Problem Ⅰ: A gutter is designed to drain the 25-year 30-min storm runoff from a
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Problem Ⅰ: A gutter is designed to drain the 25-year 30-min storm runoff from a small drainage
area in Norfolk, VA. For a computational time step of 5 min,
(1) Develop the design storm by assuming a balanced triangular distribution
(2) Can the design storm be developed using the 24-hr NRCS rainfall distribution method?
Explain your answer
Problem Ⅱ: The mean and standard deviation of the 30-min annual maximum rainfalls observed
at a rain gauge in Iowa are computed to be 10.51 mm and 5.96 mm, respectively. The Steel
Formula for the gauge’s 100-year IDF curve is
0.65
542 i
t 4.5
, where i is the rainfall intensity,
in mm/hr; and t is the rainfall duration, in min.
(1) Determine the 10-year 30-min rainfall intensity by assuming a Gumbel distribution
(2) Develop a 100-year 24-hr synthetic design storm using the NRCS rainfall distribution
method for a computational time interval of 1 hr
Problem Ⅲ: A 500 ft by 500 ft paved parking lot in Richmond, VA, has a runoff coefficient of C
= 0.85 and is to be drained by a sewer pipe with a conveyance capacity equal to the peak
discharge from a 5-year storm with a duration equal to the lot’s time of concentration (tc). The
inlet of the pipe is located at the left-bottom corner (i.e., outlet) of the parking lot. If the average
overland flow velocity is 0.5 ft/sec, determine the design peak discharge for the sewer pipe using
the Rational Equation.
Problem Sheet
13 pts
10 pt
10 pt
10 pt
3 pt
8 pts
20 pts
4−2
Problem Ⅳ: A 100-ac undeveloped watershed (including 5-ac swamps) in Iowa has a composite
CN = 50. Its 2-year 24-hr storm has a rainfall depth of 2.95 in. From the remotest location to the
watershed outlet, the overland runoff goes through sheet flow (Manning’s n of 0.25, flow path
length of 50 ft, and flow path slope of 0.001), shallow concentrated flow (flow path length of 80
ft and flow path slope of 0.005), and wide-shallow channel flow (Manning’s n of 0.035, bank-full
depth of 1.2 ft, channel length of 1000 ft, and bed slope of 0.008). If the watershed has a time of
concentration of 36 min, determine the peak discharge from a 24-hr storm with a 5.0 in rainfall
using the TR-55 Graphical Peak Flow method.
