stone riprap, articulated concrete matting, concrete blocks, gabions, and wire mesh mattresses
filled with stone. Stone riprap is most commonly used.
In cohesionless channel beds, the design of stone aprons should be based on the stone
launching to a slope of up to lV:2H. Model tests have indicated that these slopes are realistic
for sand beds, but little field confirmation seems to have been reported.
Stone sizes for launching should be the same as for slope revetment. The volume of stone
should be sufficient to cover the final scoured slope to a thickness of 1.25 times the size of the
largest stones in the specified grading. At the nose of a guide bank or spur, there should be
sufficient stone to cover the final conical surface of the scoured slope. Piers should not be
located within the launching apron slope unless it is unavoidable.
Launching aprons do not perform well on cohesive channel beds where scour occurs in the
form of slumps with steep slip faces. In such cases, bank revetment should be continued
down to the expected worst scour level, and the excavation then refilled.
A variation of these methods of toe protection is to pile the rocks in a "windrow" along the bank
line instead of excavating a trench. Then as the bank is scoured, the rocks in the windrow drop
down to pave the bank.
Windrow Revetment. Windrow revetment is an erosion control technique (Figure 6.18)
consisting of the depositing of a fixed amount of erosion-resistant material (riprap) landward
from the existing bank line at a predetermined location, beyond which additional erosion is to
be prevented.
The technique consists of burying or piling a sufficient supply of
erosion-resistant material in a windrow below or on the existing land surface along the bank,
then permitting the area between the natural riverbank and the windrow to erode though
natural processes until the erosion reaches and undercuts the supply of rock. As the rock
supply is undercut, it falls onto the eroding area, thus giving protection against further
undercutting, and eventually halting further landward movement. The resulting bank line
remains in a near natural state, with an irregular appearance due to intermittent lateral erosion
in the windrow location. The treatment particularly lends itself to the protection of adjacent
wooded areas, or placement along stretches of presently eroding, irregular bank line. The
following observations and conclusions were obtained from model investigations on windrow
revetments.
(1)
The "application rate" is the weight of stone applied per meter (foot) of bank line. The
amount of stone in the windrow indicates the degree to which lateral erosion will be permitted
to occur;
(2)
Various windrow shapes were investigated in the model investigations, and a
rectangular cross section was the best windrow configuration. This type of windrow is most
easily placed in an excavated trench of the desired width. The second best windrow shape
was found to be a trapezoidal shape. This shape provides a steady supply of stone to produce
a uniform blanket of stone on the eroding bank line. A triangular shape was found to be the
least desirable;
(3)
Studies indicated that varying the bank height did not significantly affect the final
revetment; however, high banks tended to produce a nonuniform revetment alignment.
Studies showed that the high banks had a tendency for large segments of the bank to break
loose and rotate slightly, whereas the low banks simply "melted" or sloughed into the stream.
The slight rotation of the high bank segment probably induced a tendency for ragged
alignment; and
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