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

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