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<br />FLOOD HYDROGRAPH ESTIMATION
<br />
<br />By V.B. Sauer
<br />
<br />The NFl' Program contains a procedure for com-
<br />puting a typical hydrograph that represents average
<br />runoff for a specified peak discharge, It is emphasized
<br />that this is an average hydro graph, and is not necessar-
<br />ily representative of any particular rainfaIl distribution.
<br />The average, or typical, hydrograph could be consid-
<br />ered a design hydrograph for some applications,
<br />The procedure used in NFF to compute the aver.
<br />age hydrograph is known as the dimensionless hydro-
<br />graph method. Stricker and Sauer (1982) developed the
<br />method for urban basins using theoretical techniques.
<br />Later, Inman (1987) used actual streamflow data for
<br />both urban and rural streams in Georgia, and confirmed
<br />the theoretical dimensionless hydrograph developed by
<br />Stricker and Sauer, Other investigators have since
<br />developed similar dimensionless hydrographs for
<br />numerous other States (Sauer, 1989). Except in some
<br />relatively flat-topography, slow-runoff areas, the same
<br />dimensionless hydro graph seems to apply with reason-
<br />able accuracy. The dimensionless hydrograph
<br />approach, however, is not applicable to snowmelt run-
<br />off or for estimating more complex double-peaked
<br />hydrographs.
<br />The dimensionless hydrograph method has three
<br />essential parts: (I) the peak discharge for which a
<br />hydrograph is desired, (2) the basin lagtime, and (3) the
<br />dimensionless hydro graph ordinates. In order to com-
<br />pute the avemge, or design hydrograph using the NFF
<br />procedures, the user selects the peak discharge from the
<br />NFF frequency output. The user must also provide an
<br />estimate of the basin lagtime. The NFF program then
<br />computes the hydrograph using the dimensionless ordi-
<br />nates of the hydrograph developed by Inman (1987)
<br />which are stored in the program.
<br />Basin Iagtime (LT) is defined as the elapsed time,
<br />in hours, from the center of mass of rainfall excess to
<br />the center of mass of the resultant runoff hydrograph.
<br />This is the most difficult estimate to make for the
<br />hydrograph computations. For rural basins, the user
<br />must make an estimate of lagtime, independent of the
<br />NFF program, because there are no lagtime equations
<br />currently available in NFF for rural watersheds. How-
<br />ever, Sauer (1989) has summarized basin lagtime equa-
<br />
<br />tions that have been developed for rural and urban
<br />watersheds in several States. The foIlowing statewide
<br />equations computed for rural Georgia streams by
<br />Inman (1987) are an example:
<br />
<br />LT = 4.64 A'49 SL-.21 (North of faIl line)
<br />LT = 13.6 A'43SL",31 (South of fall line)
<br />
<br />where
<br />
<br />A is drainage area, in square miles, and
<br />
<br />SL is channel slope, in feet per mile, as defined
<br />earlier.
<br />
<br />Appendix C includes a summary of equations for esti-
<br />mating basin lagtime as given by Sauer (1989) plus a
<br />few other known studies.
<br />
<br />.
<br />-
<br />i
<br />,
<br />
<br />On the other hand, the following generalized
<br />equation was developed by Sauer and others (1983) for
<br />urban basins for use on a nationwide basis:
<br />
<br />LT = O.OO3V71 (I3_BDF).34 (ST+IO)2.53 Rl2.,44
<br />IA-.20 SL",14
<br />
<br />I
<br />'~
<br />
<br />where
<br />
<br />LT is basin lagtime, in hours,
<br />
<br />L is the length, in miles, of the main channel
<br />from the point of interest to the extension of the main
<br />channel to the basin divide, and
<br />
<br />BDF, ST, RI2, IA, and SL, are described in the
<br />section "Urban Flood Frequency."
<br />
<br />The standard error for the above lagtime equation is
<br />+/- 61 percent, based on regression analysis for 170
<br />stations on a nationwide basis. For urban basins, the
<br />user has a choice of using the nationwide lagtime
<br />equation given above, or of inputting an independent
<br />estimate of lagtime.
<br />
<br />12 NlItlonwlde SUmlllllry 01 U.S. Geological Survey Reglonel Reg,..elon Equatlono lor Eotlllllltlng Mlgnltude Ind Frequency of
<br />F_ 10< Un9"llod Sileo, 1993
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