Chapter 11--Rosgen Geomorphic Channel Design

Chapter 11
Hydraulic relations
Hydraulic relations are validated using resistance
equations for velocity prediction at ungaged sites.
(Stream Hydraulics is addressed in more detail in
NEH654.06) Validation is accomplished by back calculating
relative roughness (R/D 84 ) and a friction factor
(u/u * ) from actual measured velocity for a range of
streamflows including bankfull:
⎡
⎛ R ⎞ ⎤
u = ⎢2.
83 + 5. 66 log u
⎝
⎜ D ⎠
⎟ ⎥ *
⎣⎢
84 ⎦⎥
where:
(eq. 11–1)
u = mean velocity (ft/s)
R = hydraulic radius
D = diameter of bed material of the 84th percentile
84
of riffles
u * = shear velocity (gRS)½
g = gravitational acceleration
S = slope
Measured velocity, slope, channel material, and hydraulic
radius data from various Colorado rivers using
this friction factor (u/u * ) and relative roughness
(R/D 84 ) relation are shown in figure 11–6 (Rosgen, Leopold,
and Silvey 1998; Rosgen and Silvey 2005).
Manning’s n (roughness coefficient) can also be
back-calculated from measured velocity, slope, and
hydraulic radius. Another approach to predict velocity
at ungaged sites is to predict Manning’s n from a
friction factor back-calculated from relative roughness
shown in figure 11–7 (Rosgen, Leopold, and Silvey
1998; Rosgen and Silvey 2005). Manning’s n can also
be estimated at the bankfull stage by stream type as
shown in the relationship from gaged, large streams
in figure 11–8. Vegetative influence is also depicted in
these data (Rosgen 1994).
Dimensionless flow-duration curves—Flow-duration
curves (based on mean daily discharge) are also
obtained from gage stations then converted to dimensionless
form using bankfull discharge as the normalization
parameter (fig. 11–9 (Emmett 1975)). The
purpose of this form is to allow the user to extrapolate
flow-duration curves to ungaged basins. This relationship
is needed for the annual suspended and bed-load
sediment yield calculation along with channel hydraulic
variables.
Rosgen Geomorphic Channel Design
11–12 (210–VI–NEH, August 2007)
Part 654
National Engineering Handbook
(c) Phase III—Watershed and river
assessment
Land use history is a critical part of watershed assessment
to understand the nature and extent of potential
impacts to the water resources. Past erosional/depositional
processes related to changes in vegetative cover,
direct disturbance, and flow and sediment regime
changes provide insight into the direction and detail
for assessment procedures required for restoration.
Time series of aerial photos are of particular value to
understand the nature, direction, magnitude, and rate
of change. This is very helpful, as it assists in assessing
both short-term, as well as long-term river problems.
Assessment of river stability and sediment
supply
River stability (equilibrium or quasi-equilibrium) is defined
as the ability of a river, over time, in the present
climate to transport the flows and sediment produced
by its watershed in such a manner that the stream
maintains its dimension, pattern, and profile without
either aggrading or degrading (Rosgen 1994, 1996,
2001d). A stream channel stability analysis is conducted
along with riparian vegetation inventory, flow
and sediment regime changes, limiting factor analysis
compared to biological potential, sources/causes of
instability, and adverse consequences to physical and
biological function. Procedures for this assessment are
described in detail by Rosgen (1996, 2001d) and in Watershed
Assessment and River Stability for Sediment
Supply (WARSSS) (Rosgen 1999, 2005).
It is important to realize the difference between the
dynamic nature of streams and natural adjustment
processes compared to an acceleration of such adjustments.
For example, bank erosion is a natural
channel process; however, accelerated streambank
erosion must be understood when the rate increases
and creates a disequilibrium condition. Many stable
rivers naturally adjust laterally, such as the “wandering”
river. While it may meet certain local objectives to
stabilize high risk banks, it would be inadvisable to try
to “control” or “fix in place” such a river.
In many instances, a braided river and/or anastomizing
river type is the stable form. Designing all stream
systems to be a single-thread meandering stream may
not properly represent the natural stable form. Valley
types are a key part of river assessment to understand