Erosion is the action of wind, water and ice that dislodges, dissolves or removes surface material. When material is removed from one location, it is inevitably placed somewhere else; this is called deposition. Both erosion and deposition are natural processes. But when human activities substantially alter, increase or impede the natural movement of sediment, serious damage can occur.
How does erosion occur?
Over time, even the largest mountains are worn down by weathering and erosion. Weathering is “rock decay,” which occurs through physical, chemical and biological processes. Once the minerals have been weathered and broken down, they are prone to being transported (eroded) by wind, water and ice.
The impact of raindrops on bare ground dislodges soil particles and causes erosion on the smallest scale, termed rain splash erosion. When water flows across sloped surfaces, it tends to form rivulets; these can create small channels in soft soil or sediment, and cause rill erosion. With enough rainfall on exposed earth, rivulets join together and gouge gullies in the land. Such processes can transport large amounts of topsoil and sediment from the land into water bodies.
In some places, such as prairies, deserts and coastal sand dune systems, wind causes erosion. The infamous “Dust Bowl” of the prairie states and provinces in the 1930s was caused by wind-driven erosion of soil that had been damaged by drought and intense farming practices.
As explained in coastal sediment processes
, shoreline erosion is a natural phenomenon. Bluffs are continually worn away by the action of waves and rainwater, and their sediment nourishes sandy beaches. Sand and cobble beaches themselves are part of a constant cycle. Winter storms wash away the fine material, and smaller summer waves gently replace it.
Erosion caused by rivers and streams is also basically a natural process. Especially during times of high rainfall, streams can move large amounts of sediment.
Sediment removed by erosion is deposited when the flow of wind or water slows down. For example, when a stream or river widens, the water slows and sediment that was carried along drops to the bottom. In this way, a delta is formed at a river mouth in a sea or lake.
How do human activities contribute to erosion?
Soils in this part of the world are usually covered in thick vegetation. Herbs, grasses, shrubs and trees help to prevent excessive erosion caused by heavy rains in a number of ways:
- Leaves and stems intercept the direct impact of raindrops, helping to prevent rainsplash erosion
- Roots create channels for rainwater to soak into the ground, help to anchor soils, and protect stream banks from the force of rushing water
- Decaying plant matter accumulates in and on top of the soil, creating a spongy “mat” that soaks up rain and prevents the soil from being washed away
When vegetation is removed, these natural functions are lost. Agriculture, logging, construction and road building usually involve removing existing vegetation. Unless these activities are carefully planned to minimize damage, erosion often occurs.
When human-made structures stand in the way of coastal sediment processes, problems arise. One worst-case scenario is a slope failure that damages or destroys a house built too close to the shoreline. Compaction of the soil, from the weight of structures, and excessive irrigation contribute to this situation. Vegetation removal, infilling, shoreline armouring, and construction of seawalls and groynes are practices that can cause shoreline erosion (see altered shorelines
Human activities substantially affect the energy of a stream. Impervious surfaces such as roads and parking lots cause unnaturally large volumes of rainwater to flow into streams, increasing the stream’s destructive power. Erosion can scour away the land beside a stream, and deposit large amounts of sediment downstream, in areas of slower stream flow.
How does erosion affect ecosystems?
Topsoil is formed over hundreds to thousands of years, through complex interactions among the bacteria, fungi, worms and insects that live among the roots of plants and help to break down organic matter. When topsoil is washed away by erosion, the land becomes much less fertile, and can support fewer plant and animal species.
Soil and sediment that is washed into natural water bodies degrades water quality and fish habitat in a number of ways:
- Light penetration of the water is decreased in murky water, impeding the growth of aquatic plants
- Gravel spawning beds used by fish such as Salmon and Cutthroat Trout can be buried and smothered
- Fish gills become clogged, and sediment irritates the mucous membranes of the skin and eyes, making fish more susceptible to disease and infection
Excessively high water flows associated with impervious surfaces and vegetation removal can destroy and degrade habitat for a wide range of plants and animals. This occurs when land adjacent to the stream is destroyed, or characteristics of the stream bed are altered, either by excessive erosion or deposition. The effects of habitat loss for even a few species are felt throughout the food web, since biodiversity is lost and ecosystems cannot function properly.
Paradoxically, a lack of sediment can cause damage to ecosystems as easily as excessive sediment. Some types of shoreline rely on the deposition of sediment supplied by natural erosion to retain their habitat characteristics. For example estuaries, deltas, salt marshes, coastal sand dunes and sand and gravel shorelines all require regular, natural deposition of various types of sediment. Fine sediment such as sand and silt provides the primary material in which plants root and certain animals burrow. Human-made structures such as seawalls and groynes prevent sediment transportation, and starve nearby shores of fine sediment (see coastal sediment processes
). This results in a gradual coarsening of beaches, and habitat loss for burrowing invertebrates living in the sand, and loss of a food source for the birds that feed upon them.
Dams also prevent the downstream movement of sediment, since it falls out of the water in the calm reservoir behind the dam. Areas such as floodplains and estuaries are then deprived of the gravel and sediment necessary for their proper function. For example, salmon require gravel beds for spawning, and fine sediment accumulates among tree root masses and log jams, creating new soil for riparian plants to grow in.
Erosion damage can create a window of opportunity for invasive species. Native species often cannot cope with the changes in soil characteristics and water quality. In contrast, certain invasive species may have evolved tolerances to these conditions, since they come from a different landscape. One example is Scotch Broom, which is able to grow in very nutrient-poor soils.
How does erosion affect people?
- When erosion occurs on a large scale such as a landslide, it can result in devastation of communities, roads, land and even lead to loss of life.
- Loss of topsoil can render the soil much less fertile for farming. Three quarters of the cultivated land in B.C. is considered to have a high to severe risk of erosion by water; 36 percent of farmland in the Prairies is at a high to severe risk of wind erosion.
- Habitat loss can negatively affect commercially important species such as salmon
- Excess sediment in drinking water can increase the cost and reduce the effectiveness of water treatment
- Coarsening and steepening of beaches, caused by shoreline erosion, makes them less attractive for recreation
- The effects of erosion degrade the landscape, making it less attractive from a recreational and aesthetic point of view; this can also affect the tourism economy
- Erosion damage to valuable ecosystems such as wetlands and estuaries reduces their ability to provide important services such as flood control, water storage and filtration, and protection from storm waves
What can I do to prevent erosion?
- Vegetation is a key factor in erosion prevention, particularly on steep slopes. Retaining native trees, shrubs and grasses on a site that is being developed can greatly reduce future erosion damage, and offset the cost of preserving them.
- Streamside and wetland vegetation is particularly valuable for filtering sediment (and chemical contaminants). Conserving a natural wetland, within or next to a new development, can provide valuable stormwater detention, which helps to prevent flood damage. Wetlands can also become attractive centrepieces in a public greenspace or park.
- Soil that has been compacted by heavy pedestrian traffic is more prone to erosion. Well-built trails, set back from the edge of streams, can minimize this problem. Lookouts, boardwalks and wildlife viewing areas can then be used to provide access to the water in key locations that are not susceptible to erosion.
- Machinery and vehicles also compact soils. Care should be taken during construction activities to limit and quickly remediate disturbed areas, for example by seeding with grass. When soil disturbance cannot be avoided, sediment cloth and temporary sediment ponds are examples of methods that can help control runoff of sediment into streams.
- The natural erosion of certain shorelines can be allowed to occur (without damage to property) if development is set back from the shoreline a minimum distance. This also prevents nearby beaches from becoming deprived of sediment.
- Reducing impervious surfaces is very helpful for preventing erosion damage in streams, as volume of runoff is thus greatly reduced. Permeable pavement, green roofs, and native plant gardens are just a few methods available.
- There are many agricultural practices that reduce erosion damage and limit the loss of valuable topsoil. Generally, practices that minimize tillage and irrigation are preferred.
- Bioengineering techniques can be used to control erosion. For example, "living fences," made from vegetation such as willow, help stabilize slopes, first with the structure of the fence and subsequently with growing plant roots.
- Instead of a seawall, use native vegetation to stabilize land on your shoreline property.
- When walking in the forest or along shorelines, stay on designated trails. "Short-cuts," especially on steep trails, create bare soil and potential channels for water – ideal erosion conditions.
Bioengineering Techniques for Erosion Prevention
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 shallow earth slumps and loose soil slopes and gullies ||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. |
|Pre-vegetated 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. |