TM 5-820-l/AFM 88-5, Chap. 1
(4) Standard supply. The standard intensity-frequency-duration relationship of the selected design
storm less losses for infiltration. Standard supply is usually designated by the average rainfall intensity
in inches per hour at the l-hour duration.
b. Design methods.
The design procedures
for drainage facilities involve computations
to convert the
rainfall intensities expected from the design storm into runoff rates which can be used to size the
various elements of the storm drainage system. There are two basic approaches: direct estimates of the
proportion of the average rainfall intensity which will appear as the peak rate of runoff and hydrographic
methods which account for losses such as infiltration and for the effects of flow over the surface to the
point of design. The first approach is exemplified by the "Rational Method," which is used in most
engineering offices in the United States. This approach can be used successfully by experienced designers
for drainage areas up to 1 square mile in size. ASCE Manual of Practice No. 37 and FAA AC
150/5320-5B explain and illustrate the use of the Rational Method. TM 5-820-4/AFM 88-5, Chap. 4,
presents a modified Rational Method. The second approach includes techniques to synthesize
hydrography of runoff. Where studies of large drainage areas or complex conditions of storage require
hydrography, the designer should refer to the sources listed in the Bibliography and other publications
on these subjects. The method described in paragraphs 5 through 9 of this manual and developed and
illustrated in appendixes B and C combines features from both basic approaches to determine runoff.
a. Intensity-frequency data. Studies of rainfall intensity-frequency data indicate a fairly consistent
relation between the average intensities of rainfall for a period of 1 hour and the average intensities at
the same frequency for periods less than 1 hour, regardless of the geographical location of the stations.
The average rainfall for a l-hour period at various frequencies for the continental United States,
excluding Alaska, may be determined from figure 1. Data for other locations are available from the
Office, Chief of Engineers, and the National Oceanic and Atmospheric Administration, National Weather
Service (formerly the US Weather Bureau). For Alaska, data may be obtained from TM 5-852-7/AFM
88-19, Chap. 7, and US Weather Bureau Technical Paper No. 47. Data for Puerto Rico and the Virgin
Islands and for Hawaii may be obtained from US Weather Bureau Technical Papers No. 42 and 43,
respectively. For any frequency, the l-hour rainfall intensity is considered a design-storm index for all
average intensities and duration of storms with the same frequency.
b. Standard rainfall intensity-duration curves. Figure 2 shows the standard curves that have been
compiled to express the rainfall intensity-duration relationships and the standard supply (infiltration
subtracted) which are satisfactory for the design of airfield drainage systems in the continental United
States. The curves may be used for all locations until standard curves are developed for any region
under consideration. As an example, assume the average rainfall intensity is required for a 40-minute
design storm based on a 2-year frequency in central Kentucky. From figure 1 the 2-year l-hour rainfall is
found to be 1.4 inches per hour. In figure 2, supply curve No. 1.4 is used with the 40-minute duration of
storm to determine a rainfall intensity of 1.9 inches per hour.
c. Incomplete data. In areas where rainfall data are incomplete or unavailable, the methods described
in appendix B can be used to develop design rainfall information.
d. Design frequency. Drainage systems are normally designed for the maximum runoff from rainfall
with a certain frequency of occurrence. The design frequency indicates the average frequency at which
some portions or all of the drainage system will be taxed to capacity. After the design frequency is
selected, computations must be made to determine the critical duration of rainfall necessary to produce
the maximum rate of runoff for the specific areas involved. Ordinarily, the maximum rate of runoff
occurs when all tributary areas are contributing to the system. However, in cases of odd-shaped areas
and areas containing both paved and turfed areas, peak runoff rates may occur before all areas are
contributing. Factors affecting the critical duration of rainfall are primarily the length of overland flow,
extent of surface detention, pending, and characteristics of the runoff surfaces.
e. Storms of greater severity than the design storm. The design storm alone is not a completely
reliable criterion for the adequacy of drainage facilities. Often storms more severe than the design storm
can cause excessive damage and affect operations. Therefore, the probable consequences of storms
greater than the design storm should be considered before deciding on the adequacy of facilities designed
to handle only the design storm.
6. Infiltration. Infiltration refers to the rate of absorption of rainfall into the ground during a design
storm which is assumed to occur after a 1-hour period of antecedent rainfall. Wherever possible,
determine average infiltration rates from a study of runoff records near the airfield from infiltrometer