Background
It is currently estimated that 2.2 million tonnes of topsoil are eroded in the UK annually (POST, 2006). In urban areas, the sites most at risk from erosion are brownfield and contaminated sites, as their soils often have physical properties that make them particularly prone to erosion forces (such as absent or reduced vegetation cover, soil texture and structure, or disturbed topography).
Erosion of such sites may pose a high risk through the mobilisation and transportation of toxic sediments far from site boundaries. Off-site impacts of soil erosion are often economically and ecologically of greater concern than on-site impacts. With this in mind, planning for erosion control is essential. Erosion control involves the creation of some sort of physical barrier, such as vegetation or rock, to absorb some of the energy of the wind or water that is causing the erosion. Vegetation establishment is now recognised as being a cost-effective and sustainable erosion-control technique.
Opportunities
Vegetation has the potential to protect the surface against water (raindrop impact and run-off) and wind pressures, and to modify soil hydrology. When properly established and maintained, vegetation can provide the following benefits:
- Controls erosion (stabilises the soil material), reducing soil loss (including organic matter and nutrients) and, when relevant, contaminant mobility (transport of contaminants on- and off-site), so inhibiting or reducing pollutant linkages
- Generally increases slope stability, reducing potential for landslides
- Does not require heavy machinery to install
- Is a sustainable and relatively inexpensive solution
- Encourages the establishment of wildlife habitats
- Improves the aesthetic quality of the urban landscape.
Care must be taken to ensure vegetation establishment is sustainable. Predicted changes in climate may contribute to an increase in the frequency and amplitude of erosion processes, causing existing pollutant linkages (source-pathway-receptor) to strengthen or new pollutant linkages to develop on contaminated sites. This means it is important to understand the magnitude of current and future pollutant linkages through water and wind erosion to achieve sustainable erosion control through greenspace establishment.
Practical considerations
Erosion control can be planned at different scales, depending on the distribution of the problem and the objectives of the control measure. This may mean at the individual site level, for example where a specific site is posing a risk; at the town/city scale, where a number of sites in an urban area are posing a cumulative risk; up to the catchment scale. One basic erosion-control measure is to reduce water run-off at source, and this is made possible by increasing the proportion of the ground surface covered by greenspace, as vegetated areas have higher infiltration rates than areas of hard standing. Careful planning will ensure that the objectives are achieved, whatever the scale of erosion mitigation that is necessary. See Case studies below for an example of how woodland extension/creation in urban areas and catchments can reduce erosion and sediment production, if suitable integration and planning of landscape features is undertaken.
Vegetation establishment as an erosion-control measure has been used for many years, and has improved with increasing understanding of the processes involved. In urban areas, vegetation establishment may be undertaken as part of restoration projects. In such cases, especially when land is contaminated, care must be taken to ensure that all stress factors are taken into account to provide sustainable solutions in the long term. The design of erosion-control measures should integrate the results of site investigation with the predicted ground conditions after vegetation establishment (including soil properties, species choice and contaminant mobility) as well as current and future climate pressures. Future climate pressures predicted by the UK Climate Impacts Programme can be integrated into erosion modelling to forecast long-term average erosion rates and simulate the impact of vegetation establishment to ensure the most sustainable option is selected up to the 2080s.
Specific considerations in selecting suitable species for vegetation establishment include the use of stress-tolerant species (with reference to climate and soil quality) with a naturally high production yield, which can provide a quick and efficient ground cover. General approaches may combine the addition of soil amendments, the seeding of a grass/legume mixture and the planting of trees, but the exact methodologies will depend on the specific site conditions.
Case studies
Bassenthwaite Lake
When soils are subjected to water erosion, sediments are produced and transported off-site. This may pose a risk to surface water quality; the Water Framework Directive requires that surface water should achieve ‘good ecological and chemical status’ by 2015. A risk characterisation survey published in 2005 by Defra reported that 1233 watercourses (about 20%) in England and Wales were at risk of failing the environmental objectives set up in the Directive due to sediment movement as a result of water erosion processes.
Greenspace is one land-use option that has the potential to reduce soil erosion at source. A partnership between the Environment Agency, Defra, the Forestry Commission and Lancaster University has identified how conversion of land to woodland could improve the way the land is managed with respect to sediment production. This partnership of organisations developed a catchment approach to control sediment inputs to Bassenthwaite Lake (Cumbria, England). Although the results may not be directly transferable outside Bassenthwaite, A Guide to Using Woodland for Sediment Control (Nisbet et al., 2004) provides a useful framework for addressing the erosion and sediment issue in other catchments, including peri-urban and urban catchments.
Modelling impacts of climate change on sediment production and creation of greenspace on contaminated sites
Forest Research completed a modelling study that aimed to quantify the short- and long-term impacts of climate change on pollutant linkages through water erosion and creation of greenspace on contaminated sites. The objective was to assess whether current greening strategies would be able to mitigate pollutant linkages on contaminated sites in the context of climate change. Modelling studies can examine the effects of predicted climate changes on contaminant fate and transport without the time scale and cost associated with laboratory simulations.
Site
This modelling approach was applied on a neglected metalliferous mine particularly at risk from high contaminant movement via soil erosion processes. Former mine sites often have large quantities of mineral wastes stored in spoil heaps that often have inherently high concentrations of heavy metals. Such contaminants can be mobilised from spoil heaps by weathering and leaching, and can cause harm to vegetation and wildlife as well as reducing water quality, often beyond the site boundaries.
The spoil is completely devoid of vegetation and is highly contaminated with arsenic, lead and cadmium.
Climate
Two contrasting climate data sets were simulated for the site in order to compare results for the site in its actual location (south-west England) and in a contrasting climatic zone (south-east England). The respective climate data sets were based on the UK Climate Impacts Programme data sets.
Summary of results
- The greater the climate changes, the greater the erosion rates, although there is high regional variability (e.g. lower erosion rates predicted for the South-East than for the South-West).
- By the 2080s, soil erosion rates for the case study site will increase by up to 32% if the site is located in the South-West, and 6.6% if it is located in the South-East.
- Cover management options determine the scale of erosion rates and consequent pollutant linkages. A sustainable grass cover proved to lower erosion rates by a factor mainly depending on the grass production yield.
- Grass species with a natural high production yield should be encouraged.
- Under a realistic range of grass production yields, the impact of grass cover was large enough to fully compensate for the shifts due to changes in climate.
This work was funded by the Engineering and Physical Sciences Research Council through the SUBR:IM consortium.
Services
Forest Research has extensive experience of conducting research on reclaiming disturbed land for forestry, and can provide advice and recommendations on using vegetation to control erosion at a range of sites, including contaminated sites.
Forest Research has developed UK-specific tools to estimate storm erosivities for a site or a sub-region. We can provide erosion rates for specific sites both now and in the future. This is the basis for setting up sustainable erosion-control solutions.
A modelling approach can be applied to various types of site to forecast erosion rates under various climates and control practices, including vegetation establishment. In addition to its potential to simulate different greening solutions, the modelling approach enables integration of future climate change and indicates whether practices simulated for erosion control are sustainable in the long term (up to the 2080s). Forest Research also undertakes experimental research to assess the effects of various greening techniques on erosion control.
Coppin, N.J. and Richards, I.G., eds (1990). Use of Vegetation in Civil Engineering. Butterworths, London.
Dixon, T., Raco, M., Catney, P. and Lerner, D., eds (2007). Sustainable Brownfield Regeneration: liveable places from problem spaces. Blackwell, Oxford.
Nisbet, T., Orr, H. and Broadmeadow, S. (2004). A Guide to Using Woodland for Sediment Control. Forest Research, Farnham.
Philip, R., Morgan, C. and Rickson, R.J. (1995). Slope Stabilization and Erosion Control: A Bioengineering Approach. Taylor & Francis, Oxford.
POST (2006). UK Soil Degradation (PDF-164K). POSTnote No. 265. Parliamentary Office of Science and Technology, London.
