Chapter 11--Rosgen Geomorphic Channel Design

Chapter 11
charge. Regional bankfull sediment relations versus
drainage area may be substituted if actual bankfull
measurements are impossible to obtain, but must be
extrapolated from streams of similar lithology, stream
type, and stability. Examples of such relations using
1.5-year recurrence interval discharge for suspended
sediment are shown in Simon, Dickerson, and Heins
(2004). Dimensionless flow-duration curves are also
used to produce total annual sediment yield once
dimensionless ratio sediment and flow-duration curves
are converted to dimensional relations. The examples
of predicted sediment rating curves to observed values
using a dimensionless sediment rating curve were
presented in figures 11–12 to 11–14. Changes in unit
stream power (eq. 11–7) are calculated to determine
changes in transport rate due to change in depth,
slope, and/or velocity. Dimensionless flow-duration
curves are used to generate total annual sediment
yield from the generated sediment rating curves and
bed-load transport by unit stream power.
Streambank erosion—Streambank erosion rate (lateral
erosion rate and sediment, tons/yr) is predicted
as part of the river stability assessment. The influence
of vegetative change, direct disturbance, and other
causes of bank instability is quantitatively assessed.
One of the major consequences of stream channel
instability is accelerated streambank erosion and associated
land loss. Fish habitat is adversely affected
not only due to increased sediment supply but also by
changes in pool quality, substrate materials, imbrication,
and other physical habitat loss. Water temperatures
are also adversely affected due to increases
in width-to-depth ratio due to lateral accretion. The
prediction methodology is presented in Rosgen (1996)
and in Rosgen (2001d) utilizing a Bank Erodibility
Hazard Index (BEHI) and Near Bank Stress (NBS)
calculations.
Successional stages of channel evolution—A useful
tool at this phase is the determination of various
stream type scenarios and stages of channel evolution
as depicted in figure 11–15. It is imperative to identify
the present stage of the stream and predict the direction
and consequence of change. The various stages
and scenarios depicted in figure 11–15 assist the
observer in this assessment. River channels undergo
morphological change due to various disturbance and/
or recovery (Rosgen 1996, 2001d, 2005). The assessment
phase must identify current states and scenarios.
For each state within a scenario, there are specific
Rosgen Geomorphic Channel Design
11–24 (210–VI–NEH, August 2007)
Part 654
National Engineering Handbook
morphological, sedimentological, hydraulic, and biological
relations depicted. The associated interpretations
of these relations assist in river assessments.
River stability analysis—Additional stability variables
are required for assessment, including the influence
of large woody material, flow regime, depositional
features, meander patterns, riparian vegetation,
and channel stability ratings by stream type, and are
summarized in the form shown in table 11–4.
Figure 11–15 Various stream type succession scenarios
1.
2.
3.
4.
5.
6.
7.
8.
9.
E C Gc F C E
C D C
C D Gc F C
C G F Bc
E Gc F C
E
B G Fb
B
Eb G B
C G F
D
C
C G F
C