Leaky woody barriers are constructed barriers made from woody material across a watercourse to slow flow during times of heavy rainfall and to hold more water back in the environment. The aim is to replicate the natural process of trees falling across brooks over time and to also mimic the behaviour of beavers. Beavers dam watercourses to create wetlands and deeper aquatic habitats for their lodges and for the transport of materials. This provides protection from predators and access to food resources from felled trees for their herbivorous diet. Fish and beavers have co-evolved over time and fish can navigate beaver dams successfully. High flows are best for passage, but beaver dams are not solid or regular in form and can degrade which still allows passage opportunities during low flows. With a lack of an extensive beaver population across the UK, human intervention via constructed leaky barriers can deliver the benefits of damming in areas where beavers are absent. 

Leaky woody barriers provide many benefits for the environment and help to mitigate the impacts of climate change:

Flooding: Ever increasing erratic weather events with heavy rainfall in short periods of time, means flood risk increases. Leaky barriers act as a Natural Flood Management technique to slow the flow of water and hold more water back in the environment which delays rainfall reaching larger tributaries and towns too quickly. This can contribute to spreading out a flood event in time, and reduces the peak flood height. The Environment Agency’s Working with Natural Processes Evidence Directory provides references to research and case studies across the country supporting leaky barriers having a positive flood risk management effect by reducing flood magnitude.

Water quality: Water quality is benefitted as debris and rough sediment transport can be intercepted and drop out of the water column which reduces impacts downstream. Barriers can also trap fine sediments and provide a nutrient uptake service; in one case study in Merseyside, leaky barriers reduced the average phosphate concentration by 3.6mg per litre as flows filtered through natural dams.

Habitat and biodiversity: Leaky barriers increase habitat diversity in the water environment by creating slower flowing areas, wider wetlands above barriers and areas with cascades, pools and interesting features. This variety benefits multiple different species of aquatic vegetation and invertebrates which form the base of the food chain for other animals such as the white-clawed crayfish (Austropotamobius pallipes), otters (Lutra lutra) and dippers (Cinclus cinclus). Slower flowing areas are likely to build up sediment and silt deposits which in turn can encourage marginal plants to colonise. Slower flowing areas with marginal plants may also attract species of aquatic macroinvertebrate such as demoiselle damselfly larva (Calopterygidae) that have slender bodies and are not adapted to fast flowing water. Similarly, freshwater worms (Oligachaeta) prefer slower flows and burrow into fine sediment deposits where they can survive in low oxygen conditions. Where faster flows are maintained and riffles are present, you would be more likely to find species adapted to high oxygen levels, filter feeders and grazers; Net-spinning caddisfly larvae (Hydropsychidae), blue-winged olive mayfly larvae (Ephermerellidae), blackfly larvae (Simuliidae) and stonefly larvae (Plecoptera). Fish also benefit from varied habitat types; leaky barriers can provide shelter from high flows and predators, shade, feeding opportunities, spawning and nursery sites for juveniles. Leaky barriers also provide habitat diversity out of the water such as perching sites and foraging resources for reptiles, amphibians, birds and crossing points for other mammals. In one case study from the Working with Natural Processes Evidence Directory, wood placement in a stream in the New Forest increased biodiversity by 46%.

Climate resilience: In terms of climate resilience, not only do barriers benefit communities downstream in times of heavy rainfall, but they also help in times of drought and high summer temperatures. Leaky woody barriers regulate water temperature and water levels and provide refuge pools for species during times of low flow. Holding more water in the environment for longer builds resilience to extended periods of dry weather.

Leaky woody barriers are constructed using either felled or coppiced trees, or using already fallen logs and branches. The do not aim to fully dam watercourses, but to intercept high flows, so the natural summer level is always enabled to flow and allow species to move up and downstream. There are many different construction techniques;

Substantial barriers: This technique uses large woody material perpendicular to the watercourse. This large material can be supported by existing bankside trees to prevent movement downstream. Where there are no bankside trees present, heavy and long lengths can be used where two-thirds of the length are outside of the watercourse to anchor in the environment. Stakes can also be used, but for a natural feature, it is best to use what is already present in the environment; this greatly dictates where the barriers are best to be constructed. 

Gully stuffing and brash bundles: This method uses smaller woody items packed tightly, or anchored into the bank. This is more suited to channels where the sides are steep and larger material would be difficult to anchor.

Pleaching: Pleaching is a method where a live tree is identified on the bankside and partially felled at an angle so it falls upstream into the channel. When the cut is made, it does not fully go through the central live core of the tree, so it remains alive and can regenerate at the base. This connected section provides the anchor for the tree and prevents movement downstream. 

Live willow: Willow can be coppiced and placed into the watercourse to create a live barrier which will grow over time. This method is long lasting and also provides vertical habitat.

It is important to use a variety of different methods over the length of a watercourse to vary the effects on flow and to develop complex micro-habitats throughout. A variety of methods also reduces risk of any one methodology failing and the impact accumulating along the length of a water course. The critical risk of any downstream feature or habitat is always assessed before implementing a barrier. This to ensure placement and potential failures will have no adverse effects on roads, culverts, houses, and agricultural land. At the end of a section of barriers, a terminal feature (substantial barrier) will also prevent impact further downstream if washout were to happen. 

Leaky barriers are best suited in the headwaters of tributaries where flow energy is less so the risk of washing out is reduced. Commonly in the headwaters, the adjacent environment and habitats present will allow space for water to spread out without impacting agricultural land or housing. Targeting actions in the headwaters also reduces water and debris accumulation further downstream in main rivers and any action at the top of a catchment provides benefits downstream in terms of flood risk.

In 2023 HWT began a river restoration project in the upper reaches of the Lime Brook in Lingen. Twelve leaky barriers were created over two different headwater tributaries. The pictures above are barriers during low /normal flow and the same barrier after heavy rainfall. 

From 2024, Herefordshire Wildlife Trust has been working on the Wye Adapt to Climate Change? project which has installed leaky barriers in upper tributaries of the Wellington Brook and Yazor Brook catchments. 208 leaky woody barriers have been installed in the upper Yazor Brook over 2.5km. Monitoring of the success of these interventions will come from observation post storm events, water quality testing, riverfly monitoring, river naturalness surveys pre and post interventions and individual fixed-point photography posts to assess habitat change and leaky woody barrier development over time. 

The National Lottery Climate Action Fund, the Environment Agency and Welsh Water have funded this work, delivered through a number of Trust projects across Herefordshire.