Slowing the flow – Increasing flood- plain diversity to reduce flood risk

There are a number of measures that can be done to reduce the impact of flooding. Replacing multi-arched bridges and improving flood defences in urban areas,and property level measures. Looking outside of urban areas, there is a lot that can also be done in our countryside to slow the flow of water, reducing flood peaks in those urban areas.

Looking specifically at flood plains, increasing the diversity of these can help to slow the flow of water across them when the river does flood out. A floodplain entirely comprised of just of short grazed grassland does not provide much ‘roughness’, and it is this ‘roughness’ – be it large boulders, scrub, trees, walls or hedges – which can slow the speed of the water down as it flows over the floodplain. So, increasing the complexity of the floodplain can help reduce the speed at which flood water hits our communities, and the amount of debris is carries. The best way to do this would be to plant swathes of woodland across our valley bottom land – but this is an unrealistic option – our floodplains in Cumbria are some of our prime agricultural land.

What we can do though is think about our floodplains as a patchwork quilt, the more different patches there are, the slower the water will move and the less debris it will carry. Increasing the numbers of individual trees, patches of woodland and thick hedges with the right management plans in place is encouraged as much as possible, not just for slowing down water, but for their many other benefits.

Amongst this, is where Short Rotation willow cropping can fit in, creating another different ‘patch’ on that patchwork quilt of a floodplain. It is still a crop, providing a regular income source for farmers; planted once in 25 years it can withstand flooding and help to slow the flow of water. It is very quick to plant the crop and can be established within 6 months. The SRC Willow crop at Keswick that was flooded was only 1 year old and is still intact. Environment Agency Energy crops and floodplain flows report Project Summary SC060092/ S2 concluded that the dense nature of the mature SRC Willow plantation acted like a ‘green leaky dam’ to hold water back within and immediately upstream of the energy crop plantation, and to slow the speed of water spread across the floodplain. Energy crops ability to increase hydraulic roughness is key here.

Only one year old SRC willow stems undamaged after a significant flood in December 2015.