General Principles of Erosion Protection
(6) Analysis of the present conditions and prediction of future channel migration
should take into account the magnitude of flows that occurred prior to the site
inspection, or that occurred during the time period spanned by surveys or aerial
photos that are being used for analysis. High flows tend to attack the bank
further downstream, and low flows farther upstream, because meander wave
length is directly proportional to discharge. The stream integrates the total
hydrograph over the long-term, but short-term observations may be distorted by
extreme events. Therefore, if the period of observation is weighted toward low
flows, the long-term attack may be farther downstream and more severe than
current observation would indicate. High flows during the period of observation
may have the opposite effect.
6.1.1.4 Special Considerations for Braided Streams
Protection works on braided streams may need to extend upstream and downstream
from the active erosion, because bars, chutes, and pools move more rapidly and much less
predictably than in meandering streams. Often bank erosion is associated with meandering
tendencies of major anabranches, and the likely pattern of attack can be fairly well predicted
using the rules of downvalley and lateral migration for alluvial bends. However, in other cases
bank erosion may be caused by less predictable impinging flow in side channels. The most
efficient approach on a braided stream where bank erosion is associated with anabranch flow
which impinges against the bank at sharp angles may be one of the following:
Treat problem areas as they arise by constructing spot stabilization as the
stream attacks first one spot, then another.
Temporarily divert impinging flows, either by excavating a new channel,
building temporary dikes of streambed material, or using floating
"breakwaters" to absorb the brunt of the impinging flow. However,
environmental aspects of construction activity in the streambed may not be
acceptable.
If long-term stabilization is required on a braided stream, and project constraints
preclude installing bank protection periodically as the channel changes, an adequate solution
may require the construction of continuous protection on both sides of its "braid belt."
Trenchfill or windrow revetment is well-suited for this application.
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