Management and use of large wood

Description of measure

Large wood refers to branches, trunks, root boles or entire trees that fall into the bankfull channel. Fallen trees and branches are a natural part of tree-lined river ecosystems and provide morphological diversity, localised flow diversity and areas of refuge within the channel.  Wood accumulations affect the geomorphic processes of a river system which can impact sediment storage and routing, stream bed and bank structure, velocity distributions, and sinuosity of a stream.

Application

Flood risk management has historically consisted of the complete removal of large wood from the channel to prevent the risk of blockage to maintain conveyance. Increasingly however, working with natural processes can work to use in-channel wood and debris dams to hold back flows. A management approach which takes account of flood risk policies will allow wood to be retained in the channel.

Large wood on the Cod Beck, North Yorkshire

Benefits for Water Framework Directive

Large wood can help deliver objectives of the WFD by:

  • Locally removing fine silt from the system by creating silt ’benches’ immediately upstream.This process helps to maintain faster areas of flow prevent gravels from becoming silted over, and so providing a benefit to spawning fish and macrophytes that root in coarser sediments.
  • Providing variable substrate that can form habitat for macrophytes and invertebrates and fish.
  • Introducing variability to the flow and morphology of the channel.

Emergent macrophytes growing on woody debris

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