Measured total hourly discharge rates (in cfs) from a 3.10-mi2 drainage basin are listed in the accompanying table. The hydrograph was produced by a rainstorm having a uniform intensity of 2.60 in./hr starting at 9 a.m. and abruptly ending at 11 a.m. The base flow from 8 a.m. to 3 p.m. was a constant 100 cfs. The volume of direct runoff determined as the area under the direct surface runoff hydrograph, is 1.000 cfs-hr.
Time | 8 a.m. | 9 | 10 | 11 | 12 | 1 p.m. | 2 | 3 |
Measured discharge | 100 | 100 | 300 | 600 | 400 | 200 | 100 | 100 |
(a) At what time did the direct runoff begin?
(b) Determine the net rain (in inches) corresponding to the volume of the direct surface runoff of 1,000 cfs-hr.
(c) Determine the ϕ index for the basin.
(d) Derive a 2-hr unit hydrograph for the basin by tabulating time in hours and discharge in cfs.
(e) What is the excess release time of the basin?
(f) For the same basin, use the derived 2-hr unit hydrograph to determine the direct runoff rate (cfs) at 4 p.m. on a day when excess (net) rainfall began at 1 p.m. and continued at a net intensity of 2 in./hr for 4 hr, ceasing abruptly at 5 p.m.
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