CE 372 Engineering Hydrology and Hydraulics
Fall 2006
Homework 6
Due Friday October 20
Instructions: Remember that only one person records the homework.
23. A stormwater detention basin has the following storage characteristics:
Stage (m)
8.0
8.5
9.0
9.5
10.0
10.5
11.0
Storage (m3 )
0
1041
2288
3761
5478
7460
9726
The discharge weir from the detention basin has a crest elevation of hw = 8.0 m,
and the weir discharge is given by Q = 3.29(h − hw )3/2 , where h is the stage. The
catchment runoff hydrograph is
Time (min)
0
30
60
90
120
150
180
210
240
270
300
330
360
390
Runoff (m3 /s
0.00
3.60
8.40
5.10
4.20
3.60
3.30
2.70
2.30
1.80
1.50
0.84
0.51
0.00
a. If the pre-storm stage in the detention basin is 8.0 m, estimate the discharge
hydrograph from the detention basin. Plot the inflow and outflow hydrographs
vs. time.
b. Devise a quantitative measure of flood protection and analyze the detention basin
in this problem with it.
24. A catchment with an area of 2.1 km2 has a 15-minute unit hydrograph given in the
file on the website
a. Calculate and plot the runoff hydrograph resulting from the following 80-minute
storm:
Time (min)
0-50
50-80
Depth (cm)
3.5
1.8
b. Provide at least two ways to convince me that your answer is reasonable.
25. The flow hydrograph at a channel section is given by
Time (min)
0
30
60
90
120
150
180
210
240
270
300
330
360
390
Runoff (m3 /s)
0.0
12.5
22.1
15.4
13.6
12.4
11.7
10.8
9.9
8.4
8.1
7.5
4.2
0.0
a. Compute the outflow hydrograph 1000 m downstream for X = 0.3, K = 35 min,
and ∆t = 30 min. Plot the hydrographs at the two sections vs. time.
b. Repeat the calculation in part a with ∆t = 35 min and ∆t = 10 min. Discuss any
differences with the results in part a or any unphysical behavior.
c. Repeat the calculation in part a with ∆t = 45 min and X = 0.6. Discuss any
differences with the results in part a or any unphysical behavior.