The Process
the plan meet project goals"? If the answer is yes, the process moves to the consideration of local
stabilization or local habitat features. If the goals are not met, the process then proceeds back to the first
step in the loop and repeats the design process.
It must be recognized that there is no single approach that is universally applicable to all projects.
The specific methodology must reflect the project goals, financial and man-power resources, and
environmental considerations. For this reason, the design approach discussed herein should not be viewed
as a "cookbook" process, but rather, as a logical strategy that can be adapted to the specific demands of
the project.
In this section, methodologies will be introduced and discussed which follow this generalized
approach. A step-by-step procedure is presented for each methodology which will instruct the user on
how to approach preliminary design for channel rehabilitation.
2.4.1
PRELIMINARY DESIGN METHODOLOGY - DETERMINATION OF SYSTEM
STABILITY
Whenever, a channel rehabilitation project is contemplated, one of the first issues that must be
addressed is the overall stability of the channel system being considered. Failure to consider the system
stability frequently results in costly channel designs that fail to function properly, both from an engineering
and environmental perspective. The determination of the stability of the channel system is accomplished
through the geomorphic approach discussed in Chapter 5. If the project goals are simply the stabilization
of localized instabilities, or the installation of localized habitat features, then it is important that system-wide
channel instability does not exist. If it is determined that system-wide instabilities do not exist, then the
design of the local bank stabilization or habitat features can proceed. However, if system-wide instability
exist, in the form of aggradation, degradation, or plan form changes, then it becomes necessary to first
address these system instabilities before local stabilization is considered. Likewise, if the system is currently
in dynamic equilibrium, but, if it is anticipated that the project will modify this stability, then a more rigorous
systems analysis must be performed.
2.4.2
PRELIMINARY DESIGN METHODOLOGY - COMPUTATION OF CHANNEL
FORMING DISCHARGE
The second step in the preliminary design sequence is the determination of the channel forming
discharge. The procedures for determining this value were discussed in Chapter 5. If the project goals will
not significantly modify the water and sediment yield, then the existing channel forming discharge calculated
during the geomorphic assessment can be used for the design process. For example, if the objectives of
the rehabilitation project are to simply stabilize the channel grade using low drop grade control structures,
then it may not be necessary to calculate a new post-project channel forming discharge, since the effect of
these structures would not be expected to dramatically change the water and sediment loads. However,
if the plan involves more comprehensive features such as flow control, channel enlargement, flow diversions,
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