New Environment Agency policy on woody debris in rivers

The Environment Agency have recently updated their guidance on woody debris in rivers. After many years of lobbying for a change in policy this is seen as a significant step forward in the sustainable approach to river management by organisations like OnTrent. The policy will aim to promote a better understanding of the benefits of woody debris and encourage its retention and positive use where appropriate as part of working with natural processes. It will ensure the Environment Agency take a balanced and nationally consistent approach to managing riparian trees and woody debris when maintaining assets, restoring rivers, working with partners and advising others (e.g. internal drainage boards, landowners, local authorities, rivers trusts and fishing organisations).

What is woody debris? Woody debris are branches, large limbs, root boles or entire trees that have fallen into rivers are commonly referred to as large woody debris (LWD). Accumulations of smaller branches, twigs and leaf litter are generally known as coarse woody debris (CWD). This policy covers both naturally occurring woody debris and that which is deliberately placed into river channels to restore morphological diversity.

The policy statement from the Environment Agency is summarised below:

• We recognise that woody debris can be beneficial to rivers because it helps to vary the flow and shape of the channel, creating physical habitat for many species of plants, invertebrates and fish. It can improve the resilience of river ecosystems to the impacts of climate change.
• Where we can show that flood risk to people and property is not increased, we will retain woody debris in rivers and streams when undertaking river management. We will promote its use in river restoration projects, as a rapid and cost-effective method for creating or restoring morphological diversity as required under the Water Framework Directive (WFD).
• In catchments with very little tree cover and those vulnerable to increased water temperatures, we will encourage a patchy mosaic of tree and shrub regeneration to provide the debris of the future, where this is compatible with other river uses including flood risk management.
• Good communications with landowners and river users will be essential to the successful implementation of this policy.

Introducing woody debris into river channels brings the following benefits:

•  it restores rivers to good ecological status or potential;
•  it can be locally sourced, thereby reducing costs and decreasing our carbon footprint;
•  it creates lower water velocity upstream and encourages deposition of fine sediments in marginal zones, ideal habitat for juvenile lamprey and emergent vegetation important for fly life;
•  it creates scouring of gravel substrate downstream, improving the habitat for spawning salmon, trout and lamprey, and for Ranunculus and Callitriche plant communities;
•  it gives shelter for fish, and the accumulated leaf litter is an important food reserve for shredding macro-invertebrates;
• it slows flows and provides fish refuges during flooding, whilst the scour pools and areas of slack water are important during drought. The importance of woody debris to regulate flow is likely to increase as a result of climate change, with greater flow variability predicted from lower summer flows to more rapid and extreme floods;
• scattered trees and shrubs help prevent bank erosion and reduce water temperature through dappled shading, whilst large debris dams can slow flows and help to delay flood peaks, thereby reducing flood risk. Trapped sediments provide germination zones for willow, alder and birch scrub along river corridors.

One important aspect of this policy statement will be how the Environment Agency manage risk and perception. Woody debris can increase flood risk in critical locations, for example upstream of culverts or multi-arched bridges in channels with high velocity flows. Any proposal to retain or introduce woody debris in high risk river systems would need particularly careful consideration.

The introduction of this policy statement should herald the beginning of a new era of river management. OnTrent are excited about the new possibilities across the River Trent catchment.