Question

The net precipitation (runoff) in a three-hour period was 0.6 inches. The direct runoff hydrograph that resulted from this precipitation was in the form of a triangle with a time base of 21 hours and with a peak flow of 200 cfs that occurred 9 hours from the beginning of the runoff.

(a) Sketch the direct runoff hydrograph. Be sure to label axes.

(b) Determine the runoff volume (ft^{3})

(c) Determine the drainage area (acres)

(d) Sketch the unit hydrograph – and show supporting calculations.

(e) What will the peak discharge in the stream be if a three-hour storm produces 2.4 inches of runoff? Assume baseflow is 50 cfs.

Answer #1

After a 2 hour storm, a station downstream from a 45 mi2
drainage watershed records a peak discharge of 9400 cfs and a total
runoff of 3300 acre-ft.
(a) what is the unit hydrograph peak discharge?
(b) what would be the peak runoff and design flood volume if a 2
hour storm dropped 2.5 inch net precipitation?

Rain fell over a 1490-acre watershed at a constant rate of 2.75
inches/hour from 9 to 11 AM. Stream flow data recorded each hour
are reported below. Note that the stream flow declines in dry
periods to a relatively constant lower limit of 100 cfs (ft3/s).
(a) Separate the base flow from the hydrograph. (b) When did direct
(i.e., storm) runoff begin and end? (c) Find Pe,, the depth of
excess precipitation, in inches. (d) Derive the 2-hour unit
hydrograph....

Part I. Indicate whether true or false (T or F).
____ Storm water detention ponds typically are designed to
regulate the outflow peak rate at or below a single target value,
such as the pre-development (pre-land use change) peak runoff rate
for a specified return period event. Detention storage alters the
peak but not the volume of the outflow hydrograph.
_____ Typical rating curves for weirs are concave upward.
Typical rating curves for orifices are concave downward.
____ A sediment...

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