Terrace Zone

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:

where long periods of inundation or water erosion are minimized;

where adequate moisture is available, i.e., natural precipitation is adequate for

species selected or plants are irrigated;

where there is no competing vegetation or a 30" diameter area around plant is

scalped of competing vegetation at planting time;

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.

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