Forestry Commission logo

Woodland Design for Water

Planting Woodland for Water

Introduction

Appropriately located and designed new woodland can help reduce flood risk and/or diffuse water pollution. Trees can assist in the following ways:

  • by reducing water runoff;
  • by slowing the flow when rivers are in flood;
  • by protecting stream banks from erosion;
  • by protecting sensitive soils from erosion and reducing sediment delivery to water courses;
  • by intercepting sediment and pollutants in runoff;
  • by intercepting pesticide spray drift;
  • by reducing fertiliser and pesticide usage.

The propensity for trees to deliver these benefits depends on the location for woodland creation, which can be categorised as follows:

Wider catchment woodland - planting here can help reduce fertiliser and pesticide usage; protect sensitive soils from disturbance and erosion; increase infiltration and reduce water runoff; and intercept sediment and chemical pollutants in runoff, reducing the delivery of pollutants to watercourses.

Riparian woodland - planting along watercourses can act as a buffer between rivers and the adjacent land, intercepting and removing nutrient pollutants and sediment in runoff; providing a barrier to pesticide spray drift; protecting river banks from disturbance and erosion; slowing flood flows; and providing shade to reduce thermal stress to fish and other aquatic life.

Floodplain woodland - planting here can act as a partial barrier to a river when in flood.  This helps to slow flood flows and encourages the deposition of sediment and the retention of pollutants on the floodplain

Water Constraints for Woodland Creation

In some circumstances, woodland creation can have a negative impact on water resources and/or water quality. In particular parts of the country, the high water use of conifers and short rotation energy crops can threaten local water supplies and river flows, while the ability of woodland canopies to ‘scavenge’ acid pollutants from the atmosphere can exacerbate surface water acidification. Where the scale and type of woodland planting suggests this might be an issue, the FC will seek advice from the Environment Agency.

Wider Catchment Woodland

Priority Locations for Woodland Creation within Target Catchments

WHERE?

Planting will generally be located:

  • within groundwater and surface water Protection Zones;
  • on soils at high risk of erosion or leaching chemical pollutants;
  • where temporary surface water collects and flows during heavy rain;
  • on areas receiving runoff from hard standings, on infiltration basins and on sustainable rural and urban drainage systems; and
  • downslope of erosion or chemical pollutant sources.

Objectives and Design Principles

WHY?

Planting here can help reduce fertiliser and pesticide usage; protect sensitive soils from disturbance and erosion; increase infiltration and reduce water runoff; and intercept sediment and chemical pollutants in runoff, reducing the delivery of pollutants to watercourses.

HOW?

For maximum benefit, your planting should:

  • target pollutant sources and retention zones;
  • run parallel to the land contour where the woodland is designed to intercept pollutants draining from upslope areas; and
  • have the highest planting densities along runoff pathways;
  • include an open space towards the uphill edge of the planting to enhance the trapping of fine sediment where overland flow is an issue.

Countryside Stewardship Planting Requirements

STOCKING DENSITY

Minimum 1600 sph, average 2.5m spacing, though closer spacing across runoff pathways.

OPEN SPACE

Maximum 20% of the area where fully justified, but preferably less.

SPECIES

Productive broadleaf or conifer species will provide the greatest benefits for water. Avoid larger scale planting of conifers where acidification or water resources are an issue.

Riparian Woodland

Priority Locations for Woodland Creation within Target Catchments

WHERE?

Planting will generally be:

  • next to and within 30 m either side of watercourses;
  • targeted towards stretches of watercourse at risk of receiving sediment, nutrient pollutants or pesticide spray from adjacent land;
  • along reaches of watercourse vulnerable to bank erosion; and
  • along watercourses lacking shade and where fish are thought to be at risk from thermal stress.

Where appropriate and practicable, include the construction of large woody debris dams within the watercourse to improve channel structure and aid re-wetting of the riparian zone.

Objectives and Design Principles

WHY?

Planting along watercourses can act as a buffer between rivers and the adjacent land, intercepting and removing nutrient pollutants and sediment in runoff; providing a barrier to pesticide spray drift; protecting river banks from disturbance and erosion; increasing hydraulic roughness and slowing flood flows; and providing shade to reduce thermal stress to fish and other aquatic life.

HOW?

For maximum benefit your planting will:

  • provide continuous canopy cover along the length of the riparian zone, but allowing for light/dappled shade alongside the watercourse itself;
  • include open space towards the outer edge of the new planting to enhance the trapping of fine sediment where overland flow from adjacent land is an issue;
  • be at its widest and densest where overland flow discharges from the adjacent land, and extend to include areas of active erosion and unstable slopes where possible; and
  • extend right up to the edge of the watercourse where bank erosion is an issue.

Countryside Stewardship Planting Requirements

STOCKING DENSITY

Minimum 1600 sph, average 2.5m spacing, though closer spacing in the floodplain and where overland flow discharges from the adjacent land.

OPEN SPACE

Maximum 20% of the area, though preferably less, and located primarily towards the outer edge of the new woodland and on key areas of open habitat such as wetland flushes.

SPECIES

Predominantly native broadleaves, but other productive broadleaves and some conifers may be acceptable away from the water course (see UKFS Guidelines).

Floodplain Woodland

Priority Locations for Woodland Creation within Target Catchments

WHERE?

Where possible, planting will generally occupy a significant part of the width of one or preferably both sides of the floodplain.

Planting should avoid areas:

  • where flood flows are controlled/back-up by existing restrictions such as bridges and culverts, particularly where these are vulnerable to blockage;
  • alongside stretches of main river with engineered flood defence banks;
  • where the backing-up of floodwaters could threaten local properties; and
  • within ‘washlands’.

Objectives and Design Principles

WHY?

Planting here can increase hydraulic roughness which helps to slow flood flows and encourages the deposition of sediment and the retention of pollutants on the floodplain.

HOW?

For maximum benefit, your planting will:

  • involve random spacing but, if in rows, the rows will be offset and aligned perpendicular to the flow of water in order to slow the flow;
  • reduce to 1.0 m spacing across the lowest lying/wettest parts of the floodplain and along the downstream edge of the planting to increase low level roughness and temporary flood storage; and
  • have open space that will be concentrated on the higher/drier parts of the site.

Countryside Stewardship Planting Requirements

STOCKING DENSITY

Minimum 2250 stems per hectare, average 2.1m spacing, though closer (down to 1.0m) on the lower lying parts of the floodplain and along downstream edge.

OPEN SPACE

Maximum of 20% of the area, but preferably less than this.

SPECIES

Predominantly native broadleaves adjacent to watercourses (see UKFS Guidelines) and on lowest lying/wettest areas. Productive broadleaves or conifers elsewhere, especially on higher/drier parts of the site.

 Printable format - Design Principles

Last updated: 19th February 2016