Explore our methods
A wide variety of soil bioengineering methods have been developed to treat the range of problems that are encountered. This section provides an overview of the soil bioengineering methods that have been developed and used by Polster Environmental Services Ltd. to treat common soil failures. Steep slopes can be treated with methods such as wattle fences, live reinforced earth walls, live smiles, modified brush layers and brush layers. Sites with excess soil moisture can be treated with live pole drains. Where moving water is causing a problem, live palisades, live silt fences, live bank protection or live gravel bar staking can help. Live staking can be used to re-establish strong root systems on stream banks and slopes. Several soil bioengineering techniques can be used together to solve complex problems. For instance, live pole drains can be used in combination with wattle fences to treat sites where piping failures have cause soil slumps and steep slopes. Soil bioengineering systems are built of living cuttings of pioneering plants so they start the recovery processes natural successional systems.
SOIL BIOENGINEERING FOR RIPARIAN RESTORATION
David F. Polster, M.Sc., R.P. Bio.
Paper presented at the Canadian Land Reclamation Association 2006 Conference, Ottawa, Ontario, August 20 – 23, 2006. David F. Polster, M.Sc. R.P.Bio., President, Polster Environmental Services Ltd.
Abstract: Degradation of riparian areas can be traced to a wide variety of causes from urban developments to industrial activities, including mining. Soil bioengineering, where living plant materials are used to perform some engineering function, can be an effective tool in reclaiming damaged riparian areas. Soil bioengineering can be used to treat unstable banks where modified flow regimes or riparian clearing has caused accelerated erosion. Soil bioengineering can be used to treat sites where excessive sediment deposition such as from placer mining has modified flow patterns. Soil bioengineering treatments can be used to trap sediments and to re-establish the functions of riparian vegetation including increasing bank root densities, providing shade, organic matter and insects for aquatic systems. Soil bioengineering treatments are well adapted to implementation by local stewardship groups and can provide an opportunity for low costs riparian restoration. This paper explores the soil bioengineering techniques that can be used to treat degraded riparian areas. A variety of case studies from western Canada are presented.
Key Words: Hardwood cuttings; earth retaining structures; drains; flow modifying systems; sediment control; living plants; riparian vegetation; cost-effective; local labour.
Increasing use and/or degradation of riparian areas requires development of effective strategies for restoration. Urban developments, industrial uses, including mining and forestry, as well as agricultural expansion can impact riparian areas, either directly through habitat loss or indirectly by changing hydrologic patterns and increasing peak flows. Urban development can cause significant degradation of riparian areas. Clearing riparian vegetation for houses, roads and infrastructure such as sewer lines, has had and continues to have detrimental impacts on riparian areas. Mining and forestry activities can change flow patterns and even watercourse locations. Agricultural clearing of riparian vegetation or livestock use of riparian areas can result in damaged riparian areas. All of these activities have in common the loss of riparian structure (vegetation, living and dead) and/or riparian function. Soil bioengineering treatments can be used to restore riparian areas and return lost riparian structure and function.
Soil bioengineering is the use of living plant materials to perform some engineering function (Schiechtl 1980). Riparian treatments using soil bioengineering systems can range from simple live staking to complex systems designed to stabilize eroding or slumping banks (Schiechtl and Stern 1997). The plant materials, usually dormant cuttings, used in soil bioengineering systems sprout and take root and form ideal riparian vegetation. The plants used in soil bioengineering are typically pioneering species so the vegetation established with soil bioengineering treatments is early seral and ideal for treatment of damaged or degraded sites (Polster 1989). By using early seral species the restored site is re-aligned with the natural successional trajectory for the area.
Labour for soil bioengineering projects can come from trained crews or from untrained community stewardship groups. Since the techniques are simple and the materials can be collected locally, soil bioengineering treatments are ideally suited to the treatment of small, local problems with local people. However, soil bioengineering can also be used to treat large problems. The basic premise with soil bioengineering treatments is that there is strength in numbers. Where a large eroding slope is causing a problem, rather than building some large structure such as a large retaining wall at the bottom, the soil bioengineering solution would involve a whole series of small retaining walls all the way up the slope (see wattle fences below). Similarly, when erosion problems threaten a streambank rather than building a large rip-rap slope, a soil bioengineering treatment would apply hundreds or even thousands of cuttings (see live gravel bar staking below) that would slow the near surface flood flows and thus prevent the substrate from being eroded and in some cases even allow suspended sediments to be deposited.
This paper explores the use of soil bioengineering systems for treatment of riparian areas that have been damaged, degraded or destroyed by the activities of humans. Natural erosion of river banks or lakeshores is an important process in maintaining gravels for fish spawning and for creating the early seral riparian landscape that is essential to so many biota. Soil bioengineering systems can help to re-establish the natural successional trajectory of riparian ecosystems that have been negatively impacted by humans. As ways in which humans can live lightly on the landscape are sought, soil bioengineering offers treatments that can be applied by local stewardship groups as well as municipal workers. In industrial settings, soil bioengineering offers alternatives to the traditional hard engineering that is applied for slope failures and degraded sites. The soil bioengineering systems discussed in this paper are more fully described in Polster (2005).
Soil bioengineering methods use living plant materials to build structures to stabilize the problem site. As such, the construction materials must be strong enough to withstand the forces acting on them. In addition, since the intention of building the structures of living materials is that these materials will sprout and grow, the materials must be in a condition that will promote their subsequent growth. The plant materials are typically stem cuttings and must therefore be capable of forming new roots and shoots without special mist tents and bottom heat used in nurseries for plant propagation. Willows (Salix spp.), cottonwood (Populus balsamifera L.) and red-osier dogwood (Cornus stolonifera Michx.) are the only woody native British Columbian species that have been found to reliably root from stem cuttings.
Willows and cottonwood are most commonly used for soil bioengineering projects. These species are used due to their aggressive growth on disturbed sites. The nomenclature of willows is notoriously difficult and the exact identification of the willows used in a soil bioengineering project is not necessary. Typically common willows such as Scouler’s willow (Salix scouleriana Barratt in Hooker), Pacific willow (Salix lucida Muhlenberg), pussy willow (Salix discolor Muhlenberg) and glaucous willow (Salix glauca L.) are used. Although the nomenclature of the
willows that are used is not important, it is essential that species be selected from habitats that approximate those found on the reclamation site. For instance, treatment of a dry raveling slope composed of sandy gravel would not be very effective with willows that were collected from around a marsh. Red-osier dogwood is particularly useful where the treatment site is under the canopy of other vegetation and not in direct sun.
Cuttings used in soil bioengineering projects should follow the “rule of thumb” that is, if it is not as big in diameter as your thumb it is too small. Minimum diameter of the cuttings at the tip end should be at least 2.5 cm, and larger cuttings tend to work better than smaller ones as long as they are not old and decadent. In terms of length, cuttings should be at least 40 cm long and where structures such as wattle fences are being built, the cuttings should be as long as possible. Cuttings that are 6 or 7 m in length can be used to make very strong structures. Trim all of the small branches and twigs from the cutting before using it in a structure. Where live pole drains are being built, trim smaller twigs but pencil sized twigs can be left on the cutting as long as they do not have leaf or twig buds on them.
The cuttings that are collected for soil bioengineering projects need to be handled in such as way that they will retain their viability. Keeping the cuttings cool and moist and avoiding excessive damage to the cambium will help to retain viability. In addition, soaking the cuttings in water for up to 10 days has been found (Becker 2002) to stimulate root development in un-rooted cuttings. Cuttings should be collected during the dormant period for the plant (late fall to early spring) and where timing does not permit immediate use of the cuttings, they can be stored in a cold storage facility (0 to 1 degree C) for several months as long as they are kept moist. Storage of collected cuttings in snow banks has been found to be very effective.
Wattle fences are short retaining walls built of living cuttings. Figure 1 shows the typical design for wattle fences. Wattle fences are used on sites where over-steepened slopes are preventing growth of vegetation. As the cuttings are fairly well exposed, wattle fences work best where there is ample moisture available to sustain the growth of the cuttings. Other techniques such as modified brush layers can be used where sites are drier. Wattle fences can be very effective on streambanks where the moist conditions supports the growth of the cuttings and the stepped back design allows flood flows to pass without damage. In addition, the water flows against the cuttings that provide a resistance to erosion, thus protecting the bank from erosion. Wattle fences can be used on very steep slopes as long as the slope itself is globally stable. At the University of British Columbia wattle fences have been effective at revegetating the sand cliffs with an average slope of the in-situ materials of 70 degrees (see www.serbc.info restoration projects section). Wattle fences can be particularly useful where moisture sensitive soils are sliding down the slope as they will hold the soil and allow the moisture to drain, improving the stability of the soil.
The support for wattle fences can be either stout cuttings as shown in Figure 1 or 15 mm steel concrete reinforcing bars (rebar). Where rebar is used care must be taken to avoid the hazard created with steel bars protruding from the slope. Where cuttings are used, care must be taken while installing the cuttings to ensure they are not damaged too much. Creation of pilot holes and the use of short tapping strokes to drive the cuttings in can prevent excessive damage. When cuttings are used, the cuttings as well as the cross pieces grow and contribute to the vegetation on the slope.
Live Bank Protection
Live bank protection consist of wattle fences along the bank of the stream to create a woody
buffer against further erosion. The construction of the live bank protection must be sufficiently
dense so that erosion is avoided. Sometimes twigs and trimmings from the cuttings can be used
to fill in gaps between the cuttings and thus avoid erosion. Once the cuttings used in the live
bank protection sprout and grow, the resulting vegetation provides good protection against
erosion. Live bank protection can be particularly effective along the edges of newly constructed
ditches. Brush mats (see Schiechtl and Stern 1997) can be used with live bank protection where
erosion is excessive.
Figure 2. Live bank protection shown here without backfill. Note that the ends of the
structures are carefully placed to avoid areas where the current is actively
eroding the bank.
Live palisades are large cottonwood posts installed in trenches adjacent to the eroding stream or
river where the natural riparian vegetation has been lost due to clearing or erosion. Figure 3
shows the typical design for live palisades. The key is to get the cottonwood posts down into the
water table so that the trees will grow even during dry weather. Large cottonwood posts (15 to
20 cm diameter by 3 to 4 m long) are inserted into a trench dug by an excavator a few meters
away from the actively eroding bank. The cottonwood post is expected to root along its entire
below ground length and thus produce a dense cylinder of roots that will protect the bank from
erosion as the steam encroaches on the palisade. The large cottonwood posts are placed about 50
cm apart so that the growth of the roots will overlap within one growing season. Cottonwood
roots can grow as much a 1 cm per day during the growing season (Braatne and Rood 1998).
Riparian cottonwood trees provide significant riparian benefit when they mature.
Care must be taken when establishing live palisades in gravelly alluvial materials that the
excavation of the trench for the palisades does not cause an increase in bank instability. Setting
the row of palisades well back can help reduce this problem. In some cases, two rows of
palisades can be installed in a single trench by simply laying the posts in at an angle. Where fine
silty materials are encountered, a tractor mounted post-hole auger with an extension to reach the
water table can be an effective tool for inserting the poles. In addition to the cottonwood poles,
cuttings of willow and red-osier dogwood can be added to increase the vegetation cover and
enhance the diversity.
Live Gravel Bar Staking
Excess gravel deposits in streams and rivers can occur in areas of resource development from erosion of upslope areas. These in turn cause disturbances in the stream flow that result in greater accumulations of sediments downstream. This cycle continues until the stream ends up as a broad expanse of bare gravel with a braided channel and no fish habitat. Live gravel bar staking is designed to establish the natural successional processes that would revegetate the gravel bars and eventually lead to a single channel with well-vegetated banks. The key to live gravel bar staking is to get the cuttings well into the substrate. Use of an excavator is essential (Figure 4). Cuttings should be a minimum of 1 m long and should not protrude from the gravel bar surface more than 20 cm. Large diameter cuttings (4 to 10 cm) appear to work better than smaller stock.
Figure 4. Live gravel bar staking is used to initiate natural succession on bare gravel bars. The sprouted cuttings trap small woody debris that in turn creates a flow disruption that results in deposition of sediment. Once the sediment builds to the point where the sprouts can no longer trap small woody debris there is no more sediment capture until the next year when growth of the sprout again traps woody debris.
Live shade is designed to provide an immediate vegetation cover over newly constructed offchannel fish habitat and small streams where the riparian cover has been lost. Live shade consists of tripods of living cuttings placed over the stream with the feet sunk into the banks about 50 cm so that ample moisture is assured. The cuttings need to be large enough and strong
enough to easily span the stream and to support the weight of the new growth plus whatever snowfall might be expected in the region where they are applied. Live shade can be used on natural streams and ditches where riparian vegetation has been lost due to clearing. Figure 5 shows the typical design for live shade.
Figure 5. Live shade provides an immediate vegetation cover on newly constructed fish channels and streams where riparian cover has been lost. The spacing of the legs of the tripods can control the density of the shade.
Other Soil Bioengineering Techniques
There are a variety of other soil bioengineering techniques that are typically applied to slopes (Figure 6). Modified brush layers are used to create small terraces on dry raveling slopes where conditions are too dry for wattle fences. Live smiles are used where flowing mud pushes linear structures over. Sites must be relatively moist year-round to sustain live smiles. Live pole drains are used to drain excess moisture from seepage zones causing slope problems. They act like living French drains. Live reinforced earth walls perform like traditional soil reinforced structures except the construction elements sprout and grow. Brush layers in a fill act to prevent circular failures of the fill surface by providing sheer resistance while brush layers in a cut create a wall of vegetation to prevent raveling of the cut slope material.
Figure 6. Modified brush layers (1), live smiles (2), live pole drains (3), live reinforced earth walls (4), brush layers in a fill (5) and brush layers in a cut (6) can be used to treat a variety of slope problems.
Soil bioengineering techniques can be used to treat a variety of problems that may arise in the restoration of riparian sites. These techniques can provide protection from erosion and effective riparian vegetation that is congruent with the natural vegetation in the riparian zone. In most cases these techniques are easy to install and can be constructed by relatively untrained crews or volunteer groups. The diversity of techniques available allows soil bioengineering to address a wide array of problem sites. Soil bioengineering fits nicely between traditional engineering and normal revegetation programs.
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