Building Large Woody Debris Dams

We are working in collaboration with Cumbria County Council (upstream of Cumrew and Gamblesby) and the Environment Agency (upstream of Stockdalewath in the Roe and Ive) on a pilot project to implement Natural Flood Management (NFM) options to slow the flow of water upstream of these locations.

NFM encompasses a whole suite of potential measures for slowing the flow, one of which is the creation of Large Woody Debris dams.

What are Large Woody Debris Dams?

A number of different names are used to describe this natural flood management technique, including: large woody debris, leaky dams, log jams, etc. However, the basic technique is the same.

Woody debris (such as trees or branches) are placed in the watercourse. This aims to mimic the natural process of trees or branches falling into a water course and causing a local obstruction.

This can range from simply a single large branch, up to several large trees linked together to form a substantial obstruction. Large woody debris can provide a wide range of positive effects including stabilising river banks and beds, increasing floodwater storage, improving water quality, providing habitat for fish and storing carbon.

These leaky dams contribute to flood management by creating a partial blockage in the water course during high flows, causing water levels to rise locally.
This can help reduce flooding downstream by:

- Storing water for longer upstream,

- Slowing down the flow of water,

- Encouraging local ‘out of bank flows’ where the water stored behind the large woody debris is slowly released back into the water course, further helping to even out the flow and reduce peak flows.

Large woody debris should not impinge on the ability of fish or eels to pass along the water course, nor does it affect flow rates under normal (i.e. non-flood) conditions.

Due to the relatively small volumes stored and slowed by each dam, the more of these structures are installed, the greater the potential positive impact.

Timescales
This is an ongoing project that started this summer. We will finish installing dams for this year at the end of September, but will recommence in Spring/Summer 2017.

Find out more
If you'd like to find out more about how Large Woody Debris dams work, these reports make interesting reading:

The River Restoration Centre, “Managing Woody Debris in Rivers, Streams & Floodplains” 

Forest Research (2007), "The Robinwood Robinflood report: Evaluation of Large Woody Debris in Watercourses"