is widely used in the Corps of Engineers for two reasons. First, both the flow
duration curve and the sediment discharge rating curve are process-based and can
be changed from the historical values needed for hindcasting to values needed for
forecasting water and sediment yield in the future. Also, the curves can be
defined to reflect specific components of the sediment runoff process, i.e., a
sediment discharge rating curve can be calculated for sand and gravels when
those are the types of sediment of most interest to project performance.
Sediment discharge rating curve
The sediment discharge rating curve is a relationship between water discharge
and sediment discharge as discussed in chapter 3. SAM.sed calculates the bed-
material sediment discharge rating curve needed for SAM.yld. SAM.sed will
create this rating curve in terms of sediment concentration. However, the
sediment discharge rating curve may be described in terms of tons/day by direct
input to SAM.yld. If total sediment yield is required, the sediment discharge
rating curve must be determined from measurements and directly input to
SAM.yld.
Flow duration curve
The flow duration curve is a relationship between water discharge and the
cumulative frequency each discharge occurs over a given time. It is a graphic
description of a hydrologic event. The discharge magnitudes are plotted as the
ordinates with the corresponding percents of time exceeded as the abscissas.
Care should be taken in developing this curve.
Often the flow duration curve is calculated from historical databases
containing the USGS mean daily records. If this data is used as the basis for a
flow duration curve, efforts must be made to ensure that the peak flows are
represented. This may be important in smaller streams where peak flow
durations are considerably shorter than one day. Discharge durations for events
larger than those in the mean daily record can be determined by one of several
methods. If there are flood hydrographs on record, they can be the basis for
calculating the discharge duration for the high flow events. If there is no
recorded hydrograph for a flood event that has a published peak discharge, then
the discharge durations can be determined assuming that the hydrograph had the
same shape as a flood hydrograph of record or as a synthetic hydrograph
calculated using HEC-1 or another hydrologic method. In any case, to ensure
reasonable results the peak discharges of record should be incorporated into the
flow duration curve, or the hydrograph, used in SAM.yld.
Calculations
Class intervals of water discharge are used in the integration of the flow-
duration and sediment discharge rating curves. The percent exceedance is
tabulated at each ordinate in user-defined increments that should be sufficiently
58
Chapter 4
Theoretical Basis for SAM.yld Calculations
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