Bioengineering Techniques for Erosion Prevention
What is Bioengineering?
Bioengineering involves using living plant material to build structures that reduce, prevent or repair erosion. A good description of a bioengineering structure is a "living fence." At first, the structure itself helps to prevent soil from washing away. As the vegetation grows, the plant roots further stabilize the soil.
Bio-Engineering techniques are not new; they were used in China (as long ago as 28 BC), for constructing and repairing dikes. European, Celtic and Roman people have also practiced various bioengineering techniques. Today, a resurgence of this old knowledge is occurring. With proper design, there are many advantages to bioengineering techniques over conventional "hard" structures: they can be more effective for controlling erosion; they are aesthetically pleasing; they are usually self-maintaining and less expensive; and they can be used to create streamside habitat for wildlife and fish. Furthermore, while human-made structures such as concrete walls break down over time, bioengineering structures grow stronger as plants mature.
A word of caution: many factors may contribute to the erosion of a particular slope, and restoration will not be successful unless the root causes are addressed. Erosion, and slope instability in particular, is a complex and potentially dangerous problem. The advice of a professional should always be obtained, especially for large projects; the information and references provided here are meant simply as an introduction to this fascinating topic.
For stream banks, the most common type of bioengineering used is a “wattle fence” constructed of willows. Live willow poles make up the fence. When the willows take root and grow, they provide continuing erosion protection. In most cases this bioengineering technique is easy to install. Work can be done with the help of neighbours and volunteers.
An example of bioengineering: wattle fencing after installation (above) and after four months of growth (below)
Willows should be harvested from a natural area, which will typically require permission of the landowner. The time of year is important – the willows need to be harvested when they are dormant. Bioengineering is typically done in the fall, once the willows have lost their leaves. It can also be done in spring before leaf-out. It is important to do this technique at a time of year where there will be sufficient moisture for the willow poles to survive and take root.
How do Bioengineering techniques work?
Bioengineering combines principles of ecology, hydrology, geology and physics. The basic idea is to harness the natural properties of vegetation to stabilize soil, while well-designed structures prevent the slope from failing and allow the plants time to establish. Plant roots bind and anchor soils. Decaying plant material makes the soil sponge-like, and encourages water to infiltrate the ground rather than running off and causing erosion. Structures such as "live pole drains" may also be designed to divert or direct drainage.
The importance of vegetation to stream function and health cannot be overstated.
- Plant roots in and along the stream bank prevent soil from being washed away
- Large roots and fallen trees help to dissipate the (potentially destructive) energy of flowing water
- Shrubs and trees shade the stream and keep the water cool, benefiting aquatic organisms including fish
- Vegetation provides habitat (and in some cases, food) for insects, birds, mammals, fish and amphibians
Bio-Engineering thus not only helps to solve a specific problem, i.e. erosion, it also helps to restore the overall condition of the stream.
Specific conditions of the site must also be carefully assessed to choose the most appropriate technique and plant variety. In BC, willows (Salix species), cottonwood (Populus balsamifera) and Red Osier dogwood (Cornus stolonifera) are the native species most commonly used.
What are the applications of Bioengineering techniques?
A number of techniques have been used for various types of applications.
The diagrams here illustrate some examples, and the applications are
summarized in the table below.
Wattle Fencing (left) and Live Pole Drain (right). (Diagrams courtesy
of Polster Environmental Services Ltd.)
| Technique |
Application |
Description |
| Wattle fence |
Over-steepened slopes where vegetation cannot naturally establish |
Long cuttings (e.g. willow) laid horizontally and supported with larger vertical plant stakes or rebar. Soil is filled in behind the fence; creates a series of terraces. |
| Live bank protection |
Along stream banks, to protect against further erosion. |
Wattle fences are contoured around bends in the stream, in areas that are susceptible to erosion; soil is then backfilled behind the fences. |
| Live palisades |
Adjacent to a stream or river where the natural vegetation has been removed. |
Large posts of a tree such as cottonwood are sunk in trenches some distance back from the edge of the stream. |
| Brush layering |
Stabilizing shallow earth slumps and loose soil slopes and gullies |
Benches are excavated in the slope, willow branches are laid, on a slight angle, on the benches; branches are covered with soil, with just the tips sticking out. |
| Coconut fiber fascines |
Stabilizing the base of a stream bank or shoreline. |
Fascines (fibre “log rolls”) are placed along the base of a bank; they collect sediment and help to stabilize the bank; riparian vegetation may then be planted in and alongside the roll. |
| Prevegetated mats |
Lakeshores and wetlands |
Plants are grown on mats made of a slowly biodegradable material such as coconut fibre; the mats are then simply placed on the slope. |
| Live gravel bar staking |
Unnaturally large gravel bars amid braided streams (caused by upstream resource activities) |
Using an excavator, live stakes are inserted in the gravel bar, in order to start recolonization of natural vegetation, and a return to a single stream channel. |
References & Links
Websites
- Bioengineering for Hillslope, Streambank and Lakeshore Erosion Control: U of Nebraska
- Soil Bioengineering Guide for Streambank and Lakeshore Stabilization
Articles
- Soil Bio-Engineering Techniques for Riparian Restoration (PDF
) - On Course with Nature: Soil Bioengineering: A Natural Approach to Stream Bank Stabilization (PDF )
- Bioengineering Techniques for Streambank Restoration: a Review of Central European Practices (PDF )
- Soil Bioengineering for Upland Slope Protection and Erosion Reduction: US Department of Agriculture (PDF )