A planned 5.68-ac subdivision is sketched in Figure. The soils are generally sandy, and th...

A planned 5.68-ac subdivision is sketched in Figure. The soils are generally sandy, and the only runoff will occur from the directly connected (i.e., hydraulically effective) impervious street and driveway surfaces shown in the figure. (Only the 20 × 30 ft portion of each driveway that drains to the street is shown.) The street is 30 ft wide and the cul-de-sac has a radius of 30 ft. For storm drainage, the plan is to let the stormwater run along the street gutters in lieu of installing a pipe. For purposes of this problem, the entire drainage system can be treated as overland flow. The street slopes from an elevation of 165 ft at the center of the cul-de-sac to 160 ft at the entrance to the subdivision and has a Manning roughness of 0.016.

Drainage regulations specify a 5-yr return period design. Local IDF curves can be approximated functionally as

where

i = rainfall intensity (in./hr),

tr = duration (min),

a, b = constants for different return periods.

For this hypothetical location, a = 160 and b = 18 for a return period of 5 yr.

(a) Estimate the directly connected impervious area (ac).

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(b) Estimate the maximum drainage length along the DCIA (ft).

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Figure

(c) Determine the kinematic wave parameters α and m.

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(d) Estimate the 5-yr peak flow at the outlet. Assume that the impervious surface experiences no losses and ignore the geometry of street gutters (treat the street as a flat surface).