Woody debris dams to slow the flow of water
Leaky dams—also known as large woody debris dams or leaky barriers—are a simple yet powerful tool in the world of natural flood management (NFM). These structures work with nature, not against it, helping to reduce flood risk while enhancing river habitats.
Leaky dams mimic the natural blockage caused by fallen trees and branches in rivers. They can be as simple as a single branch or as complex as several tree trunks secured together.
In the Eden catchment, we favour a few different types such as:
Large woody debris dams—whole tree trunks sourced locally and placed across small streams. These are anchored with stakes and wire, set above the normal water level so fish and regular flow aren’t disrupted,
Beaver-inspired dams – taking our cue from these wonderful escosystem engineers, we lay lots of smaller branches and tree trunks across the stream and build up the height. These don’t have a gap underneath them for water to flow freely, instead the water percolates through the branches.
True to their name, leaky dams don’t block water completely. Instead, they slow it down, allowing only a limited amount to pass through at a time. This helps reduce flood peaks during heavy rainfall.
Leaky dams are best suited to small watercourses less than 2 metres wide. That’s why we focus on installing them in upland becks and streams, where they can make the biggest impact.
They’re also cost-effective and low-maintenance. Occasionally, they need to be cleared of debris and sediment to keep water flowing through the gaps—preventing overflow and maintaining their effectiveness.
Leaky dams offer more than just flood protection:
We’ve even trialled placing horizontal tree trunks on hillsides to intercept storm runoff before it reaches the stream—slowing water higher up the catchment.
