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General Principles of Erosion Protection Other Considerations
Application of the preceding discussion to the selection of upstream and downstream
limits of stabilization work should consider the following concepts:
(1) The downstream limit is usually more critical than the upstream limit, since the
scour pools associated with the normal pool-bar pattern tend to move
downstream, and bank failure is often associated with these pools. Although
beginning the upstream end of work at the precise point where erosion presently
begins carries some risk that erosion will later occur upstream of that point, cost
savings may make that risk acceptable, because often a bar will migrate
downstream, changing the area of erosion at that point into one of deposition.
Conversely, placing the downstream end of the work at the present limit of
erosion carries a high risk that the work will be outflanked by subsequent erosion
downstream of that point.
(2) Although the preceding concept applies to most situations, a caution regarding
the upstream limit should be observed: if a serious mistake is made in assessing
the future migration pattern of the stream, and the stabilization work is
outflanked at the upstream end, then subsequent deterioration of the integrity of
the stabilization project may occur rapidly.
(3) Model tests under the Section 32 Demonstration Erosion Control Program (U.S.
Army Corps of Engineers, 1981) indicated that erosion protection in a bend
should be extended downstream from the point of tangency a distance of at least
1.5 times the width of the approach channel into the bend. This can be used for
general guidance if data on channel behavior at a specific location is unavailable
or unreliable. Studies by Parsons (1960) provide similar insight.
(4) The transition from the stabilized bank to the natural bankline can be made rather
simply at the upstream limit. The details depend on the type of protection being
used, but in general a slight increase in the strength and/or a shallow "key-in" will
be sufficient for armor-type protection. Indirect protection can simply be turned,
or "feathered," into the bank, with a slight recess being good insurance.
(5) In contrast, more elaborate precautions are advisable for the transition to the
natural bankline at the downstream limit. The most important precaution is to
insure that the work is not stopped prematurely, as discussed above. Beyond
that, it is advisable to "key-in" and/or increase the strength of armor revetment.
For indirect bank protection works, a pronounced "tuck-in" should be provided,
perhaps with a liberal application of stone if conditions are severe. The
alternative is to be prepared to reinforce the work at some time in the future if
scour at the downstream limit threatens to outflank the protection. Such an
approach may be sound if project authority, rate of erosion, and the potential
consequences of miscalculation allow it.
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