General Principles of Erosion Protection
6.2.4 VARIATIONS IN RIVER STAGE
Although this is, strictly speaking, a hydraulic variable, its primary application to
design is geotechnical, since the susceptibility of the bank to mass failure, leaching, and
piping, is to some degree a function of the rate of drawdown of the water level. The
magnitude and timing of the variation also influences the constructability of different
techniques.
6.2.5 TOP ELEVATION OF PROTECTION
A major design parameter concerns the determination of the elevation to which
erosion protection works should be constructed. The most conservative approach for armor
revetments is to set the top elevation at design flowline plus a margin for freeboard. This
equates to the top of the levee for leveed channels, or to the top of the riverbank where there
are no levees, or where they are well protected by vegetation or by distance from the channel.
In many situations, this criteria is too conservative and would result in excessive cost and
reduced environmental suitability. Unless erosive velocities are believed to exist at high
elevations, and the consequences of even minor erosion are unacceptable, consideration
should be given to designing the top elevation of protection at a more frequently occurring
flowline. Other factors that should be considered in order to decide on the lowest and least
costly, yet effective, elevation are shown in Figure 6.4 and are listed below:
Stage duration;
Severity of overbank flow;
Erodibility of upper bank material;
Type of protection and slope of the bank; and
Consequences of failure.
These factors also influence the top elevation of indirect protection, although the most
conservative elevation for indirect protection is normally considered to be the elevation of top
of the river bank rather than the elevation of the design flood flowline.
Little quantitative guidance is available for applying these factors. An exception to
this occurs in situations where the primary purpose of the work is protection against wave
action. In those situations, fairly rigorous procedures have been developed to compute wave
height and run-up for use in designing top elevation of protection. This is usually not the
critical condition for streambank protection, but the references provided in 6.2.6 provide
specific guidance for situations where it is the critical condition. In other cases, merely
considering qualitatively the factors listed above and discussed below is an aid to intelligent
decision making.
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