generating the flow frequency distribution should be set equal to the critical discharge for the initiation of

bed load transport.

6)

Calculate Flow Frequency Distribution

The frequency of occurrence for each discharge class is determined from the record of observed

flows. The frequency units should reflect the time base in the flow record. For example, if mean daily flows

have been used then the frequency is expressed in days. If a regional flow duration curve has been

developed for an ungaged site, then the frequency for each discharge class must be calculated using the

equation for the curve, which is usually a power function. This can be achieved by calculating the geometric

mean discharge of each discharge class and deriving the frequency from the equation of the curve.

7)

Check For Extreme Flow Events

It is recommended that all discharge classes display flow frequencies greater than zero and that

there are no isolated peaks in individual classes at the high end of the range of observed discharges. If this

is not the case, it is likely that either the class interval is too small for the discharge range, or the period of

record is too short. Both zero frequencies and extreme flow events (outliers), can be eradicated by

reducing the number of classes or using logarithmic class intervals, as described in the later section on

Evaluation and Troubleshooting, but noting the cautions in each case. In either case, Steps 5 and 6 are

repeated to generate the flow frequency distribution for the new class intervals.

1)

Determine Sediment Data Availability

Sediment transport data are required to generate the bed material load rating curve. These data

may be obtained from measurements at a gaging station if the gage is in close proximity to the study site and

the sediment record at the gage is representative of the sediment load there. Otherwise, sediment transport

data must be derived for the study site.

2)

Define Composition of Bed Material Load

It is recommended that wash load (generally defined as particles less than 0.063 mm) be excluded

from the data set used to develop the sediment rating curve. If the bed material load moves both as bed

load and suspended load, then bed load and suspended load measurements are required to determine the

bed material load. If measured load data are insufficient, appropriate equations in a hydraulic design

package, for example SAM (Thomas *et al*., 1994), can be used to generate bed material loads for selected

discharges.

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