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General Principles of Erosion Protection
If impermeable dikes are constructed of a material such as stone, which will launch
into a developing scour hole, the size of the scour hole will tend to be self-limiting. However,
impermeable dikes are often less effective in inducing deposition than permeable dikes, and
are likely to produce more concentrated flows and higher velocities locally, which tends to
offset the positive effect of self-launching.
6.3.2.6 Retards
For rigid retards, such as non-adjustable fencing or piling, some designers assume
that the impedance to flow provided by the structure, and subsequent landward deposition
of sediment, will prevent toe scour from endangering bank stability. Thus they make no
specific provision for limiting toe scour. This approach is sometimes successful, but it is not
recommended unless the following conditions are met:
Pile penetration and size are designed for the condition of maximum predicted
scour;
The structure will be monitored frequently and reinforced if necessary;
Suspended sediment load of the stream is large, so that deposition behind the
structure is likely to occur; and
The distance from the toe of the bank to the structure is at least twice the
predicted scour depth.
One of the following approaches may be more efficient and effective:
Locate the retards in a trench excavated to the predicted scour depth;
Install a self-launching toe section of stone or other material; and
Secure a flexible mattress to the riverward side of the retards
Flexible retard designs, such as jacks, tire-posts, and trees, allow part; or all of the
retard structure itself to displace downward as scour occurs. Use of this technique requires
secure connections between retard units and between the retard structure and the landward
anchors, or extra penetration of piling, depending on the specific design. Allowance must be
made in selecting retard height so that the downward displacement will not leave the upper
bank exposed to significant erosion during high flows. Otherwise, it may be necessary to
maintain the work by placing additional units on top as the originals units displace downward.
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