Chapter 11--Rosgen Geomorphic Channel Design

Chapter 11
FLOWSED—Continued
Rosgen Geomorphic Channel Design
11–40 (210–VI–NEH, August 2007)
Part 654
National Engineering Handbook
Convert dimensionless flow duration to dimensioned flow duration—The bankfull discharge is multiplied by
each of the ratios to convert dimensionless data to actual discharge values representing mean daily discharge
for each percentile. An example of a dimensioned flow-duration curve using bankfull discharge to convert from
the dimensionless relation (fig. 11–21) is shown in figure 11–24.
Calculate annual sediment yield for both suspended and bed-load sediment—This is accomplished by taking
the dimensioned flow-duration curve and multiplying flow increments for duration of time in days by the sediment
yield associated with that flow. Enter these calculations in the FLOWSED worksheet (table 11–11).
Calculate flow-related sediment yield—This calculation is accomplished using the output of the flow-duration
curves showing the increase in magnitude and duration of flow. The post-treatment flows are routed
through the calculation in the FLOWSED worksheet (table 11–11). The excess water calculation output from
the WRENSS snowmelt model (EPA 1980) or a similar model integrates the flow with flow-duration changes.
Dimensionless flow-duration curves are also converted to dimensioned values by multiplication of the bankfull
discharge value. Reference conditions for watersheds in relative hydrologic recovery are compared to watersheds
where streamflow has been increased or decreased by change in vegetation or by reservoirs and/or
diversions.
Stormflow models, such as TR–55, need to be used to compute new bankfull values, converting dimensionless
values to new dimensioned flow durations. It is important to calibrate the bankfull discharge, as the precipitation
probability for a given antecedent moisture content and runoff curve number that generates the bankfull
discharge needs to be determined. Any greater flow will be distributed on flood plains or a flood-prone area if
the stream is not entrenched. Thus, flow-related sediment changes are determined by the use of dimensionless
sediment rating curves and dimensionless flow-duration curves. Other appropriate models can also be used for
this step, based on the user’s familiarity with the various models selected. The output required, regardless of
the model, is bankfull discharge and pre- and post-treatment flow-duration curves.