Traditional streambank stabilization Biotechnical streambank stabilization Channelization alters: shape pattern slope bed morphology cover Ecological effects: loss of riparian habitat loss of fish habitat reduced diversity influence on fish reproduction, processes, etc. Minimizes erosion reinforcing the soil with tensile fibers of the root mass increasing shear strength by reducing porepressure through transpiration anchoring the slope through deep root penetration into more stable strata Reduces overland flow Reduces rate of flow in the stream channel increase roughness Traps sediment absorbs water and filters sediment runoff To attain optimum stabilization vegetation must establish quickly and solidly on streambanks Projects with vegetation alone stabilization is vulnerable in early stages and increases as vegetation becomes established Projects with rock and vegetation rock will provide protection as vegetation becomes established Li, M. and Eddleman, K.E. 2002. Biotechnical engineering as an alternative to traditional engineering methods: a biotechnical streambank stabilization design approach. Landscape and Urban Planning 60 (2002): 225-242. Other resources from the USACE Engineer Research and Development Center, Ecosystem Management and Restoration Research Program http://www.erdc.usace.army.mil/ Stream Corridor Restoration handbook (FISRWG, 1998) http://www.nrcs.usda.gov/technical/stream_restoration/ Wattles Function reduces overland flow, traps sediment, adds roughness Make-up & installation tube stuffed with rice or wheat straw with an outside net made of jute, nylon, or photo degradable materials diameters range from 8-12 25-30 feet long staked in trenches on the contour of the embankment 3-5 years longevity (biodegradable) Photos from http://www.strawwattles.com/ 1
Coir rolls Function traps sediment, reduces overland flow, reduces flow velocity, wetland plants (rooted sprigs, cuttings) incorporated into coir roll where roots become interlocked in fibers, provides food and cover for fish along the water line Make-up and installation coconut fiber rolls bound together e with twine and/or polypropylene to form cylindrical structures diameters range from 8-12" placed at low water line 4-8 years longevity (biodegradable) Coir rolls con t Allen & Fischenich, 2000 http://el.erdc.usace.army.mil/elpubs/pdf/sr0 4.pdf Photos from Allen & Fischenich, 2000 Fascines Function traps sediment, velocity, protects banks from shallow slide failures Make-up and installation long bundles of live woody vegetation buried in a streambank in shallow trenches placed parallel to the flow of the stream. The plant bundles sprout and develop a root mass that will hold the soil in place and protect the streambank from erosion. cuttings are bound together in bundles 6-8 inches in diameter and 4-20 feet in length. Photo from http://www.dnr.state.oh.us/water/pubs/fs_st/stfs14.htm Fascines con t Limited data that have been collected for shear or velocity tolerances of LF and IF structures (largely empirical information collected from constructed projects), are summarized in Tables 1 and 2. Live fascine Inert fascine (Above table based on USDA SCS, 1992) Photo from http://www.ext.vt.edu/pubs/forestry/420-155/figure12.html Sotir & Fischenich, 2001 http://el.erdc.usace.army.mil/elpubs/pdf/sr31.pdf Brush layering Function stems provide frictional resistance to shallow slides or mass wasting; protruding stems serve to break long slopes into shorter slopes and retard runoff erosion; live cuttings eventually root and provide a permanent reinforcement Make-up and installation alternating layers of live branches and earth. branches protrude beyond the face of the slope. typically constructed concurrently with the construction of cut or fill embankments. Branchpacking Vegetated geogrids (aka Vegetated Reinforced Soil Slope structures) Function traps sediment, velocity, minimize bank erosion Make-up and installation layers of live cuttings incorporated with natural or synthetic geogrids/geotextiles can use on steeper slopes than brush layering 2
Vegetated geogrids (aka Vegetated Reinforced Soil Slope structures) Sotir & Fischenich, 2003 http://el.erdc.usace.army.mil/elpubs/pdf/sr30.pdf Live cribwall Function effective on outside bends where high strength is needed to reduce erosion; provides excellent habitat Make-up and installation box-like interlocking arrangement of untreated logs or timbers; structure filled with backfill material and layers of live cuttings that root inside the crib structure & extend into the bank slope effective where a steep slope face or a more vertical structure is needed http://www.abe.msstate.edu/csd/nrcs-bmps/pdf/streams/bank/livecribwall.pdf and http://www.dnr.state.oh.us/portals/7/pubs/fs_st/stfs17.pdf Brush mattress Brush mattress con t Function traps sediment, adds roughness, reduces flow velocity; not suitable for slopes experiencing large mass wasting Make-up and installation layer of woody branches (e.g., willow, dogwood, alder) placed on a bank face, often with a live fascine, coir roll, and/or rock toe at base of bank mattress held in place with twine or wire, live stakes, and dead stout stakes Photo from FISRWG, 1998 Photo from Allen & Fischenich, 2000 Allen & Fischenich, 2000 http://el.erdc.usace.army.mil/elpubs/pdf/sr23.pdf Live stakes & joint planting Live stakes & joint planting con t Function enhance aesthetics of existing rock riprap; provide better habitat, increase strength of riprap Make-up and installation rock riprap with live stakes tamped into joint or opening between rocks Live stakes can be used alone without rock riprap Willow stake specifications: Schaffr & Lee, 2003 http://el.erdc.usace.army.mil/elpubs/pdf/er02.pdf Sotir & Fischenich, 2007 http://el.erdc.usace.army.mil/elpubs/pdf/sr35.pdf 3
Live stakes & joint planting con t Tree revetment Function traps sediment, velocity Make-up and installation series of whole, dead trees cabled together and anchored by earth anchors in the streambank semi-permanent; inexpensive & uses readily available material; used in combo with other biotechnical treatments Cost/Strength Matrix Sotir & Fischenich, 2007 http://el.erdc.usace.army.mil/elpubs/pdf/sr35.pdf Root wads Root wads con t Cost/Strength Matrix Photos from FISRWG, 1998 Function traps sediment, velocity, redirects high-velocity flow core, provides wildlife & fish habitat Make-up and installation logs or rootwads anchored on streambanks effective on meandering streams with out-of-bank flow conditions sustain high shear stress if installed properly should be used in combo with other biotechnical treatments Photos from FISRWG, 1998 Root wads con t Design specifications Orientation of the root wad fan to velocity vectors: Root wad spacing: As a general rule, a spacing of 3 to 4 times the projected length of the rootwad is adequate. RECPs Function minimizes erosion Make-up and installation single-plastic-net or natural fiber straw (coir, excelsior) blanket typical slopes are 3:1 to 4:1with low flow rates blankets are staked down, usually seeded with grass, native vegetation provide further stabilization single-plastic-net straw blanket will last three to four months on average & costs $0.50 to $1.50/yd. 2 natural fiber blankets last one to two years, and the cost is up to $2.30/yd. 2 Photos from http://www.forester.net/ecm_0305_recp.html Sylte & Fischenich, 2000 http://www.swf.usace.army.mil/pubdata/environ/regulatory/other/links/stream/rootwadcomposites.pdf 4
RECPs con t Blankets are typically staked down with live stakes Post-project monitoring should include collection of the following data: Channel cross sections Water surface elevations Channel w/d ratio Streambank and bed erosion rates Longitudinal profile Aerial photos Reasons why biotechnical stabilization may fail: Selection of unsuitable vegetation species Very large flood before root system establishment Drought/inadequate rainfall during plant establishment Poor soil conditions for plant to root and grow Inadequate site preparation, grading, and drainage control Livestock grazing Insect infestation Plant disease Types of failure: Erosion of bank toes, leading to failure of overlying bank Erosion of soil along the banks Mass movement of saturated cohesive banks incapable of free drainage Flow slides in saturated silty and sandy soils Erosion of soil by groundwater seepage out of the bank Erosion of the upper bank or the river bottom due to wave action Freeze-thaw action Abrasion by ice or debris Shrinking and swelling of clays Biotechnical limitations: Methods require time to reach their full strength Dormancy requirements for live cuttings and growing season requirements for seeding makes timing projects difficult Short growing seasons in some regions can negatively affect establishment and strength Streambanks with excessive flow velocity and turbulence require high shear strength that biotech methods may not provide With limited technical design guidelines, even well executed vegetative protection may not achieve the same degree of confidence or offer as high a safety factor as structural protection Warmer regions utilize biotech methods less than climatic counterparts, so fewer qualified contractors will be available 5