Problem

A planned subdivision near Tallahassee can be represented conceptually as shown in Figure...

A planned subdivision near Tallahassee can be represented conceptually as shown in Figure P6-20.

The inner rectangle represents DCIA, consisting of rooftops, streets, sidewalks, driveways, and so on. The dimensions of the inner rectangle are 1000 ft wide by 2500 ft tall. The pervious area behaves like “lawns, sandy soil, average slope.” The DCIA has characteristics of streets and roofs. “High-end” values of runoff coefficients should be used.

Figure

The inlet time for flow from the far upper end of the catchment (e.g., an upper corner of the pervious area) is 2 hr. The inlet time just for the DCIA is 20 min.

(a) What is the 25-yr outflow, by the rational method? Remember to compute runoff both for the total area and for just the DCIA.
(b) Use the USGS regression equations for Tallahassee [Eq.] and compute the 25-yr peak flow. Observe that this value is close to the rational method estimate for DCIA, from part (a).
(c) A corrugated metal pipe (CMP) culvert (n = 0.024) will receive the 25-yr flow from the catchment, using the rational method of part (a). If the pipe slope is 1%, what standard U.S. diameter (see Problem) should be used?

Equation

Problem 1

The subdivision of Problem 2 drains to the upstream end of a circular pipe that has an n value of 0.013 and a slope of 0.005 ft/ft. Determine the size of pipe needed to carry the flow from the 5-yr storm. (Note: Standard diameters in the United States start at 12 in. and increase in 6-in. increments to 60 in., then continue to increase in 12-in. increments.)

Problem 2

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).
(b) Estimate the maximum drainage length along the DCIA (ft).
Figure
(c) Determine the kinematic wave parameters α and m.
(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).