General Approach to Bank Stabilization
and channel characteristics. The drawback of this approach is that a
significant change in the sizes of the sediment mixture may induce
some channel instability by changing the river regime and triggering
some response in channel shape, planform, and/or slope. These are
effects which cannot be precisely predicted.
Land loss from overland erosion and gullying is reduced.
4.1.1.2 Reservoirs
The primary purpose of reservoir construction is usually flood control or water
supply, but reservoirs also may be designed specifically to induce channel stability. Reservoirs
may also be designed specifically to trap sediment. These are sometimes called "debris
basins," and require periodic removal of sediment until the basin is stabilized (U.S. Army
Corps of Engineers, 1991).
The effect of reservoirs is to reduce peak discharges and sediment supply to the
downstream channel. In this regard, their impacts can be viewed qualitatively as the same as
land treatment, but the effect is of much greater magnitude. The potential benefit to channel
stability is, therefore, much greater, but so is the risk of induced instability due to a change
in river regime.
A reduction in peak discharge often reduces bank instability by inducing deposition
at the channel margin in the form of berms. In effect, the channel adapts to a lower effective,
or dominant, discharge by shrinking. However, reducing the sediment supply to the stream
also often induces channel degradation downstream, which can actually lead to mass
instability by increasing bank heights. Reducing peak discharge and lowering the flowlines
in the downstream channel may also induce tributary instability by lowering their effective
base level. This may trigger a reversal of main channel response and lead to its eventual
aggradation due to increased sediment supply from tributaries (Biedenharn, 1983).
The effect of reservoirs on rapid changes in river stage can also be significant. The
nature of the effect on bank stability depends on the shape of the stage hydrograph of the
stream before reservoir construction and the manner in which discharges from the reservoir
are regulated, as discussed in 4.1.5.2.
If there is consumptive use or interbasin transfer from reservoirs, for irrigation or
water supply, the potential for channel response is further complicated by the reduction of
total volume of flow as well as peak discharge.
Bank failure upstream of reservoir impoundments will be decreased by the reduction
in flow velocities and bank shear stresses for the length of channel affected by the
impoundment. However, associated raising of flowlines due to channel aggradation may
create flood control and environmental quality problems.
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