiltration rates depend on type of soils, veg-
off events after accounting for losses and atten-
etal cover, and the use to which the areas are
uation of the flow over the surface to the point of
subjected. Also, the rates decrease as the duration
design. The first approach is exemplified by the
of rainfall increases. Typical values of infiltration
Rational Method which is used in the large ma-
for generalized soil classifications are shown in
jority of engineering offices in the United States.
It can be employed successfully and consistently
table 2-1. The soil group symbols are those given
in MIL-STD-619, Unified Soil Classification Sys-
by experienced designers for drainage areas up to
tem for Roads, Airfields, Embankments, and
1 square mile in size. Design and Construction of
Foundations. These infiltration rates are for un-
Sanitary and Stem Sewers, ASCE Manual No.
compacted soils. Studies indicate that compacted
37, and Airport Drainage, FAA AC 150/5320-5B,
soils decrease infiltration values from 25 to 75 per-
explain and illustrate use of the method. A mod-
cent, the difference depending on the degree of
ified method is outlined below. The second ap-
compaction and the soil type. Vegetation gener-
proach encompasses the analysis of unit-hydro-
graph techniques to synthesize complete runoff
ally decreases the infiltration capacity of coarse
hydrography.
soils and increases that of clayey soils.
b. To compute peak runoff the empirical formula
c. Peak rates of runoff are reduced by the effect
of transitory storage in watercourses and minor
Q=C(1-F)A can be used; the terms are defined
2-2
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