A detention basin is designed to attenuate the runoff from 5.16 acres comprising 4.65 acres of roadway/pavement, 0.28 acres of lawn, and 0.23 acre of brick pavers. The allowable discharges from the detention basin are calculated at 7.94, 10.90, and 15.88 cfs for the storms of 2-, 10-, and 100-year frequency, respectively. Elevation–discharge–storage relations for the detention basin are listed in the following table:

a. Perform calculations to determine the critical storm duration, namely the storm that requires the largest detention storage. Use the IDF curves in Figure 3.1b in Chapter 3.

b. Perform routing computations for the critical storm.

c. Compare the computed peak outflows from the detention basin with the allowable discharge from it.

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Elevation (ft) Discharge (cfs) Storage (ft) 869.5 0 0 870.0 0.8 1,150 870.5 2.0 3,400 871.0 2.7 5,670 871.5 4.1 7,950 872.0 6.0 10,200 872.5 8.4 12,600 873.0 11.0 15,200 873.5 13.8 17,950 874.0 17.4 20,980