TM 5-820-1/AFM 88-5, Chap. 1
studies or from similar acceptable information. Suggested mean values of infiltration for generalized soil
classifications are shown in table 1. The soil group symbols are those given in MIL-STD-619.
Infiltration values are for uncompacted soils. Studies indicate that where soils are compacted, infiltration
values decrease; the percentage decrease ranges from 25 to 75 percent, depending on the degree of
compaction and the types of soil. Vegetation generally decreases infiltration capacity of coarse soils and
increases that of clayey soils. The infiltration rate after 1 hour of rainfall for turfed areas is
approximately 0.5 inch per hour and seldom exceeds 1.0 inch per hour. The infiltration rate for paved or
roofed areas, blast protective surfaces, and impervious dust-palliative-treated areas is zero.
Table 1. Infiltration rate for generalized soil
classifications (uncompacted)
Infiltration,
Soil Group
inch per hour
Description
Symbol*
7. Rate of supply. Rate of supply refers to the difference between the rainfall intensity and the
infiltration capacity at the same instant for a particular storm. To simplify computations, the rainfall
intensity and the infiltration capacity are assumed to be uniform during any specific storm. Thus the
rate of supply during the design storm will also be uniform.
a. Average rate of supply. Average rates of supply corresponding to storms of different lengths and
the same average frequency of occurrence may be computed by subtracting estimated infiltration
capacities from rainfall intensities represented by the selected standard rainfall intensity-duration curve
in figure 2. For convenience and since no appreciable error results, standard supply curves are assumed
to have the same shapes as those of the standard rainfall intensity-duration curves shown in figure 2.
For example, if supply curve No. 2.2 in figure 2 were selected as the design storm and the infiltration
loss during a l-hour storm were estimated as 0.6 inch, supply curve No. 1.6 would be adopted as the
standard supply curve for the given areas.
and unpaved areas having different infiltration capacities. A weighted standard supply should be
established for the composite drainage areas by weighting the standard supply curve numbers adopted
for paved and unpaved surfaces in proportion to their respective tributary area. An example is given in
appendix B.
8. Runoff. The method of runoff determination described herein is based on an overland flow model.
Details are given in appendix B.
a. Overland flow. The surface runoff resulting from a uniform rate of supply is termed "overland
flow." If the rate of supply were to continue indefinitely, the runoff would rise to a peak rate and remain
constant. Ordinarily, the peak rate is established after all parts of the drainage surface are contributing
to runoff. However, in cases of odd-shaped areas and areas containing both paved and turfed areas, peak
runoff rates may occur before all areas are contributing. The elapsed time for runoff to build to a peak is
termed the "time of concentration," which depends primarily on the coefficient of roughness, the slope,
and the effective length of the surface. When the supply terminates, the runoff rate diminishes, but
continues until the excess stored on the surface drains away.
b. Effective length. The effective length to the point under consideration must account for the
effects of overland and channel flow and for the differences in roughness and slope of the drainage
surface. Methods for determining effective length are presented in appendix B.
supply curves No. 2.0 and 2.2. Curves for other supply rates are given in appendix B (figs B-7 through
of supply or time of concentration, tC; the effective length of overland flow, L; and the resulting
maximum rate of runoff. The curves are not complete hydrography for any specific design storm, but are
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