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A Guide to Using Woodlands for Sediment Control

Date: 2004

Evidence type: Research

Organisations: Forest Research, Department for Environment Food and Rural Affairs (DEFRA), Environment Agency, Forestry Commission, Lancaster University

Author(s): Nisbet, T., Orr, H. & Broadmeadow, S.

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Description

Integrated land use planning and management can resolve sediment problems, woodland especially can provide an effective solution. It is widely acknowledged that soils under woodland are generally well protected and are often improved. The development and implementation of best practice as exemplified by the Forestry Commission's Forests & water guidelines means that the planting and establishment of new woodland offers an effective approach to reducing sediment losses in problem areas. Woodland has been shown to benefit sediment control in the following ways:

 

  • By providing physical shelter from the wind
  • By reducing water run-off
  • By increasing the entry of rainwater into the soil
  • By improving soil strength and stability

Woodland is one land use option that has the potential to reduce soil erosion at source, to limit the delivery of sediment to watercourses, to protect river banks from erosion and to encourage sediment deposition within the floodplain. This guide describes the catchment approach that is being developed by a partnership of organisations in Cumbria in northwest England to control sediment inputs to Bassenthwaite Lake. Although the results may not be directly transferable outside of Bassenthwaite, the guide provides a useful framework for addressing the sediment issue in other catchments.

 

Bassenthwaite Lake lies in the Lake District National Park and is of high conservation importance. This makes it very vulnerable to disturbance and highly sensitive to pollution. A key issue is the high level of soil erosion in the catchment caused by a range of pressures, including overgrazing, land cultivation, drainage and human trampling.

Key Messages

Less than 1% of the observed bare ground was associated with woodland, confirming the advantages of woodland for soil protection and sediment control.

Green infrastructure type
Woodland
Climate change role/function
Reducing soil erosion

Several woodland options for sediment control are discussed.

These included:  

- Large-scale woodland planting on soils classed as having a high or medium vulnerability to erosion.  

- Targeted planting of woodland on and immediately around areas of bare ground (particularly downslope to retain mobilised sediment).  

- Targeted planting of riparian woodland along river reaches with a medium or high risk of bank erosion, especially on those that are actively eroding.  

- Medium-scale planting and restoration of floodplain woodland around the confluence of major tributaries and the main inflows into Bassenthwaite Lake.

Green infrastructure type
Woodland; Water courses
Climate change role/function
Managing riverine flooding; Reducing soil erosion

Another important consideration was whether woodland was appropriate for a given location. This required an assessment of a range of possible constraints.

- Areas designated for their special landscape character. All of the catchment lay within the Lake District National Park, placing restrictions on the location, scale and type of new woodland planting.  

- Sites designated for their special conservation interest as an open habitat, e.g. SSSIs, NNRs, SPAs and SACs. A large part of the catchment of Bassenthwaite Lake is a candidate SAC on the basis of the high ecological value of the predominantly upland grassland and heath vegetation (Lake District High Fells).This occupies some 66% of the area classed as being highly vulnerable to erosion. A further 7% of the latter area is designated as non-woodland SSSIs. However, there could be significant scope for riparian woodland along upland valleys/ghylls.  

- Existing woodland, which occupied 12% of the catchment. It is notable that only 1% of the high soil erosion vulnerability class is under woodland. Similarly, only 12% of the river length identified as highly vulnerable to bank erosion is occupied by riparian woodland.  

- High ground above the natural tree line (29% of catchment >450 m), although there may still be a role for a dwarf tree cover such as juniper and some riparian trees/scrub woodland.  

- Areas of Common Land, except for juniper and riparian woodland.  

- Sites of special archaeological interest, e.g. Scheduled Ancient Monuments.  

- Riparian areas where access is required for river maintenance and flood management.

Green infrastructure type
Woodland
Climate change role/function
Managing riverine flooding; Reducing soil erosion

Identification of where new woodland could best aid sediment control.

The end result was the identification of the following opportunities for woodland planting to assist sediment control:  

- 95 ha of land below 450 m with a high vulnerability to erosion, mainly comprising two areas in the central and southern part of the catchment with some potential for woodland extension.  

- 37 km of river length with a high vulnerability to bank erosion. Assuming that buffer zones of 10, 20 and 40 m width would be appropriate for protecting the banks of first, second and third order stream channels, respectively, this presented a total area of 223 ha of land that would benefit from the planting of riparian woodland. There was a low potential to build on existing riparian woodland.

Green infrastructure type
Woodland; Water courses
Climate change role/function
Managing riverine flooding; Reducing soil erosion

A key need is to improve the synergy between woodland and agricultural grants as well as advisory services.

The establishment of demonstration woodlands is being considered as a way of communicating to local landowners the advantages for sediment control and conserving the soil resource.

Green infrastructure type
Woodland; Agricultural land
Climate change role/function
Reducing soil erosion
Document Analysis

Complete

Level of document
National
Geographical area to which document refers
United Kingdom
Is 'green infrastructure' mentioned?
No
Relevant to climate change...
Adaptation
Is it relevant to other (non-climate change) benefits of green infrastructure?
Yes
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