Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
The clumps of emergent aquatic plants mentioned above that were placed in the coir rolls
were grown from seedlings placed in a coir wrapping and allowed to develop hydroponically
(in water without soil, but with nutrients added). This leads to a well-developed, but light and
easily transportable plant unit with roots readily established and poised to grow in a planting
medium, such as the coir roll or in a soil substrate.
Coir fiber mats made in various thicknesses are also used in the splash zone. These are
often prevegetated at the nursery with emergent aquatic plants (Figure 25 a-c) or sometimes
sprigged (use of single or multiple rooted stems inserted into substrate) with emergent aquatic
plants harvested from local sources. When prevegetated at the nursery, the fiber mats have
the advantage of being light and can be lifted in rolls or smaller mats and transferred directly
to the planting site where immediate establishment is required. They are usually tied into or
keyed into whatever is used as the toe material. In the example on the North River above,
1-inch thick mats were prevegetated and tied into the coir rolls. Coir fiber mats have the
attributes of high tensile strengths, the ability to trap sediment, they are pH neutral, they
facilitate root development because of the fiber network, and they are slow to biodegrade.
These types of vegetated coir mats have also been used on dredged material in coastal
environments with wave environments. Knutson et al. (1990) reported successful trials of
sprigging emergent aquatic plants into such mats. This success was attributed, in part, to the
attributes mentioned above, such as sediment entrapment. The blankets trapped sediment
very well on the North River which aided plant establishment.
Single-stemmed sprigs and clumps of emergent aquatic plants and flood-tolerant grasses
or grass-like plants, e.g., rushes, sedges, can be planted shoreward of hard rock toes, coir
rolls, and fiber mats. They can even be used in lieu of the fiber mats if the site-specific
conditions are appropriate. This may mean that the soils are more cohesive, i.e., have more
clay in them, the stream discharges at that level are not as high.
Our focus in the splash zone, so far, has been on use of emergent aquatic and other
herbaceous plants. Woody plants are also used in the splash zone. For these, wetland plants
are used that can also withstand periods of dryness. The woody plants should be those that
can sprout roots and branches from the stem. These include willow, some species of alder,
dogwood, and several other species. Several possible species are listed by the Georgia Soil
and Water Conservation Commission (1994) and Gray and Sotir (1996). Sometimes, woody
plants may be all that are suited to the splash zone. At times, the bank geometry is very steep
down to the normal flow level without a shallow water zone for emergent aquatics or, the
stream system has extreme fluctuations and large silt loads that would drop sediment on
emergent aquatics and bury them.
Bioengineering techniques that utilize woody plants include: brushmattress, brush
layering, vegetative geogrids, dormant post method, dormant cuttings, and dormant root
pads. All of these are usually used in combination with hard structures or materials that either
deflect the current away from the bank or protect the toe and upper and lower ends.