CE 372 Engineering Hydrology and Hydraulics
Fall 2006
Comments on homework 5
19. Most groups managed the curve number and runoff calculations well. Several groups
computed infiltration incorrectly; always return to the water balance. In this case,
there is an initial abstraction Ia = 0.2S that must be included in the calculation.
20. Almost all groups chose the estimate from the kinematic wave equation. Because the
estimates from the Kerby equation was close to (but less than) that from the KWE,
one group chose the Kerby value for design to be conservative. Many groups computed
the channel flow time incorrectly—usually by confusing seconds and minutes.
21. Almost all groups realized that because the rational method requires the entire basin
to contribute to runoff, the duration must be greater than the time of concentration.
22. Most groups did not approach part c correctly. Parts a and b are for the original
design with no parking lot. In part c, you must compute the runoff two ways: (1)
from the directly connected impervious area (parking lot) alone and (2) from the
entire composite catchment. The latter requires a composite C, computed from an
area-weighted average. You should also use a composite n.