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

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

fcd.maricopa.gov
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Drainage Design Manual for Maricopa County Hydrology: Unit Hydrograph Procedures Figure 5.6 SYNTHETIC TIME-AREA RELATION FOR URBAN WATERSHED Figure 5.7 SYNTHETIC TIME-AREA RELATION FOR NATURAL WATERSHEDS 5-20 August 15, 2013

Drainage Design Manual for Maricopa County Hydrology: Unit Hydrograph Procedures Time, as a percent of Time of Concentration (1) 5.6 S-GRAPHS Table 5.4 VALUES OF THE SYNTHETIC DIMENSIONLESS TIME-AREA RELATIONS FOR THE CLARK UNIT HYDROGRAPH Contributing Area, as a Percent of Total Area Urban Watersheds (2) Natural Watersheds (3) 0 0 0 0.0 10 5 3 4.5 20 16 5 12.6 30 30 8 23.2 40 65 12 35.8 50 77 20 50.0 60 84 43 64.2 70 90 75 76.8 80 94 90 87.4 90 97 96 95.5 100 100 100 100.0 HEC-1 Default (4) An S-graph is a dimensionless form of a unit hydrograph and it can be used in the place of a unit hydrograph in performing flood hydrology studies. The concept of the S-graph dates back to the development of the unit hydrograph itself, although the application of S-graphs has not been as widely practiced as that of the unit hydrograph. The use of S-graphs has been practiced mainly by the U.S. Army Corps of Engineers, Los Angeles District, and the U.S. Bureau of Reclamation (USBR). An example of an S-graph from Design of Small Dams (USBR, 1987) is shown in Figure 5.8. The discharge scale is expressed as percent of ultimate discharge (Q ult ), and the time scale is expressed as percent lag. Lag is defined as the elapsed time, usually in hours, from the beginning of an assumed continuous series of unit rainfall excess increments over the entire watershed to the instant when the rate of resulting runoff equals 50 percent of the ultimate discharge. The intensity of rainfall excess is 1 inch per duration of computation interval (Δt). An equivalent definition of lag is the time for 50 percent of the total volume of runoff of a unit hydrograph to occur. It is to be noted that there are numerous definitions for lag in hydrology and the S-graph lag should not be calculated by methods that are not consistent with this definition. Ultimate discharge is the maximum discharge that would be achieved from a particular watershed when subjected to a continuous intensity of rainfall excess of 1 inch per duration (Δt) uni- August 15, 2013 5-21

<strong>Drainage</strong> <strong>Design</strong> <strong>Manual</strong> for Maricopa County<br />

<strong>Hydrology</strong>: Unit Hydrograph Procedures<br />

Figure 5.6<br />

SYNTHETIC TIME-AREA RELATION FOR URBAN WATERSHED<br />

Figure 5.7<br />

SYNTHETIC TIME-AREA RELATION FOR NATURAL WATERSHEDS<br />

5-20 August 15, 2013

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