Page 393 - Planning and Design of Airports

344 Airp o r t D e sign

severe storm occurring very infrequently would undoubtedly cause
some damage if the system were designed for a storm of lesser severity.
However, if serious interruptions in traffic are not anticipated, a system
designed for the larger storm may not be economically justified. Taking
these factors into account, the FAA recommends that for civil airports
the drainage system be designed for a storm whose probability of occur-
rence is once in 5 years [2]. The design should, however, be checked
with a storm of lesser frequency (10 to 15 years) to ascertain if serious
damage or interruption of traffic would result from such a storm. Drain-
age for military airfields is based on a 2-year storm frequency [8].
Ordinarily no ponding is permitted on paved surfaces, but in the
intervening areas ponding is permitted, provided it will not result in
undesirable saturation of the subgrades underneath the pavements.

Determining the Intensity-Duration Pattern
for the Design Storm
The determination of the amount of rainfall which can be expected at
the site of the airport is the first step in the design of a drainage sys-
tem. Rainfall intensity is expressed in inches per hour for various
durations of a particular storm. The expected frequency of occurrence
is also an important factor to consider. The severity of storms is
related to their frequencies; a storm which is expected to occur once
in 100 years will be more severe than one having a frequency of occur-
rence of once in 5 years.
David L. Yarnell of the U.S. Department of Agriculture conducted
extensive investigations concerning rainfall intensities, durations, and fre-
quencies throughout the United States [16]. West of the 105th meridian,
where the Yarnell information is not as complete, the National Weather
Service has compiled rainfall data which appear in Refs. 12 to 14.
Yarnell developed rainfall intensities for 5-, 10-, 15-, 30-, 60-, and
120-min durations for a storm which can be expected to occur once in
5 years and the intensities for a 1-h duration for storms whose expected
frequencies of occurrence are once in 2, 5, 10, 25, 50, and 100 years.
The intensities for a duration of 1 h for frequencies of 2, 5, 10, 25, 50,
and 100 years are shown in Fig. 9-1.
The 1-h intensity does not by itself portray the intensity-duration
pattern of a storm. The Corps of Engineers made extensive studies of
rainfall patterns in the United States and found that irrespective of
frequency, the intensity-duration patterns of storms were largely
governed by their 1-h intensities. That is, two storms of different
frequency of occurrence whose 1-h intensities are equal will have
similar intensity-duration patterns. This is shown in Fig. 9-2. For
example, if the 1-h intensities of storms whose frequencies were 5,
10, or 15 years were all exactly 2.0 in/h, the intensity-duration pat-
terns would be expected to follow the pattern indicated by the curve
labeled 2.0.