Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
e.
How deep is the stream along the eroding bank? Willow posts must penetrate to
a depth that is deeper than the water near the eroding bank. There should be a shelf
or at least a sloping bank that allows willow posts to penetrate at least 2 feet deeper
than the deepest water at the shore or the posts will be undercut below the root
zone. If this cannot be achieved by the willow posts, then some kind of hard toe,
like a rock revetment, should be used to prevent scour beneath the posts. The
length of the willow posts will depend on the water depth as well as the dryness of
the soil above the stream level.
f.
How wide is the stream channel at the erosion sites when compared to stable
channels upstream and downstream? The channel with vegetation at the erosion
site(s) should not be narrower than stable channels upstream or downstream;
otherwise, vegetation could choke the channel and cause other erosion problems.
g.
Do you have a source of large willows close to the site? Costs are less when willow
stands are close because of less transportation costs. Also, there is less chance of
mortality due to long durations of handling and possible drying of the willow.
h.
Will the site be wet during dry summers? Willow posts require considerable water
while the roots are becoming established from the root primordia on the stems. For
dry sites, such as in the western states of the United States, tops of willow posts
should be only 1-2 feet above ground and they should penetrate into at least the
capillary zone of the groundwater table. Figure 10 shows willow posts being used
in eastern Montana on the upper Missouri River in combination with a line of coir-
covered haybales for toe protection. In similar cases, care should be taken to ensure
the posts are cut off not more than two feet above ground and that they penetrate
the groundwater.
i.
Can you keep cattle and other animals, domestic or wild, away from the posts
during the first summer? Willows and other plants produce food for regrowth from
leaf photosynthesis. If these sprouting branches with leaves continue to be browsed
or if the tops of the plants continue to be cut off by beaver during the first growing
season, they could die. It is best to prevent this by keeping cattle off of the area and
either trap beaver off the area or spray the willow stems with organic beaver
deterrent sprays, made with such constituents as mountain lion urine. It should be
noted, however, that beaver damage during subsequent years of development may
only promote resprouting of branches from the main stem and actually promote a
shrubby-like plant. This is a positive effect from a surface roughness perspective
whereas the many branches slow the current and promote sedimentation that can
lead to other plant colonization.
j.
Have debris jams or trees and logs forced floodwater into the eroding bank? These
must be removed at least to the point where they are not directing water into a
bank. Trees and logs can be moved parallel to the bank and cabled to dead men.
B-56
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