Chapter 11--Rosgen Geomorphic Channel Design

Chapter 11
Base-level change—A key part of channel stability
analysis. Degree of channel incision (lowering of local
base level) is determined by the ratio of the lowest
bank height divided by maximum bankfull depth,
called the bank height ratio. A stream may not be
entrenched (vertically constrained), but may be partially
incised, leading to entrenchment. A grade-control
structure requirement is often associated with partially
incised channels (Rosgen 1997a).
Direct disturbance and riparian vegetation—The direct
disturbance of stream channels must be offset by
correcting dimension, pattern, profile, and often channel
materials. Levees adjacent to both banks should
be set back allowing room for a flood plain. Riparian
vegetation change is not only a major cause of instability
and loss of function, but is a key solution in restoration
and natural channel design. Riparian vegetation
reestablishment should contain the correct overstory
and understory species to be compatible for a self-sustaining,
long-term solution.
Biological assessments—Biological assessments that
describe fish species, food chains, diversity with broad
categories of ecoregions, and stream types (habitat
units) are currently collected with the assessment level
for identifying biological potential. Limiting factor
analysis provides information that identifies specific
problems that may be corrected by changed management
and/or restoration.
It is readily apparent that this procedure involves extensive
field observations and an extensive data base
followed by a thorough and detailed analysis. All of
this must be completed prior to restoration planning,
as it forms much of the foundation for what follows.
It is important to understand the various causes of instability
responsible for loss of physical and biological
function and corresponding loss of value. Recommendations
that follow are critically linked to land uses,
disturbance regime, and other problem sources. The
flowchart (fig. 11–10) depicts the assessment criteria
of channel stability.
Rosgen Geomorphic Channel Design
11–26 (210–VI–NEH, August 2007)
Part 654
National Engineering Handbook
(d) Phase IV—Passive recommendations
for restoration
A first priority in restoration is to seek a natural recovery
solution based on changes in the variables causing
the instability and/or loss of physical and biological
function. Changes in land use management can influence
riparian vegetation composition, density and
vigor, flow modifications (diversions, storage, and
reservoir release schedule modifications based on the
operational hydrology), flood control measures, road
closures/stabilization, hillslope erosional processes,
and other process influences of river stability. Often, a
change in management strategies can be very effective
in securing stability and function. This often has to be
determined based on the recovery potential of various
stream types and the short- and long-term goals associated
with the stated objectives (including costs). The
alternative of self-stabilization is always a key consideration
in any stability assessment. The time-trend
aerial photography from phase III may help to provide
insight into stream recovery potential following disturbance.
Successional stages of channel adjustment (fig. 11–15)
can also assist at looking at natural recovery potential.
It is very important to ensure that objectives are met
through effectiveness monitoring required to provide
the documentation on the nature, magnitude, rate, and
consequences of natural recovery. If natural recovery
potential is poor and/or does not meet specific objectives,
phase V would be appropriate (Rosgen geomorphic
channel design methodology).
(e) Phase V—The stream restoration
and natural channel design using the
Rosgen geomorphic channel design
methodology
Phase V involves combining the results of the previous
phases. A good design can only follow a good assessment.
It is preferred not to patch symptoms, but rather
provide solutions to restoration that will offset the
cause of the problem and allow for the river to be selfmaintaining.
The practitioner must be very familiar
with the processes involved in hydrology, hydraulics,
sedimentology, geomorphology, soil science, aquatic
habitat, and riparian vegetation. Due to the inherent
complexity, it is usually necessary to obtain technical