The Process
At this point, a word of caution is needed about re-establishing meanders in a previously
straightened reach. While this generally a commendable goal, and one that may be achievable in certain
circumstances, it is usually not as simple as is often purported, particularly in large scale projects, or where
severe system instability exists or has existed in the past. For this reason, it is important to consider the
stability of the reaches immediately upstream and downstream of the project reach. This is an essential,
yet often overlooked component of the design process. If the unstable project reach is bounded on the
upstream and downstream ends by stable reaches, or if there exist some sort of man-made or geologic
controls on both ends of the reach, then the reach may be much more manageable. Consider the example
of a channel reach that is undergoing significant channel widening and has been converted from a
meandering to a braided channel due to overgrazing, along the reach. If the channel upstream and
downstream of this reach is stable, then elimination of the overgrazing problem, and the re-establishment
of the old meander pattern may an achievable goal. Now consider a project where the goal is to restore
a five mile segment of a 20 mile straightened reach that has experienced 15 feet of degradation. Several
approaches could be identified to meet project goals, but each would have inherent problems. For
example, simply constructing a new sinuous channel at the existing channel elevation would not re-establish
the natural hydraulic connection between the channel and the floodplain. This problem could be overcome
by abandoning the old channel and constructing a new channel in the floodplain. However, transitioning into
the downstream reach would pose a serious problem that probably would require expensive and possibly
environmentally unacceptable grade control structures to drop the flow from the new channel into the old
channel.
2.4.6
SEDIMENT IMPACT ASSESSMENT
The purpose of the sediment impact assessment is to assess the long-term stability of the restored
reach in terms of aggradation and/or degradation. This can be accomplished using a sediment budget
approach for relatively simple projects or by using a numerical model which incorporates solution of the
sediment continuity equation for more complex projects.
2.4.7
PRELIMINARY DESIGN METHODOLOGIES - MEETING PROJECT GOALS AND
FINAL DESIGN
At this point in the preliminary design, the overall design process is reviewed. The preliminary
channel design is evaluated to insure that project goals such as reducing sediment loads, flood control, or
environmental enhancements are met. If not, the design process is re-visited to insure the project goals are
met with a stable channel design (Figure 2.4). With a satisfactory design, final local and system wide
designs can be initiated. Alternatives for satisfying project goals which were identified at project planning
stage can be designed and implemented.
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