Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
for a few days. The rock toe extended from the bed to about 1/3 the height of the bank
(Figure 8). This treatment has been successful in this type of setting after several floods
exceeding the top of the bank.
Rock toes are also used streamward or just below other materials such as hay bales or
geotextile rolls. In one example, Omaha District recently used rock riprap below a large hay
bale cylinder covered with a fabric (rope mesh) made from woven fibers of coconut husks
called coir. The riprap weighed about 1.5 Tons/ft and was about 3.5-ft deep. Then,
vegetation in the form of dormant willow poles (discussed below) was placed above this
(Figures 9 and 10).
In another example, a rock roll (Figure 11) was used on the Rhine River in Dusseldorf,
Germany, below an installation of wetland vegetation grown in geotextile mats made from
coir. The large rock was wrapped in a polyethylene type of rope mesh and installed in the
following fashion: 1) a ditch is dug; 2) the rope mesh is placed in the ditch so that enough
of it is overhanging the ditch on the riverward side to wrap around the rock and have some
left on the shoreward side on which to place more rock; 3) the rock is placed on the rope
mesh; 4) the rope mesh is wrapped around the rock with a portion of it running up the
shoreward side; and finally 5) more rock is backfilled on top of the rope mesh to hold it all
firmly in place. This rock roll serves to protect the treatment from undercutting. The rope
mesh contains smaller rocks and strengthens the system and is similar to the function of
gabions which are discussed below. It should be mentioned that this whole system of rock
rolls and geotextile mats with wetland grasses or grass-like plants, such as sedges, were
placed in between large rock transverse dikes (Figure 12). The dikes were already there
before this treatment was installed and divert river currents away from the banks. The rock
roll (toe protection), the transverse dikes, and the geotextile coir mats, work together to
obtain wetland plant establishment and erosion control. Prior to the installation of plants,
even though the transverse dikes were present, an asphalt revetment used to control erosion
failed because water got behind the asphalt and pushed it out. This system has been in place
from 1991 to present and has withstood a large flood in 1994, the largest in the last decade,
with more than a 7 m fluctuation above normal flow. The flood overtopped the treatment for
several months. Because of the wetland plants' flood tolerance, the rock toe, and transverse
dikes, they survived and are still controlling erosion. A key wetland plant species instrumental
in the treatment's success was a sedge, Carex hirta2.
1
Herr Lothar Bestmann, personal communication, President, Ingenieurbiologie,
Wedel,Germany, May 9, 1996.
B-23
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