Chapter 11--Rosgen Geomorphic Channel Design

Chapter 11
analysis for both the reference reach and project
reach. This analysis of the reference, stable condition
is compared to the potentially disturbed
reach. To maintain stability, a stream must be
competent to transport the largest size of sediment
and have the capacity to transport the load
(volume) on an annual basis. These calculations
provide a prediction of sediment competence as
required in steps 23 through 26.
Step 27 Compute sediment transport capacity.
Following this analysis, the depth and/or slope
may need to be adjusted by recalculating steps 14
through 27.
FLOWSED and POWERSED are sediment supply/
sediment transport models that predict the following:
• total annual suspended sediment yield
• total annual suspended sand sediment yield
• total annual bed-load sediment yield
• potential aggradation and/or degradation
• flow-related annual sediment yield due to
changes in streamflow magnitude and duration
The models are based on the use of dimensionless
reference sediment rating and flow-duration
curves. The normalization parameters include:
• bankfull discharge
• bankfull stage bed load
• suspended and suspended sand sediment
The appropriate dimensionless sediment curves
are selected for representative stream types and
stability ratings. The dimensionless flow-duration
curves are developed from representative hydrophysiographic
province data from USGS stream
gage data.
The FLOWSED model reflects sediment supply
and generates the total annual sediment yield for
both suspended and bed load. Changes in flow are
also reflected in flow-duration curves and corresponding
sediment yield. To determine annual
sediment yield, near-bankfull stage values must be
field measured to convert dimensionless sediment
and flow-duration curves to actual values.
Rosgen Geomorphic Channel Design
11–36 (210–VI–NEH, August 2007)
Part 654
National Engineering Handbook
The POWERSED model compares sediment transport
capacity from a stable, reference condition
by predicting transport rate change due to channel
hydraulics. The hydraulics reflect potential change
in morphological variables such as channel width,
depth, and slope. The corresponding changes in
flow resistance are used to predict velocity, shear
stress, and unit stream power (velocity multiplied
by shear stress). Sediment rating curves from the
FLOWSED model are converted from discharge
to unit stream power for a wide range of flows.
Revised values of annual sediment transport can
then be compared to the reference condition from
the subsequent change in the hydraulic geometry
of the stream channel and corresponding response
in sediment transport. Any flow modifications can
also be simulated by revised flow-duration curves.
Detailed descriptions and model tests are provided
for FLOWSED/POWERSED in Rosgen (2006).
This analysis is complicated and detailed. However,
it can be computed by spreadsheet or commercially
available computer programs
(RIVERMorph® 4.0). The basis of the calculations
and model descriptions, however, are described
to better understand how the models work. Table
11–9 lists the data required to run the FLOWSED
and POWERSED models. With these data, the
user can generate average annual sediment yields
(tons/yr).