Quantcast Design Considerations (Cont.)

Indirect Techniques for Erosion Protection
allowance for subsequent settlement of the backfill) then letting nature take
its course afterwards. The most sophisticated approach is to fill all voids in
a stone root to the extent possible by flushing sand into the voids, then
placing engineering fabric over the top of the stone and sand, then
completing the backfill with compacted lifts of silt or clay, then vegetating
the backfill and adjacent disturbed areas.
Bank height or bank height plus a scour allowance can also be used as a
starting point for designing approach (2) above. The length of armor
downstream of a dike should be a multiple, perhaps three for average
conditions, of that dimension. Upstream paving is optional, but the distance
need not exceed bank height. Normally, the bank toe just upstream of a
dike is a depositional area. For designing stone paving, the guidance in
Appendix A can be referred to, but because that guidance is not intended for
application to highly turbulent situations, stone size and thickness should be
greater than that which would be designed for a riprap blanket not adjacent
to a dike, perhaps a multiple of 1.5 or 2.
Stone is an excellent choice for a root dike material, even if the dike itself is of
other materials, because in other than mild erosion situations, the ability of the dike root
to adjust to scour is critical.
In severe conditions, dike roots or armoring of bankheads can become large cost
items, which is part of the reason why dikes can be more expensive than conventional
bank armoring in those cases.
(g) Structural scour protection prevents undermining and failure of rigid dikes, and fortifies
dikes of an adjustable material such as stone against unacceptable loss of elevation or
Alternative approaches to structural scour protection are to:
Place a blanket (sometimes called an "apron") of adjustable armor or a
flexible mattress on the bed under and adjacent to the dike. As with
bankhead armor, this blanket or mattress should be of a stronger design than
if it was being used at the same site not adjacent to a structure. USACE
(1981) found that an apron of stone or gabion mattress did not reduce the
depth of scour at the tip of a dike, but did enhance the stability of the
structure by moving the scour away from it.
Place extra material at the end and on the side slopes of the dike. The extra
material will launch into a scour hole and limit its extent, thus leaving the
dike length and elevation intact. This approach is simpler to construct than
an apron, but allows the scour to approach close to the dike. For a stone
dike, it would consist of a crown wide enough for stone to launch into the
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