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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
5.6.1 Limitations and Applications
S-graphs are empirical, lumped parameters that represent runoff characteristics for the watershed
for which the S-graph was developed. S-graphs that are developed from recorded runoff
data from one watershed can be applied to another watershed only if the two watersheds are
hydrologically and physiographically similar. In addition, a study for the Flood Control District of
Maricopa County (Sabol, 1987) has demonstrated the shape of S-graphs is significantly affected
by storm characteristics, particularly the maximum intensity of the rainfall. Therefore, it may not
be advisable to adopt S-graphs that have been developed from one hydrologic zone and to apply
those to watersheds in other hydrologic zones because of possible differences in rainfall characteristics
in the two zones that may affect the shape of the S-graph. Application of S-graphs
requires the selection of an appropriate S-graph and the estimation of one parameter, basin lag.
Four S-graphs have been selected for use in Maricopa County and a method to estimate lag is
provided.
The USBR has revised the Flood Hydrology Studies chapter of Design of Small Dams (USBR,
1987), and it has identified S-graphs for application in six generalized regional and physiographic
type of watersheds. The USBR has issued a Flood Hydrology Manual (Cudworth, 1989) that
contains extensive discussion of flood hydrology in general, and S-graphs in particular. Both of
these references should be consulted before using S-graphs. The S-graph has been adopted as
the unit hydrograph procedure by Orange County and San Bernardino County, California, and
selected S-graphs are presented in the hydrology manuals for those counties. The S-graphs in
those hydrology manuals have been selected primarily from S-graphs that previously had been
defined by the U.S. Army Corps of Engineer, Los Angeles District from a rather long and extensive
history of analyses of floods in California.
An S-graph can, in theory, be used in any application for which an unit hydrograph can be used.
In practice an S-graph must be first converted to a unit hydrograph, and this can be done by one
of two methods. First, the S-graph can be converted to a unit-hydrograph manually; or second,
the S-graph can be converted to an unit hydrograph by use of the DDMSW program. The
DDMSW program outputs the HEC-1 input file with the S-graph converted to a unit hydrograph,
and the unit hydrograph is written to a HEC-1 input file using the UI (given Unit Graph) record.
The use of DDMSW greatly facilitates the use of S-graphs.
Although the S-graph is completely dimensionless and does not have a rainfall excess duration
associated with it, the unit hydrograph does require the specification of the duration. In general,
the same rules and recommendations apply to the S-graph as were made for the Clark Unit
Hydrograph; that is, the duration (computation interval, NMIN) selected for the development of
the unit hydrograph from a S-graph should equal about 0.15 times the lag. A duration (NMIN) in
the range 0.10 to 0.25 times the lag is usually acceptable.
5-24 August 15, 2013