Stream Bank Protection
Stream bank protection or stabilization involves the placement of material along or beside banks to prevent erosion. This can be accomplished by armoring the bank with a hard surface (e.g. "rip-rap"), by modifying the bank using "soft-armoring" or bioengineering methods, or by placing structures in the stream to divert the current or to reduce the erosive effects of flow energy on stream banks.
Any stream bank protection method utilized should seek to provide the greatest degree of natural stream and floodplain function achievable through application of an integrated, ecological approach. In general, “soft” bioengineering techniques are preferred over “hard” armoring methods. The selection of protection measures is typically determined by an analysis of the mechanisms and causes of stream bank failure, reach conditions, and habitat impacts. Large woody debris (LWD) should be included as an integral component of all stream bank protection treatments in almost all cases. Generally, the LWD should be large (>1-foot in diameter and >10-feet in length) and coniferous, with an intact and untrimmed root wad in place to provide functional refuge habitat for aquatic biota.
Guidance for stream bank stabilization can be found in the Washington Department of Fish and Wildlife (WDFW) Integrated stream bank protection guidelines (ISPG) manual.
Stream bank stabilization methods
The following is a list of stream bank stabilization methods may be used individually or in combination, depending on the site conditions and project goals:
- Native vegetation, including trees, shrubs, and groundcover.
- Bioengineering techniques, including deformable soil reinforcement, consisting of soil layers or lifts strengthened with biodegradable rolled fabric materials.
- Wattles (long bundles of coir or other biodegradable fiber) used individually or in layers to provide a terraced arrangement to stabilize slopes and provide growth medium for riparian plants.
- Bank reshaping and slope grading used to reduce a bank slope angle without changing the location of its toe, increase roughness and cross-section, and provide more favorable planting surfaces.
- Flow spreaders, consisting of one or more rows of trees and/or shrubs and accumulated debris used to spread flow across the floodplain.
- Floodplain roughness (e.g. trees, shrubs, LWD, and brush barriers) used to reduce the likelihood of avulsion in areas where natural floodplain roughness is poorly developed or has been removed.
- Engineered log jam (ELJ) structures, consisting of a collection of LWD used to create structural and hydraulic complexity and redirect flow, provided that the ELJ is anchored primarily by the weight and shape of the structure itself. Rock and/or cabling may also be used to stabilize the ELJ.
- Barbs (also referred to as vanes or weirs) when designed as follows:
- No part of the flow-redirection structure may exceed bank full elevation, including all rock buried in the bank key.
- The flow-redirection (barb) structure shall be built primarily of wood or otherwise incorporate LWD at a suitable elevation in an exposed portion near the tip of the structure. Placing the LWD elements near stream banks in the depositional area between flow-direction structures to satisfy this requirement is not approved, unless those areas are likely to be greater than 1 meter in depth, sufficient for salmon rearing habitats.
- The trench excavated for the bank key above the ordinary high water elevation shall be filled with soil and topped with native vegetation.
- The maximum flow-redirection structure length must not exceed 25% of the channel width at ordinary high water.
- Rock shall be placed individually without end dumping.
- If two or more flow-redirection structures are built in a series, place the farthest upstream barb in accordance with the technical standards established by WDFW (see above link).
- Include native riparian planting as a project component.
- All actions intended for stream bank protection will be designed to provide the greatest degree of natural stream and floodplain function achievable through the application of an ecological approach to bank and channel protection.
- LWD will be included as an integral part of all stream bank protection treatments.
- Use of rock, stone and similar “hard” materials should be avoided or minimized, but may be allowed for the following uses:
Whenever feasible, the rock placed below ordinary high water must be large, round (river) rock and may not impair natural stream flows into or out of secondary channels or riparian wetlands.
- As ballast to anchor or stabilize LWD.
- To construct a footing, facing, headwall or other protection for certain types of existing structures.
- To construct a flow-redirection (barb) structure (each barb will incorporate large wood and woody riparian planting as part of the project).
- Structures built entirely or primarily of rock, concrete, steel or similar materials intended to prevent bank failure at design flows above the high water mark, such as revetments, bulkheads, groins, buried groins, and rock toes are strongly discouraged except in very limited situations. The use of dikes, groins, buried groins, drop structures, weirs, riprap, rock toes, and similar structures to stabilize stream banks also are discouraged.