By Fran Southgate
Living Landscapes Advisor
Last week we posted a blog about our working washland near Plumpton. One week on and our working washland is complete. The weather has obliged us by torrentially raining for a few days, filling it with water, so that we can show you exactly how it works.
When you look at the site, you would never imagine that such a small, innocuous valley could make such a huge contribution to reducing flooding locally, and to biodiversity. But witnessing just one of these projects in action makes you realise just how easy it would be to capture and store millions more litres of water in our countryside, in flood storage that would benefit people and wildlife.
At the final count, the additional flood water storage water that the site creates is a whopping 350,000 litres (350 tonnes) of water, and that’s BEFORE the site floods! Once it floods, the flood storage in the valley could be anything from around 350,000 litres to 1,350,000 litres of extra flood storage, depending on the height of the flood. This flood water is slowed down and spread out across the land, allowing silt and other debris to drop out of the flood, making it a much less dangerous flood peak, benefitting soil fertility and reducing pollution and siltation in our rivers.
When we started this project, we thought that the majority of the water flowing into the site would be flood overflow from an adjacent stream. What has happened instead is that it has filled up rapidly from rainfall running off the surrounding grassland slopes.
This is particularly interesting. In a natural meadow like this, it is widely accepted that the rough grassland will slow down rainfall enough to enable most of it to filter into the soil and away into aquifers rather than flowing over the land surface. However, the intensity of the rainfall meant that instead, there was very obviously a lot of overland surface water flooding filling up the new washland, way before any actual river flooding took place. The soil on the site is some of the best I’ve seen, so it is not soil compaction creating this flood run off, it is simply the volume of water falling from the sky onto a slope in a short space of time.
So our new washland is helping to store both river and ‘land’ flooding, and in doing so is creating some really nice restored wetland habitat. Seeing so much water appear in the washland overnight was fantastic. I can’t wait to see what wildlife shows up next spring.