Page 421 - Planning and Design of Airports

Airport Drainage 365

time t for the area directly tributary to inlet 3 been larger than the
c
sum of the inlet time for the area tributary to inlet 4 plus the flow time
to inlet 3, the former would have established the duration of the storm
for the design of the pipe from inlet 3 to inlet 2.

Typical Example—Ponding
If ponding is permissible, the first step is to establish the limits of the
ponding area. From a grading and drainage plan, the volumes in the
various ponds can be computed. These volumes are then expressed
in terms of cubic feet per acre of drainage area, as shown in column 9
of Table 9-8. The actual and equivalent L values are determined in
the same manner as for the case of no ponding, with one exception. The
actual L is measured to the mean edge of the pond rather than to the
drain inlet. The actual and equivalent effective lengths are listed in
columns 12 and 13.
The Corps of Engineers has developed charts which yield drain
inlet capacities to prevent ponds from exceeding certain specified
volumes. Typical charts are shown in Figs. 9-12 and 9-13. The vol-
umes are computed for various supply curves (Fig. 9-2), assuming
the slope of the basins forming the drainage areas is 1 percent. The
supply curves represent the intensity-duration pattern for storms
whose 1-h intensities correspond to the supply curve numbers. The
volumes of runoff for a specific supply curve are computed in a
manner similar to the procedure used by the FAA. The cumulative
volumes of runoff are compared with the various capacities of drain
inlets to arrive at the volumes of storage shown in Figs. 9-12 and
9-13. Since the volumes of runoff depend on L and S, charts must be
prepared for a wide range of L values. Figures 9-12 and 9-13 show
drain inlet capacities for L equal to 100, 200, 300, and 400 ft. Addi-
tional charts have been prepared for L = 0, 40, 600, 800, 1000, and
1200 ft [8].
The physical significance of the charts may be described by refer-
ence to the following example. Suppose that L for a large drainage
area is 100 ft and that the runoff pattern corresponds to supply curve 2.
Assume that the maximum permissible ponding is 300 ft /acre of
3
3
drainage area. From Fig. 9-12 a pipe which has a capacity of 1.0 ft /s
per acre of drainage area would be adequate to prevent the pond
3
from exceeding a volume of 3000 ft during any part of the storm. The
dashed lines labeled 4 are equal to rates of supply corresponding to a
duration of 4 h. Although smaller drain inlets are possible, it is felt that
the sizes corresponding to a duration of 4 h are about the minimum
from a practical standpoint.
The required drain inlet capacities for the drainage layout in
Fig. 9-11 were obtained from Figs. 9-12 and 9-13 and are tabulated in
Table 9-8. Note that the time of concentration is not a factor in these
computations.