CE 372 Engineering Hydrology and Hydraulics
Fall 2006
Comments on homework 2
General: Remember that groups are supposed to have one recorder per homework. After
this warning, I will impose the 25% penalty that is listed on the course syllabus. See also
an email for some reasons for having one recorder per homework.
5. Remember that the frequency factor I had you use holds only for an infinite data set;
for sample sizes between 15 and 100 you can use the table in the book. . .Intuition
might suggest that the flows computed from the annual exceedance series would be
higher than those computed from the annual maximum series because the exceedance
series uses the largest values. While the mean for the exceedance series is higher than
the mean for the maximum series, the standard deviation is much lower.
6. Remember that with the log-Pearson type III or other PDF applied to the logarithms,
you must take the logarithms first and then calculate the statistics (mean, S.D., and
skewness).
7. There were a few small errors (e.g., reading the chart for a1 , b1 , and c1 incorrectly;
using t in hours in the Chen formula; etc.). . .The answers to this problem illustrate the
precision in the Chen method: Answers ranged from 10.3 to 19.0 years with a mean of
13.9 years and standard deviation of 2.6 years. (P.S.: How did the U.S. accidentally
drop a hydrogen bomb into the Mediterranean Sea?)
8. Again, the precision of the Chen method depends on the reading of the maps and
10
chart. I found R110 = 2.3 inches, R24
= 4.5 inches, R1100 = 3.3 inches, a1 = 32.7,
b1 = 10.25, and c1 = 0.83. Most groups mentioned that the IDF curves reproduce the
expected behavior (e.g., intensity decreases with increasing duration and decreasing
return period). A couple of the groups also checked their computed values of the R’s
against the values they read from the maps. For example, my computed values are
within 5% of the map values.
10
9. Most groups found R110 , R24
, and R1100 from the rainfall records and then used the
Chen maps to find the city. You could have computed a, b1 , and c1 and compared to
the Wenzel values, but that was not necessary.
10. If you need the 10-year storm for one of the cities in the table for the Wenzel method,
then Wenzel’s method is better. Otherwise, use the Chen method. Many groups
forgot that the Wenzel method is only for the 10-year storm.