General Principles of Erosion Protection
In contrast, the discharge which governs the long-term behavior of a stream, and thus
many aspects of the design of bank stabilization work, is exceeded rather frequently, but there
is no consensus on how to best define it. It is variously called the "dominant" or
"channel-forming" or "effective" discharge, but is in fact an abstract quantity because in
nature no single steady discharge will reproduce the morphological and sedimentary features
which are formed by the varying discharge of a natural stream. However, it is often
considered to be about equal to bankfull discharge on streams that are neither aggrading or
degrading.
In practical terms, the design discharge is the flow which would stress bank
stabilization work most severely over a short period of time. It is desirable to quantify it and
to use it to size the armor layer if we are using an armor technique for which criteria exist.
However, for many other protection methods, determination of a design discharge will be
academic because no criteria exists to apply it.
6.2.2 TRACTIVE FORCE AND PERMISSIBLE VELOCITY
Tractive force and permissible velocity are two parameters that are commonly used
to quantify stress on the boundaries of a channel, whether the boundaries are formed in
sediment or consist of a protective armor. The highest stresses usually occur under the design
discharge. Except for riprap and some manufactured products, little precise guidance exists
regarding the limiting tractive force for erosion protection materials. For materials for which
no precise guidance exists, demonstrated performance under comparable conditions is the best
guide.
6.2.3 SECONDARY CURRENTS
Flow at channel bends in meandering channels and alongside bars and confluences in
braided channels is sharply three-dimensional. Velocities in the plane normal to the axis of
primary or longstream flow are termed secondary currents and coherent patterns of these
currents, termed secondary cells, can influence the distributions of primary velocity and
tractive force that erode the banks.
However, the patterns of secondary cells, especially close to eroding banks, are poorly
understood. It is known that plunging flow close to the outerbank in natural meanders often
promotes deep toe scour and that the sweeping effect of inward directed secondary currents
near the bed promotes point bar growth at the inner bank.
These processes are important to the growth of meanders, but more to the point here,
they are fundamental to the mechanisms of bank failure, and influence the effectiveness of
many types of bank and channel stabilization work. Unfortunately, secondary currents can
usually be addressed in design only indirectly, by letting the stream integrate them into its
behavior, along with all the other geomorphic, hydraulic, and geotechnical processes.
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