|
|
||
 Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
plant into the roots (Mitsch and Gosselink, 1986). Therefore, they can extend roots into
deeper water than many other types of plants, such as woody plants. Reeds, such as common
reed (Phragmites australis), and sedges, such as bulrushes (Scirpus spp.), also protect
streambanks in various ways. Their roots, rhizomes, and shoots bind the soil under the water,
sometimes even above the water (Seibert 1968). In the reed zone, as Seibert (1968) defines
it, they form a permeable underwater obstacle which slows down the current and waves by
friction, thereby reducing their impact on the soil. Active protection of the bank can be
ensured by reeds only in an area which is constantly submerged (Seibert 1968).
It should be mentioned that common reed is often considered a pest in the U.S. where
it has been observed as a monotypic plant that does not offer habitat diversity. The authors
would submit that this is true where there is not much of an elevation and hydrologic gradient.
In other words, on shallow flats that become periodically inundated, it can thrive. However,
when it is on a shoreline and becomes inundated over about 18 inches, it is often replaced by
other more water tolerant species. One should use caution on where this plant is used and
match it to one's objectives.
Various wetland grasses, sedges, and other herbs were used in this zone as a part of a coir
geotextile roll in an urban park setting in Allentown, Pennsylvania. The main vegetative
components of erosion control of the stream embankment are: lake sedge (Carex lacustris),
stubble sedge (C. stipata), and woodland bulrush (Scirpus sylvaticus). Other minor
components used for diversity and color included: rice cut-grass (Leersia oryzoides), other
sedges (C. lata, C. lanuginosa, C. hysterina, and C. prasina), softstem bulrush (Scirpus
validus), blue flag iris (Iris versicolor), and monkey flower (Mimulus ringens). The latter
two species were provided primarily for additional diversity and color (Siegel, 1994a). Siegel
reported that these plants, along with bioengineering methods such as the coir roll, stabilized
a streambank that was subjected to storm events. In fact, the methods were designed to
accentuate and enlarge the existing floodplain to act as a buffer zone for floods associated
with storms greater than the 25-yr event (Siegel, 1994b). The vegetation list above only gives
one examples of types of species that were used for erosion control in the splash zone, i.e.,
flood-tolerant and fast growing grasses and sedges. Care should be exercised in selecting
species that are adapted to the project's geographic area. Local university botanists and
USDA Natural Resources Conservation Service (NRCS, formerly Soil Conservation Service)
district personnel can be consulted for suitable species.
Herbaceous emergent aquatic plants, like those shown in Figure 6, must be used on a
streambank that has a geometric shape conducive to such plants. Caution must be used on
streams that have heavy silt loads that could suffocate plants. These plants must grow in
fairly shallow water, from +45 to -152 cm (Allen et al., 1989). Sometimes, it is impossible
or impractical to find or shape a stream to match those conditions. Then, flood-tolerant
woody plants, like willow (Salix spp.), dogwood (Cornus spp.), and alder (Alnus spp.) are
used in the splash zone. Again, a good rule of thumb is to look at the natural system and
observe what is growing there and try to duplicate it.
B-20
|
||
|
||
