Natural flood management methods

Using wetlands to reduce flooding naturally

With climate change likely to bring more frequent and intense flooding for the UK, it’s recognised that traditional defences like concrete dams, embankments and walls are no longer going to be enough to protect us. We need alternative solutions.

WWT is working to develop a more sustainable and efficient way to manage flooding using wetlands. Natural flood management, or NFM, is an approach that uses natural features in the landscape like ponds, floodplains and wet woodlands. Some of the measures are designed to slow down and stop rain water accumulating and flowing across the surface, others act upstream to slow down the water flow and reduce the peak rainfall surge.

This approach works in three main ways. Firstly, features like ponds help create temporary storage which will fill up during a flood and then empty slowly. Secondly, NFM helps slow the flow by increasing the resistance to surface and in channel water flow, for example by planting trees and hedgerows and creating woody dams. Thirdly NFM features work by increasing water losses, either by encouraging more water to drain into the ground or be lost back into the atmosphere.

WWT’s ‘Two Valleys’ project is working in Somerset, in the Monksilver and Doniford Catchment with local landowners. We have:

• Installed over one thousand metres of hedgerow
• 35 metres of hedge bank
• Planted more than one thousand trees
• Created three wet woodlands with ponds
• Built 91 woody dams
• Downstream we have introduced over a hundred new measures such as cross drains, scrapes and bunds

Different natural flood management interventions and how they work

Hedgerows and hedge banks

Hedgerows create a physical barrier to surface water runoff from the land. By reducing the speed at which water moves across the surface they give it more time to soak into the soil. This slowing down of surface water runoff also reduces the effect of soil erosion, allowing any transported sediment to settle out. Hedge banks are raised up hedges with built-in overflow pipes that increase the physical obstruction to water movement. During really intense rainfall the overflow pipes divert excess water and reduce the likelihood of bank collapse if water accumulation is extreme. Hedgerows have the indirect benefit of acting as wildlife corridors letting birds and small mammals move between habitats.

Woodlands

Woodlands help in a number of ways. They intercept rainfall and use it directly (a process called evapotranspiration). Their roots also help the water filter down and soak into the soil. Woodlands can also slow surface runoff and reduce sediment transport down hillslopes by increasing the resistance to flow. Woodlands protect the soil from disturbance and improve soil structure through the action of tree roots and high inputs of organic matter. Mature woodlands enhance soil infiltration and water storage capacity which can reduce surface run-off, soil erosion and sediment transport.

Woody dams

Woody or leaky dams mimic the natural obstruction caused by trees and branches falling into a river and they work brilliantly at slowing down water flow. Normally water can pass through or under them, but during heavy rainfall when water levels rise, they become physical barriers, forcing flood waters to spill out on to the surrounding banks. When this happens the water is slowed down, allowing more of it to soak away or ‘infiltrate’ into the ground. This slowed down and infiltrating water takes longer to reach downstream communities, reducing the flood peak and associated devastation.

Wet woodlands and ponds

Wet woodlands, or carrs, are now some of our rarest wooded habitats. You can find them growing alongside streams and rivers where they become wetted when nearby streams flood. As well as providing the benefits of a regular woodland (see above), because they’re boggy and ephemeral they also allow different trees like alder, willow and birch to thrive, along with the ferns and mosses that flourish below. They can happily exist as small enclaves within larger, drier woodlands, providing a sink for transported nutrients and sediments to be deposited and temporarily holding high water flow. Ponds become the land’s water storage tanks.

Scrapes and bunds

Scrapes and bunds help by temporarily storing water, so increasing the flood plain storage capacity and can additionally act as silt traps to reducing the effects of soil erosion and allowing the redistribution of valuable top soil.