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.
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 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.
Figure 3. Live palisades consist of a row (or rows) of large cottonwood posts sunk into the water table. Smaller cuttings of willow and red-osier dogwood are inserted in the trench as it is backfilled to provide some diversity to the riparian stand as it develops.
Live Gravel Bar Staking
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 off-channel 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.