A revetment (Fig. 10.26) is similar to a seawall in the sense that it is a shore-parallel structure that can prevent storm erosion but is ineffective against structural erosion. Compared to a seawall, a revetment is more gently sloping (e.g. 1:2 or 1:4), can have either a rough or smooth surface and can also be applied over a limited vertical distance.
Two aspects call for some remarks, viz.:
- Slope of revetment in relation to depth of scour hole;
- Level of upper end of revetment.
For rather smooth revetments it has been demonstrated that the depth of the scour trench depends on the slope characteristics. During tests in the Delta Flume of Delft Hydraulics (now Deltares) it turned out that with a slope of 1:3.6 a deeper scour hole was found than with a slope of 1:1.8. The deepest point of a scour hole is not always found at the intersection point between revetment and cross-shore profile (see Fig. 10.27 for a sketch). For a rough slope a smaller scour depth may be expected than for a smooth slope.
The level of the upper end of a revetment, above which the natural dune or cliff is unprotected, determines the amount of dune erosion above that level, but also to some extent the depth of the scour hole. The higher the level to which the revetment is applied the smaller the dune erosion is, but by implication also the deeper the scour trough (since no sediment is supplied from the dunes). If the level of the upper end of a revetment is equal to the storm surge level (or lower than that level), no reduction in dune erosion is found compared to a situation without any protection.
Summarizing, in a situation without longshore transport gradients, revetments as well as seawalls can be used to limit the rate of storm-induced erosion. The structures physically prevent the loss of material from the dunes or land and thus reduce the dune or mainland retreat. The potential scouring in front of the structures has to be taken into account in the design.