TM 5820-4/AFM 88-5, Chap 4
in appendix D. This equation is known as the mod-
In such cases, the upper portion of the drainage
ified rational method.
areas should be ignored and the peak flow com-
(1) C is a coefficient expressing the percentage
putation should be based only on the more effi-
to which the peak runoff is reduced by losses (other
cient, downstream portion.
than infiltration) and by attenuation owing to
(3) For all durations, the infiltration rate is
transitory storage. Its value depends primarily on
assumed to be the constant amount that is estab-
surface slopes and irregularities of the tributary
lished following a rainfall of 1 hour duration. Where
area, although accurate values of C cannot readily
F varies considerably within a given drainage area,
be determined. For most developed areas, the ap-
a weighted rate may be used; it must be remem-
parent values range from 0.6 to 1.0. However, val-
bered, however, that previous portions may re-
ues as low as 0.20 for C may be assumed in areas
quire individual consideration, because a weighted
with low intensity design rainfall and high infil-
overall value for F is proper only if rainfall in-
tration rates on flat terrain. A value of 0.6 may
tensities are equal to or greater than the highest
be assumed for areas left ungraded where mean-
infiltration rate within the drainage area.
dering-flow and appreciable natural-ponding ex-
In design of military construction drainage sys-
ists, slopes are 1 percent or less, and vegetal cover
tems, factors such as initial rainfall losses and
is relatively dense. A value of 1.0 may be assumed
channel percolation rarely enter into runoff com-
applicable to paved areas and to smooth areas of
putations involving the Rational Method. Such
substantial slope with virtually no potential for
losses are accounted for in the selection of the C
surface storage and little or no vegetal cover.
coefficient.
(2) The design intensity is selected from the
c. Where basic hydrologic data on concurrent
appropriate intensity-duration-frequency rela-
rainfall and runoff are adequate to determine unit
tionship for the critical time of concentration and
hydrography for a drainage area, the uncertain-
for the design storm frequency. Time of concen-
ties inherent in application of the Rational Method
tration is usually defined as the time required,
can largely be eliminated. Apparent l0SS rates de-
under design storm conditions, for runoff to travel
termined from unit-hydrograph analyses of re-
from the most remote point of the drainage area
corded floods provide a good basis for estimating
to the point in question. In computing time of con-
loss rates for storms of design magnitude. Also,
centration, it should be kept in mind that, even
flow times and storage effects are accounted for
for uniformly graded bare or turfed ground, over-
in the shape of the unit-hydrograph. Where basic
land flow in "sheet" form will rarely travel more
data are inadequate for direct determination of
than 300 or 400 feet before becoming channelized
unit-hydrographs, use may be made of empirical
and thence move relatively faster; a method which
methods for synthesis. Use of the unit-hydro-
may be used for determining travel-time for sheet
graph method is particularly desirable where de-
flow is given in TM 5-820-1/AFM 88-5, Chap 1.
signs are being developed for ponds, detention res-
Also, for design, the practical minimum time of
ervoirs, and pump stations; where peak runoff from
concentration for roofs or paved areas and for rel-
large tributary areas is involved in design; and
atively small unpaved areas upstream of the up-
where large-scale protective works are under con-
permost inlet of a drainage system is 10 minutes;
sideration. Here, the volume and duration of storm
smaller values are rarely justifiable; values up to
runoff, as opposed to peak flow, may be the prin-
20 minutes may be used if resulting runoff ex-
cipal design criteria for determining the dimen-
cesses will not cause appreciable damage. A min-
sions of hydraulic structures.
imum time of 20 minutes is generally applicable
for turfed areas. Further, the configuration of the
d. Procedures for routing storm runoff through
reservoir-type storage and through stream chan-
most remote portion of the drainage area may be
such that the time of concentration would be
nels can be found in publications listed in appen-
lengthened markedly and thus design intensity
dix E and in the available publications on these
and peak runoff would be decreased substantially.
subjects.
2-3
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