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 Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
velocities in the vicinity of the root wads were 12 fps and yet willow clumps installed in with
the root wads and the root wads themselves did not fail.
Deflector dikes are any constructed protrusion into the water that deflect the current away
from the eroded bank. These consist of: transverse dikes, hardpoints, groins, bendway weirs,
and stream barbs. They are usually made of rock, but other materials such as logs or trees
can be used. As mentioned above in the Dusseldorf, Germany, example, bioengineered
treatments often use vegetation between deflector dikes. The dikes and the bioengineered
treatments work as a system to stabilize the streambank. Transverse dikes differ from
hardpoints or groins by projecting further out into the stream. Bendway weirs and stream
barbs are low rock sills. Flows passing over them is redirected so that the flow leaving the
structure is perpendicular to the centerline of the structure. Derrick (1996) describes the
construction and use of bendway weirs both on the Mississippi River and on smaller streams
in northern Mississippi. In the latter case, bendway weirs were successfully used, in part, with
a dormant willow post method of stabilizing the streambank (to be discussed below). Shields
et al. (1995) describe the benefits to aquatic habitats on small streams in northern Mississippi
by use of such weirs. The structures increased pool habitat availability, overall physical
heterogeneity, riparian vegetation, shade and woody debris density. To design deflector dikes
with vegetation, persons are needed with training both in hydraulic engineering and
bioengineering working as a team. Hydraulic engineers should be consulted for design,
construction, and placement of the deflector dike and bioengineers or someone with training
in botany should be consulted for use and placement of the vegetation.
A combination of materials, as mentioned above, can be used in the toe zone. Deflector
dikes can be used with plants incorporated in the dike system for erosion control as well as
fisheries habitat. Figure 21 shows a schematic of a coir geotextile roll. As illustrated in the
figure, it is used in combination with rock at the base and around the ends with some openings
for the ingress and egress of fish and other aquatic organisms. The coir is stuffed into a rope
mesh material made either out of coir itself or of polyethylene. The roll is planted with
emergent aquatic plants. The coir accumulates sediment and biodegrades as plant roots
develop and become a stabilizing system. Figure 22 shows several on a German stream.
Each structure serves to redirect the current away from the bank so that vegetation can be
installed in between. The plants in the structure furnish shade and cover for aquatic life.
While the rock of the structure would be in the toe zone, the roll and the aquatic plants would
be on top of the rock and abreast of it. The roll would actually grade into the next higher
zone, the "Splash Zone."
Splash Zone
The coir roll mentioned above can also run parallel to the bank with rock in the toe zone
providing the foundation and additional protection at the base of the roll itself. Sometimes,
the coir roll is all that is used in the toe zone when currents or waves are not strong or big
B-36
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