Biological shore protection
Biological shore protection techniques use vegetative and natural materials for stabilization and protection. Biological techniques rely on plant materials as the main structural elements in a shoreline protection system. Biological shore protection techniques are comprised of living and/or organic materials, such as native grasses, sedges and forbs; live stakes and posts; jute netting; and coir fiber rolls and mats. All materials used in biological shore protection techniques must be biodegradable.
Biological Shore Protection Techniques fall into three categories: bank treatments, integrated toe protection, and biodegradable/temporary breakwaters.
A brush mattress is a procedure that uses live cut branches along the slope of an eroding shoreline. The cut ends of branches are placed in a trench at the toe of the slope and anchored with a wattle. A grid of stakes and jute rope, wire or other material secures the branches. The live cut branches sprout and take root, thus stabilizing the shoreline with a dense matrix of roots. Additional toe protection may be needed to resist scour and undercutting.
This method requires the use of live, rootable vegetative cuttings, often willow (Salix spp.) or other species, to revegetate eroding shorelines. The cuttings are tamped into the soil, sprout and take root, stabilizing the streambank with a dense matrix of roots. Toe protection may be required where scour is anticipated. Live stakes are appropriate for the repair of small earth slips and slumps that are frequently wet. They can also be used to stake down erosion control materials.
The plant material is installed during the fall or spring when the original plant (and consequently cuttings from it) are dormant. Woody plants that have good rooting characteristics make good staking plant stock. Stakes are generally 2 to 3 feet long and 1/2 to 1 1/2 inches in diameter and can be collected from sections or branches of plants from donor sites. Stakes should be flat cut on the top and diagonal cut on the bottom so they will be installed correctly. Staking can be used alone and with other planting techniques. Typically, if stakes are used alone on the slope, they will be spaced across the slope as recommended for each species and slope situation. Each row should have the same spacing but should alternate stake positions so that if you look down or up slope, no two consecutive rows should have stakes directly above or below one another (a diamond pattern). Stake rooting will be most effective if the stake is not positioned vertically but positioned at an angle off horizontal so that rooting can occur more effectively along the entire below-ground length.
Stakes are typically placed into predrilled holes using rebar sections, which are slightly smaller than the diameter of the stakes. Gently tap stakes into holes with soft mallets or other instruments.
This technique uses alternating layers of live cuttings and compacted backfill along the slope of an eroding shoreline. The branches are oriented perpendicular to the slope contour to reinforce the earth and mass stability of the slope. This "terrace" reduces the length of the slope of the shoreline. The live cuttings sprout and take root, thus stabilizing the shoreline with a dense matrix of roots. This technique is most applicable to areas subjected to cut or fill operations or areas that are highly disturbed and/or eroded. Layering provides the best technique to achieve soil reinforcement to resist potential shallow-seated landsliding events. Brush layers act as live fences to capture debris moving down the slope. Some toe protection such as a wattle, coir fiber roll, or rock may be necessary. This method is often used to repair small localized slumps and holes in shorelines.
Fiber rolls are cylindrical tubes composed of coconut husk or excelsior fibers bound together with coconut or jute twine or plastic netting. The fiber roll product protects the bank by stabilizing the toe of the slope and by trapping sediment from the sloughing bank. Cuttings and herbaceous riparian plants are planted into and behind the coir fiber rolls. It is appropriate where moderate toe stabilization is required in conjunction with the restoration of the shoreline, and the sensitivity of the site allows for only minor disturbance. By the time the roll decomposes (approximately 6 to 10 years), riparian vegetation will have stabilized the lakeshore. It may not be appropriate for sites with high velocity flows, along high-energy shorelines, or large ice buildup.
Biodegradable or Temporary Breakwaters
Temporary breakwaters are installed offshore of the shoreland to provide an area of calm water, usually when new erosion protection designs and shoreland plant installations are becoming established. The breakwater may be temporary if it is constructed of biodegradable materials, like jute, coir fiber, willow stakes, etc. or if the structure will be removed after a set period, like at the end of the growing season.