Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Application
9.4.3 User Notes
9.4.3.1 Clark Unit Hydrograph
1. The Clark Unit Hydrograph procedure was developed from a database that included
both urban and natural (undeveloped) desert/rangeland watersheds. The primary
application of the Clark Unit Hydrograph is for urban watersheds, but it is also applicable
for undeveloped desert/rangeland watersheds. In general, the Clark Unit Hydrograph
is not applicable to agricultural fields or steep mountain watersheds.
2. The following limitations apply to the Clark Unit Hydrograph procedure.
a. The recommended drainage area limit is 5 square miles with a maximum of 10
square miles.
b. The calculated T c should not exceed the duration of rainfall excess.
c. The calculated T c should not be longer than 1.5 hours.
If a drainage basin does not meet any or all of the preceding limitations, then the following
options are available:
• Subdivide the drainage area into smaller subbasins such that all of these subbasins
satisfy the limitations.
• Use the S-Graph method, provided the drainage basin satisfies the limitations
of that method.
• Justify the use of an alternative approach.
3. Time of concentration as defined in this manual is the travel time, during the corresponding
period of the most intense portion of rainfall excess, for a floodwave to
travel from the hydraulically most distant point in the watershed to the point of interest.
The determination of the hydraulically most distant point is made in regard to both
length and slope. In other words, the hydraulically most distant point is not necessarily
the longest length, but may be a shorter length with an appreciably flatter slope.
4. When calculating the T c for a natural watershed, with slopes greater than 200 ft/mile,
use Figure 5.4 to adjust the slope. The use of the adjusted slope should be considered
when determining the T c of the hydraulically most distant point.
5. T c is a function of rainfall excess and must be recalculated for each desired frequency
or design storm duration.
August 15, 2013 9-45