General Principles of Erosion Protection
The degree to which these three techniques can be applied is a matter of judgement,
and will depend on time and funds available for the analysis, the scope of the project, the
consequences of an inaccurate estimate of toe scour, and upon the personal and institutional
experience which can be brought to bear on any specific problem.
6.3 TOE PROTECTION
Toe protection is essential to the success of bank protection work, although it may
not be a massive element of the work if the exposure to scour is relatively mild. Once a
prediction of the amount of toe scour to be expected has been made, a variety of methods are
available to accommodate it in the design. In this section, general guidance for all types of
protection is provided.
6.3.1 BASIC OPTIONS
There are two basic options. First, to "dig it in" by extending the toe of the
protective works into an excavation at or below the predicted scour depth, or at the elevation
of a non-erodible material, if such material is present within the practical limits of excavation;
or second, to "let it self-launch" by designing the work so that, as scour occurs, the
protective material can launch or flex downward sufficiently to prevent the scour from
moving inshore and causing geotechnical instability of the bank.
The "dig it in" approach is most often used with an armor revetment. Its primary
disadvantage is that excavation in a flowing stream, and precise placement of an armor
material in the excavation, is often difficult and costly, and sometimes impossible. Ten feet
is sometimes used as a rule of thumb for the limit of conventional excavation techniques
underwater. Beyond that depth, either dredging or dewatering with a cofferdam may be
required for excavation and armor placement.
Sheet-pile retaining walls and pile-supported indirect protection structures which are
designed to withstand maximum scour can be considered special cases of the "dig it in"
approach.
The "self-launching" approach offers economy and ease of construction by allowing
the stream rather than the contractor to perform the excavation. However, it does require a
larger volume of material in the toe section than if the toe is placed in an excavation, since the
launching process may be irregular. As a result, the cost of material may in some cases negate
savings in operational cost. Therefore, if site conditions permit easy mechanical excavation
to the predicted scour depth, the "dig it in" approach may be the least costly overall.
The "self-launching" technique also offers the considerable advantage of providing a
built-in scour gauge, particularly if the top of the launching section is visible above water. If
it is underwater even at low stages, it can be surveyed by accurately located soundings. If it
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