Discharge rates for a flood hydrograph passing the point of concentration for a 600-acre drainage basin are given in the table below. The flood was produced by a uniform rainfall rate of 2.75 in./hr, which started at 9 a.m., abruptly ended at 11 a.m. and resulted in 5.00 in. of direct surface runoff. The base flow (derived from influent seepage) prior to, during, and after the storm was 100 cfs.
Time | 8 a.m. | 9 | 10 | 11 | 12 | 1 p.m. | 2 | 3 | 4 | 5 | 6 | 7 |
Measured discharge | 100 | 100 | 300 | 500 | 700 | 800 | 600 | 400 | 300 | 200 | 100 | 100 |
(a) At what times did direct runoff begin and cease?
(b) Determine the ϕ index (in./hr) for the basin.
(c) Estimate the time of concentration (excess release time) for the basin.
(d) At what time would direct surface runoff cease if the rainfall of 2.75 in./hr had begun at 9 a.m. and had lasted for 8 hours rather than 2?
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