General Principles of Erosion Protection
a lengthy time period for completion of modeling, and the high cost of
model operation. Numerical and physical models are much more useful
for studying hydraulics of flow, changes in the bed, and "generic"
meandering than for predicting long-term channel migration for specific
locations. The major limitations are that meander evolution includes a
random element, and that bed and bank materials are seldom uniform, so
that it is impossible at present to predict the migration and evolution of a
particular meander bend through heterogeneous flood plain sediments.
Experience in geomorphological interpretation is required for the designer to fully
exploit these sources of information. There are at present no "cookbook" solutions.
6.1.1.2 Minimum Length of Protection
The upstream and downstream limits of the work in terms of the minimum length of
bank to be protected depends upon the nature of the erosion problem and the scope of the
project. Generally, the scale of the solution can be classified as one of the following, listed
in ascending order of required length of protection:
The streambank immediately adjacent to a threatened structure.
The length of streambank which is retreating rapidly enough that localized
protection would not guarantee adequate protection for the required project
life.
For comprehensive projects which require fixing a great length of streambank
in a stable position for navigation, flood control, irrigation, or other long-term
project purposes, the minimum requirement is to stabilize the entire bankline
which must remain in its present position or in some other predetermined
position so that navigation channel alignment, flood control works, irrigation
structures, or other project features are not threatened within the required
project life.
Figure 6.3 illustrate these situations.
Even if the minimum requirement is stabilization of a single point on the streambank,
a geomorphic analysis of channel migration as described in 6.1.1.1 should be performed to
provide insight into the stream dynamics that define the instability problem and its solution,
and to predict future problems that might arise due to channel migration. Only then can the
location of the protection required to meet the specific need be assured.
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