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

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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
Drainage Design Manual for Maricopa County Hydrology: Unit Hydrograph Procedures 5.6.2 Sources of S-Graphs S-graphs for Maricopa County have been selected from a compilation of S-graphs for the Southwestern United States (Sabol, 1987a) and an evaluation of S-graphs (Sabol, 1993a) used in the Unit Hydrograph Study (Sabol, 1987b). The sources of S-graphs for that compilation were reports and file data of the U.S. Army Corps of Engineers, Los Angeles District, and the USBR, as well as data collected for the Unit Hydrograph Study from gaged watersheds in Walnut Gulch, Tucson, Albuquerque, Denver, and Wyoming. 5.6.3 S-Graphs for Use in Maricopa County The four S-graphs selected for use in flood hydrology studies in Maricopa County are the Phoenix Mountain, the Phoenix Valley, the Desert/Rangeland, and Agricultural S-graphs. The Phoenix Mountain S-graph is to be used in flood hydrology studies of watersheds that drain predominantly mountainous terrain, such as Agua Fria River above Rock Springs, New River above the Town of New River, the Verde River, Tonto Creek, and the Salt River above Phoenix. Although the Corps of Engineers developed a separate S-graph for Indian Bend Wash, it is nearly identical to the Phoenix Mountain S-graph, which may also be appropriate for Indian Bend Wash. The Phoenix Valley S-graph is appropriate for flood hydrology studies of watersheds that have little topographic relief and/or urbanized watersheds. However, the Clark method is still the preferred unit hydrograph method for use in urban areas in Maricopa County. The Desert/ Rangeland S-graph is appropriate for use in natural areas with little to moderate relief, such as foothills, distributary flow areas, and other undeveloped desert areas. The Agricultural S-graph as the name suggests should be used for areas under agricultural crops like cotton, wheat, or vegetables. Table 5.6 summarizes the four S-graphs and describes their general areas of applicability. The four S-graphs are shown in Figure 5.9 and the coordinates of the graphs are listed in Table 5.5. The selection of S-graph should be made based on a comparison of the watershed of interest to the watershed(s) used to develop the various S-graphs. August 15, 2013 5-25
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