General Principles of Erosion Protection
6.2.6 WAVE, VESSEL, AND ICE FORCES
These forces are seldom the dominant cause of bank failure on most streams.
Therefore, they do not usually present the critical design case. However, in some situations
the erosion induced by these forces, and the effects of these forces on bank protection works,
can be highly visible and significant. There are several references that address design of
protective works in detail, and numerous more specialized papers and publications. Two
general references that provide an excellent introduction to the topic are U.S. Army Corps
of Engineers (1984) and Hemphill and Bramley (1989).
6.2.7 PREDICTION OF TOE SCOUR
Degradation is a long-term, large scale process, but toe scour is usually associated
with the impacts of high flows over short reaches of channel.
This chapter deals with situations where the design of bank stabilization proceeds
under the assumption that the channel will not suffer significant future degradation. Similarly,
the various methods for predicting toe scour deal with local scour as a separate process from
degradation.
The general approaches to predicting toe scour are:
Analytical, using one or more of the relationships that have been proposed by
various researchers;
Empirical, using experience from similar situations; and
Modelling (numerical and/or physical).
Illustrating the uncertainty of analytical approaches, Copeland (1983) cites seven
different equations proposed by as many researchers for the specialized case of predicting
scour at spur dikes. There is disagreement even as to the significant factors involved, and
certainly disagreement in the results.
As a result of the limitations of analytical approaches, an empirical approach is often
considered to be more reliable. However, adequately documented experience may be
unavailable in particular situations. In such cases, the designer must apply one or more of the
analytical or modelling approaches.
The limitations to modelling that were discussed with respect to channel migration
also apply to the prediction of toe scour, although not to as great a degree. Two-dimensional
numerical models are required, and three-dimensional numerical models would be preferable
if available. Physical models must be large scale.
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