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

Drainage Design Manual for Maricopa County
Hydrology: Application
6. If hand calculating the T c , perform the following:
a. Compute incremental rainfall excess for each time step using HEC-1. Rank
the rainfall excess values by ordering from largest to smallest.
b. The average rainfall excess intensity, i, is estimated by summing the first ten
(10) largest rainfall excess values and dividing the result by 10 times the time
interval, NMIN, in hours.
7. If a time-area relation is not specified in the HEC-1 model, then the HEC-1 default
time-area relation is used which, in general, is not recommended for use in Maricopa
County.
9.4.3.2 S-Graph
1. The recommended S-Graphs for Maricopa County (i.e. Phoenix Mountain, Phoenix
Valley, Desert/Rangeland, and Agricultural) should only be applied to large natural
watersheds. The Phoenix Valley S-Graph is also applicable to large urban watersheds.
This is, in part, due to the fact that the original database in Arizona applied the
methodology to large watersheds. As a lower limit of application a watershed area of
5 square miles can be considered.
2. K n should be selected from the best available information. General guidance and
some regional data is available from the U.S. Army Corps of Engineers (Appendix
D.1). A broader range of data for watersheds in Maricopa County is provided in the
USBR Flood Hydrology Manual (Cudworth, 1989). The S-Graph study (Sabol, 1987)
contains lag and watershed characteristics data that are not generally contained in
other publications. These sources should be consulted when selecting K n .
3. The manual discusses two slightly different forms of the lag equation (Equation
(5.11)), one by the U.S. Army Corps of Engineers and one by the USBR. The form of
the equation that corresponds to the source used in the selection of K n should be
used.
4. The length to the basin centroid (L ca ) is measured along L to a point on L that is opposite
(perpendicular to the flow path) the basin centroid. L ca is not measured to the
centroid unless the centroid happens to lie on the flow path line (L).
5. The transformation of an S-Graph to a unit graph is a function of the selected computational
time interval (NMIN). If a new NMIN is desired a new unit graph must be
recalculated.
9-46 August 15, 2013