which, in this example, is 23 minutes. To determine the maximum rate of runoff, proceed vertically

upward from the effective length to the intersection of the rate of supply line and proceed horizontally to

the left to the maximum rate of runoff, which is 1.2 cfs/acre of drainage area.

capacity of the drain serving the area, runoff rates exceeding the drain capacity must be stored

temporarily. As soon as the rate of inflow into the pending basin becomes less than the drain capacity,

the accumulated storage may be drawn off at a rate equal to the difference between the drain capacity

and the rate of inflow into the basin. The general relation between inflow, storage, and outflow is

(1) The rate of outflow from a pending basin is affected by the elevation of the water surface at

the drain inlet serving the area. The rate of outflow increases as the head on the inlet increases.

However, because of the flat slopes of airfield areas, the surfaces of the storage ponds surrounding drain

inlets are usually very large in comparison to the depth of water at the inlets. The rate of outflow

through a particular drain inlet would be approximately constant as long as the rate of runoff and

accumulated storage are sufficient to maintain the full discharge capacity of the drain inlet. The rate of

outflow equals the rate of inflow into the pond until the full discharge capacity of the drain inlet is

attained.

(2) To illustrate these assumptions, reference is made to the curves shown in figure B-16 and the

computations in table B-4. Hydrography 1 and 2 are developed as for figure B-5. Hydrograph 3 of

figure B-16 represents the constant rate of outflow corresponding to inflow hydrography 2, when the

drain-inlet capacity is assumed to be 1.25 cfs/acre of drainage area. Storage volume can be calculated

from the area between curves 2 and 3. The volume of storage above outflow hydrography 3 and below

hydrography 2 that would be accumulated at successive intervals of time under these conditions is

indicated by curve 4 of figure B 16. The maximum storage that would accumulate under these particular

conditions is 1,350 cu ft/acre of drainage area. The end of the accumulation period occurs approximately

43 minutes after the beginning of runoff.

Society of Civil Engineers, *Transactions, *Vol 110) and discussed in a(1) and (2) above have been included

in the preparation of figures B-17 through B-21. These graphs are presented to facilitate the

determination of the drain-inlet capacity (diagram A) and the critical duration of supply (diagram B) for

reflects the time associated with both the overland flow and the time to obtain maximum temporary

storage. The diagrams presented in figures B17 through B-21 have been prepared for use with effective

lengths reduced to n = 0.40 and S = 1.0 percent. As an example of the use of these figures, assume:

Effective length of overland flow = 300 feet.

Maximum storage allowable = 1,000 cubic feet per acre (cu ft/acre) of drainage area.

Rate of supply = 3.0 inches per hour.

(1) From the 3.0 inches per hour line on the top portion of figure B-19, proceed vertically upward

to the intersection of the 1,000 cu ft/acre of drainage area maximum storage capacity and then

horizontally to the left to the intersection of the minimum design drain-inlet capacity of 2.8 cfs/acre of

drainage area. To determine the critical duration of supply, t=, proceed as before to the intersection of

the maximum storage capacity on diagram A; then move horizontally to the right to the intersection of

the maximum, storage capacity on diagram B, and then vertically downward to the intersection of tC at

30 minutes.

(2) If the drain-inlet capacity of an outlet has been previously established and the temporary

an effective length of 400 feet, offers a quick check on the example presented in table B 4 and figure

B-16.

c. *Minimum drain-inlet capacity. *Curve 4 in diagram A (figs B 17 through B21) represents the

minimum drain-inlet capacities that are considered desirable, regardless of the volume of storage that

may be permitted. The drain-inlet capacities represented by curve 4 of diagram A are equal to the rates

of supply corresponding to durations of 4 hours on the standard supply curves given in figure 2. If the

drain-inlet capacity indicated by curve 4 is adopted in a particular case, some storage may result in the

pending basin during all storms less than 4 hours in duration that produce rates corresponding to the

given standard supply curve.

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