Page 398 - Planning and Design of Airports

Airport Drainage 347

If the drainage system is to be designed for a storm whose expected
frequency of occurrence is once in 5 years and if detailed data concern-
ing the intensity-duration pattern are nonexistent, the pattern can be
approximated from Fig. 9-2, provided the 1-h intensity is known. It
goes without saying that if sufficient rainfall data are available at an
airport site, the intensity-duration frequency data should be devel-
oped from this information rather than from other sources. Rarely,
however, does a site have such complete rainfall information.

Determining the Amount of Runoff by the FAA Procedure
The FAA analysis of airport surface drainage revolves about the solu-
tion of the rational method expression

Q = CIA (9-1)
3
where Q = runoff from given drainage basin, ft /s
C = ratio of runoff to rainfall
I = rainfall intensity for time of concentration of runoff, in/h
A = drainage area, acres

Examples and charts illustrating the FAA procedure for design
have been taken largely from the FAA [2].

Time of Concentration
The time of concentration is defined as the time taken by water to reach
the drain inlet from the most remote point in the tributary area. The
most remote point refers to the point from which the time of flow is the
greatest. The time of concentration is usually divided into two com-
ponents: inlet time and time of flow. The inlet time is the time required
for water to flow overland from the most remote point in the drain-
age area to the inlet. The time of flow is the time taken by the water to
flow from the drain inlet through the pipes to the point in the system
under consideration. Sometimes the inlet time will be the time of con-
centration; at other times the time of concentration will be the sum of
the inlet time and time of flow.
The time of flow can be computed by the use of well-established
hydraulic formulas. The inlet time is obtained largely empirically
from the relationship

2
D = kT (9-2)
Where D = distance, ft
T = time, min
k = dimensional empirical factor which is dependent on
slope, roughness of terrain, extent of vegetative cover,
and distance to drain inlet

Inlet times can be estimated from Fig. 9-3.