|
|
||
 Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
Again, as it was in the earlier parts of this report, emphasis should be placed on prevention
of flanking of the bioengineering treatment. In this case, either contour wattling or brush
layering treatments should be protected with some kind of hard structure both upstream and
downstream of the treatment. If natural hard points, such as large boulders, rock
outcroppings, or hard geological strata, are not present, then one should consider use of a
rock refusal. This would be rock riprap that starts at the bottom of the bank, continues up
the bank, and is keyed into the bank (Figure 4).
Terrace Zone
This zone, as mentioned earlier, is rarely flooded and usually not subjected to erosive
action of the stream except during occasional flooding. When flooded, it receives overbank
flooding with return flows that can cause gullying and rilling to occur on the fall of the
hydrograph. It is in this zone that vegetation is needed with deeply penetrating roots to hold
the bank together, such as larger flood-tolerant trees. Grasses, other herbs, and shrubs can
be planted in between the trees, depending on their shade tolerance. Bioengineering, per se,
is not normally used in this zone unless there are deep gullies that have occurred as a result
of return flows or slopes still occur in this zone that are 3H:1V or greater. In these cases,
branch layering or contour wattling treatments are often employed across the gully or on the
contours of the slope.
Care should be taken in using large trees in this zone. They should be planted far enough
back from the bank that their shade does not kill out the vegetation in the splash and bank
zones. Narrow channels, especially, can be completely shaded from one side. When trees are
planted in this zone, they are planted either as container-grown (potted) or bare-root plants.
Suggestions vary on the size of container-grown plants. Leiser (1994) suggests using
containers with a minimum size of 9 cubic inches with a depth of 8 inches and a maximum size
of no larger than one quart milk carton. Plants in larger containers increase the cost for
purchase and planting substantially. Survival is frequently reduced because of limited root
systems in relation to size of the tops of the plants (Leiser, 1994). The important thing to
remember is to have a container with growing medium well filled with roots so that the roots
and medium form a cohesive unit when removed from the container.
Woody materials (Hoag 1994b), whether they be grown in containers or derived from
cuttings, should be used only in the bank and terrace zones when the following conditions
exist:
a.
where long periods of inundation or water erosion are minimized;
b.
where adequate moisture is available, i.e., natural precipitation is adequate for
species selected or plants are irrigated;
c.
where there is no competing vegetation or a 30" diameter area around plant is
scalped of competing vegetation at planting time;
d.
where plants have a low risk of physically being pulled or eroded out due to shallow
rooting system during the first year after being planted.
B-66
|
||
|
||
