TM 5-820-1/AFM 88-5, Chap 1
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.
a. Temporary storage or pending. If the rate of outflow from a drainage area is limited by the
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
(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.
b. Drain-inlet capacity-storage diagrams. The concepts presented by G. A. Hathaway (American
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
(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
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.