Slow The Flow has long been convinced that Leaky Woody Dams are a great way to help reduce the speed with which water collects downstream, reducing the flood peak in the valley bottom. Results from our own monitoring project at Hardcastle Crags can be seen at https://slowtheflow.net/scientific-monitoring-suggests-leaky-dams-are-a-success-at-hardcastle-crags/.
The evidence base continues to build; this guest blog post from Joshua Little reflects on his PhD research at Cardiff University:
When Storm Dennis tore through South Wales in 2020, it left a trail of devastation in its wake. Many communities in upland valleys faced severe flooding – and it wasn’t just the big rivers that caused it. Small, steep streams, often overlooked, were among the main culprits. These headwaters, where rivers begin, hold huge potential for reducing flood risk if we work with nature rather than against it.
My PhD research at Cardiff University focuses on one nature-based solution that’s gaining traction: using leaky wooden barriers in small headwater streams. These simple structures can help slow the flow of water and trap sediment before it reaches vulnerable areas downstream. But how effective are they really – and what should we consider when installing them?
Why focus on Headwaters?
Headwater streams are the capillaries of our river systems. In England and Wales alone, first- and second-order streams make up 126,338 km – an incredible 73.4% of the entire river network. If we want to manage flooding at the landscape scale, it makes sense to start where the majority of water begins its journey.
Slowing the flow at the source not only helps reduce peak flows further downstream, but also spreads out the energy of storm events – making flood events less intense and more manageable.
What Are Leaky Barriers?
Leaky barriers – also known as woody debris structures or natural dams – are made from logs, branches, or brushwood placed across streams. They’re designed to let water pass through, but more slowly. Often installed by hand using locally sourced materials, they mimic natural wood accumulations in rivers, offering both hydrological and ecological benefits.
Their low cost, low carbon footprint, and compatibility with natural processes make them an ideal solution for rural catchments looking to build resilience without expensive infrastructure.
The image below shows a barrier in a South Wales headwater stream during Storm Dudley. This site has been central to my research, which tracks how barriers respond to storms and seasonal changes over time. The picture of the barrier on the left, for instance, has a full backwater, reducing flow speed and increasing storage behind it. On the right, we’re using field-based hydrometry to measure how much this barrier has slowed the water down. Both pictures demonstrate the great potential of in-stream wood to reconnect the floodplain.
While these examples show good design complexity, they also highlight the need for thoughtful placement. Less frequent spacing of larger barriers may hold back more water, but if one fails, it could cause a surge downstream. A better approach is using smaller, more frequent interventions to slow the flow in increments – more safely and effectively.
Slowing the Flow – But How?
The principle is simple: interrupt the flow with roughness, reduce the velocity. But we wanted to understand how much impact these barriers really have. By combining field data and lab flume tests to measure how leaky barriers reduce water momentum – essentially, we can predict how much they resist flow during different conditions.
The results of using leaky barriers for momentum reduction are promising. Especially during low to moderate flow events, we’re seeing measurable reductions in velocity and momentum. Over time, this helps flatten the flood hydrograph – lowering peak flows and extending the duration, which reduces downstream flood risk.
Beyond Floods: Capturing Sediment
Another major benefit is sediment capture. During storm events, headwater streams can transport large amounts of sediment downstream, where it can clog culverts, degrade habitats, and pollute rivers.
Leaky barriers can act like mini sediment traps. Over time, material accumulates behind them – stabilising streambeds, reconnecting floodplains, and even supporting plant growth.
The two images below show a barrier in the Brecon Beacons that filled completely with sediment over just three months. From a natural processes perspective, this is a great outcome. It forces water to be spread laterally, creating new wetland habitat. These barriers can boost biodiversity while also reducing downstream flood impacts.
That said, not all landowners want water spreading across fields or paths. In these cases, adding a small notch or low-flow path can keep water moving through while still delivering the core benefits.
Looking Ahead
As I near the end of my PhD, I’m more convinced than ever that natural flood management isn’t just good science – it’s common sense. Leaky barriers won’t stop floods entirely, but they can play a vital role in building more resilient landscapes.
We need to think beyond engineering. Slowing the flow is about more than wood in streams – it’s about a mindset shift. One that embraces complexity, uncertainty, and the idea that working with nature can be just as effective, if not more so, than resisting it.
If you’re a practitioner, policymaker, landowner, or just someone who wants to make a difference, I’d love to hear from you. Feel free to reach out or follow my work through Cardiff University’s Hydro-Environmental Research Centre.
By Joshua Little, PhD Researcher, Cardiff University
