Page 415 - Planning and Design of Airports

360 Airp o r t D e sign

The infiltration rate is dependent largely on the structure of the
soil cover, moisture content, and temperature of the air. The infiltra-
tion rate is not constant throughout the duration of the storm, but is
assumed so in the computations. It is felt that such an assumption is
reasonable, especially when the soil is near saturation.
The infiltration rate for paved surfaces is usually assumed to be
zero. Infiltration rates for other types of surfaces and soil cover must
be estimated from experience. A value of 0.5 in/h has been suggested
for turfed areas. Thus, if the rainfall intensity on a turfed area were
2.0 in/h, the rate of supply σ would be 1.5 in/h.
Standard Supply Curves
By use of Eq. (9-4) maximum rates of runoff q for rates of supply σ of
0.8, 1.0, 1.6, and 1.8 in/h are shown in Figs. 9-8 and 9-9. Maximum
rates of runoff are also shown for rates of supply of 0.4, 0.6, 1.2, 1.4,
2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, and 3.4 in/h [8].
Maximum rates of runoff for the curve labeled supply curve no. 1.0
(Fig. 9-8) were obtained in the following manner. From Fig. 9-2 the
intensities of runoff for various durations corresponding to the curve
labeled 1.0 are obtained. These intensities are entered as σ in Eq. (9-4),
and L is varied to produce the family of curves shown in Fig. 9-8. The
curve labeled σ is supply curve no. 1.0, obtained from Fig. 9-2. The
dotted line labeled t represents the maximum rate of runoff q which
c
would occur from an elemental area with various effective lengths L.
For example, the maximum rate of runoff from an area whose effec-
tive length L is 60 ft is 2.0 ft /s. Multiplying this rate by the drainage
3
area yields the maximum total discharge Q.
Figures 9-8 and 9-9 were prepared for n = 0.40 and S = 1 percent.
If these charts are to be used for other cases, the actual effective L for
the area under study must be converted in terms of L for n = 0.40 and
S = 1. A conversion chart is shown in Fig. 9-10. For example, if the
actual n = 0.30 and S = 2 percent and the effective length L is 400 ft,
then the equivalent effective L for n = 0.40 and S = 1 percent is 140 ft.
Typical Example—No Ponding
In the Corps of Engineers procedure, a reach of drain pipe is always
designed for a storm whose duration is equal to the time of concen-
tration for the drainage area above the pipe. The time of concentra-
tion corresponds to the time necessary to produce maximum flow
into a particular inlet (which is the same as the time necessary for
water to reach an inlet from the most remote point in the area) plus
the flow time in the pipe.
To clarify the computation of runoff by the Corps of Engineers
procedure, the following example is presented.
Consider the drainage areas shown in Fig. 9-11. The 1-h intensity
of the design storm is assumed to be 2.0 in/h. The infiltration rate for
the turfed areas is assumed to be 0.5 in/h. The retardance coefficient