CE 372 Engineering Hydrology and Hydraulics
Fall 2006
Comments on homework 4
16. The problem with the approach described in part c is that it assumes the infiltration
rate is equal to the capacity from the beginning of the storm. The simplest way to
tell that 98 mm is incorrect for part d is that it exceeds the value computed for the
case with immediate ponding.
17. Few groups did this problem completely correctly. Most problems occurred in the
last period, in which the intensity briefly drops below the capacity and then ponding
begins again. (By taking the infiltration rate to be the intensity at the beginning of
period 4, one assumes that all of the previously ponded water has run off immediately.)
The graphs were also troublesome for groups. My graphs are below.
110
Intensity
Infil. capacity
Infil. rate
100
Rate (mm/h)
90
80
70
60
50
40
30
0
0.5
1
Time (h)
1.5
2
1
Time (h)
1.5
2
100
90
Precipitation
Infiltration
Runoff
80
Depth (mm)
70
60
50
40
30
20
10
0
0
0.5
18. Few groups did this problem completely correctly. Delayed ponding will not occur
when i > f0 , but it may occur when fc < i < f0 . It will not occur if the ponding time
is larger than the duration of the storm. For this soil, however, ponding occurs for any
intensity between fc and f0 . From the Chen method, one can obtain the exceedance
probabilities for the intensities of fc and f0 . Then the probability of delayed ponding
is the difference between the probability that i > fc and the probability that i > f0 .