intermediate turfed areas bordered by paved runways, taxiways, or aprons. Runoff from paved areas

ultimately passes over turfed slopes to reach the pending areas and drain inlets, and is retarded in a

manner similar to runoff that results from precipitation falling directly on the turfed area. Inasmuch as

the time required for water to flow from the average paved area is normally very short (5 to 10 minutes),

the length of the paved area can be disregarded or given very little weight in estimating the value of L

for a composite area.

particular area varies as the size of the storage pond fluctuates during storm runoff. As water

accumulates in the relatively flat storage area during storm runoff, the size of the pond increases rapidly

and progressively reduces the distance from the edge of the pond to the outer limits of the drainage

area. In the majority of cases, it is satisfactory to estimate the value of L as the distance from the outer

limits of the drainage area to the average limits of the pending area during the period of design-storm

runoff. If the drain inlet is not located near the centroid of the drainage area, the value of L can be

estimated approximately as the average distance to the limit of the pending area, which corresponds to a

depth equal to two-thirds of the maximum depth caused by the design storm.

and maximum rate of runoff for the various supply curves presented in figure 2. The curves portray the

data presented in the flow curves shown in figure B-2 through B-4 in another format. Table B-3

illustrates the computational procedure. The runoff values obtained are assumed to be the maximum

because surface storage is negligible. Actually, the maximum runoff would normally occur a short time

after the rainfall excess or rate of supply ceases. For practical purposes, however, the maximum rate of

overland flow can be assumed to occur at approximately the same time that the rate of supply ends.

storm, but represent the peak rates of runoff from individual storm events of various durations, all of

which have the same average frequency of occurrence. The duration of supply corresponding to the

greatest discharge for a particular standard supply curve and value of L in these figures is defined as

experience indicates that adopting minimum values for tC of 10 minutes for paved areas and 20 minutes

for turfed areas in the actual design of storm drains is feasible and practical. For combined turfed and

paved areas, minimum values of tC are to be used even though the calculated effective length of overland

flow indicates a shorter critical duration of supply. For combined turfed and paved areas, where only the

minimum values of tC are of concern, the following equation should be used in selecting tC:

(eq B-3)

where

area paved, acres

area turfed, acres

c. *Consolidated design curve. *The data presented in figures 3 and B-7 through B-14 with respect to

peak runoff rates and critical durations of supply have been consolidated into one diagram, figure B-15.

Use of figure B-15 is not as precise as using figures 3 and B-7 through B-14, but figure B-15 may be

applied to most drainage problems. The following example is provided to illustrate the use of figure

B-15. Assume an effective length of overland flow of 315 feet and a rate of supply of 1.0 inch per hour.

To determine the critical duration of supply, project a line vertically upward from the effective length to

the intersection of the tC curve and proceed horizontally to the right to the critical duration of supply

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