Selection of Site-specific Stabilization Techniques
188.8.131.52 Horizontal Increments
This approach consists of initially stabilizing only that length of bank which is the
highest priority, then stabilizing the remainder of the project on a delayed schedule. This
approach does not decrease total project cost. In fact, the total cost of a project is likely to
be higher, but expenditures will be spread over a longer period. This approach is common
on comprehensive projects, and it can be used with any technique, but avoiding disaster in the
interim requires a reliable forecast of channel migration.
The probable increase in total cost is a result of having to mobilize on the same site
more than once, having to tie-in to existing work after the first phase, and perhaps having to
repair damage at the ends of the earlier work prior to extending it.
A variation on this approach can be applied to protection work which utilizes
vegetation. Several varieties of vegetation can be established initially, then the most
successful varieties can be used in a later phase to complete the work.
This section presents a rational procedure for identifying the preferable erosion
protection approach for a proposed project. This procedure provides a means for considering
all of the factors which are relevant to the selection, provides a basis for objective decision
making, and provides a safeguard against major oversights occurring in the selection process.
The concept has been used in the Lower Mississippi Valley Division of the U.S. Army Corps
of Engineers to compare standard protection techniques to other potentially useful techniques.
The procedure is flexible in that it can be adapted to either a very disciplined and
thorough approach, or to a very informal and rudimentary approach. The procedure is
iterative, requiring only the level of effort and number of iterations that are necessary to
ensure a competent selection. Estimates of costs need be only to the level of detail that is
appropriate for each iteration.
A matrix is the fundamental element of the procedure. The matrix is composed of the
factors of effectiveness, environmental suitability, and economics which were discussed in
Sections 5.1 through 5.3, along with all of the alternative protection methods which are
available for use on a project. The contents of the matrix and the definition of the pertinent
factors can be changed to satisfy a particular project. The matrix can also be expanded into
sub-matrices as appropriate for a specific project. For example, environmental factors can
be listed in detail in a sub-matrix. Also, in some cases it will be appropriate to subdivide the
streambank into two or more zones of elevation, to provide for the selection of a composite
or "hybrid" protection technique.
A suggested general matrix is shown by Table 5.1. A beginning point for applying the
matrix to a proposed project is to first eliminate factors that are irrelevant, and protection