Ecosystem and streambank restoration projects use a wide variety and large quantities of native vegetation to economically restore wetlands and streambanks, while providing adequate armoring to withstand the velocities and shear stresses from river flows. Vegetative materials include: wetlands or uplands seed mixes; bare rooted seedlings or saplings; balled and burlap trees; dormant live cuttings (brush, stakes, poles) and potted plants. This photo shows a dormant live willow stake that has sprouted in Spring after being driven into a bare bank during the dormant. This stake is about ½” diameter, and projects about 12” from the bank. The stake is buried 3’ to 4’ into the bank. Native cuttings can also be used to provide brush layers, fascines, dormant live stakes and poles and vegetation soil reinforced systems.
The use of vegetation can reduce the extent of riprap armoring, while still providing the necessary resistance to shear stresses and velocities on the upper banks of a stream. The upper range of permissible shear stresses of soil bioengineering measures has been found to range from 1 to 5 lbs/square foot, with a corresponding range of permissible velocities from 3 to 10 feet/second. Velocities and shear stresses need to be determined through hydrologic and hydraulic modeling of the river reach to determine worst case velocities and shear stresses. See link to publication below: http://el.erdc.usace.army.mil/elpubs/pdf/sr29.pdf
On many of the Cuyahoga Valley National Park projects, we utilized native plant materials found within the park, rather than having the contractor provide cuttings from off site. These included: sandbar willow; black willow; red ozier dogwood and some others. The Park’s landscape architect would identify areas within the park from which the contractor could harvest plant materials. This is both economical and environmentally sustainable, since the harvested plant material will eventually grow back, and the cost to cut and deliver the plant material from within the park is significantly lower than purchased material. One should be aware not to specify plant material that is under distress from an invasive species, as is the case with Green Ash. At one time cuttings from Green Ash were specified for use in the upper zone of restored riverbanks until it became known that ash trees were being harmed by the Emerald Ash Borer. Plant material cannot be relied upon to provide armoring below mean water level. In those areas, a riprap revetment should be used, and should extend up the bank to a level corresponding with the channel forming discharge or ordinary high water.
Plant material, when properly engineered, is an environmentally sustainable means to effectively stabilize riverbanks.
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