Selection of Site-specific Stabilization Techniques
"Take the work to the river" by using equipment mounted on barges or other
craft, to place stone, flexible mattress, or rigid structures underwater, with the
upper bank protection being constructed in a conventional, less costly manner.
"Let the river come to the work" by placing a stone fill on a smooth alignment
behind the existing bank. As bank erosion reaches the stone fill, the stone
displaces downward until the eroding bank is sufficiently armored to prevent
further erosion. This extremely useful technique is known as "trenchfill"
revetment when the stone is placed in an excavated trench, and as "windrow"
or "falling apron" revetment when it is placed on top of the existing ground.
5.1.4 FORESHORE LIMITATIONS
If grading the bank to provide geotechnical stability or to provide a suitable surface
for the placement of surface armor is expensive or impractical because of structures near the
streambank, or because of restrictions on rights of way, then a technique must be selected
which leaves at least part of this "foreshore" or "berm" intact. Restrictions on disposing of
excavated material may also create a need to minimize the amount of bank excavation. Two
approaches which may be suitable in these situations are:
If immediate stabilization of the bank must be guaranteed, and even a limited
amount of bank grading is prohibited, then the bank must be restored, usually
with a retaining wall or longitudinal stone bulwark with backfill.
If a limited amount of bank grading can be performed, then a cut-and-fill
technique may be adequate and cost-effective. This approach requires careful
attention to the protection used on the filled portion of the bank, through the
provision of toe protection, a filter layer or fabric, and using an armor material
which can adjust to moderate bank subsidence. A noncohesive fill material is
best suited to this technique because it can more readily be compacted to
prevent subsidence than can a cohesive material. Full compaction is costly,
but providing some degree of compaction by traversing the fill with tracked
equipment or rollers as the fill is being placed is usually worthwhile, since it
greatly reduces future subsidence at minimal expense.
5.1.5 CHANNEL ALIGNMENT
Avoiding major realignment of the channel by adapting to the existing general
alignment is usually less expensive than realignment, and has the advantage that it changes
stream characteristics less. However, navigation considerations, the presence of existing
structures, or the unfavorable hydraulic conditions created by an extremely short radius bend
or highly irregular bankline, may require a deviation from the existing alignment. If