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Wednesday, 26 February 2020 14:06

Naturally designed 'leaky dams' prevent flooding

A natural "leaky dams" flood management scheme, which was only installed in Tudeley Woods near Tunbridge Wells, Southern England in January this year, has functioned effectively during extreme weather conditions in Kent caused by Storms Ciara and Dennis which swept across the UK in February, according to the South East Rivers Trust (SERT). In partnership with local landowner The Hadlow Estate, the scheme has been introduced to protect properties in Five Oak Green, downstream of the Alder Stream catchment, from future flooding.

"As a child, I grew up in a cottage in the woods here," says Harry Teacher, whose family has owned the 900 acres of Tudeley Woods since the 1840s. "It's important to me that we do our best to protect and preserve this beautiful place."

Tudeley Woods Leaky dams 3And that is why visitors to the woods, which are run as an RSPB nature reserve, are finding that the streams here are slowed by a set of intricately-constructed "leaky dams". In total, there are 50 such structures, as Harry explains: "It's all part of an innovative project to help tackle flooding in nearby homes with the addition of fallen timber in the streams, to slow the flow of water."

Harry and his wife Kate head up the 4,000-acre Hadlow Estate, which came into Harry's family when it was first owned by his ancestor Sir Isaac Goldsmid, more than 170 years ago. It was Harry's father James, himself an eminent conservationist, who first invited the RSPB into Tudeley Woods in 1987.

The Natural Flood Management Project, as it is known, has been carried out by the Hadlow Estate in partnership with the South East Rivers Trust (SERT), a local environmental charity bringing rivers back to life across the South East of England. The project has been funded by the European Union Interreg North Sea FRAMES project, Defra and the Environment Agency, and is one of a programme of Natural Flood Management projects across the River Medway catchment.

Dean Morrison, the trust's Natural Flood Management Project Officer explains: "What we have found is that, in heavy rain, water rushes down from the headwaters and rapidly increases the flow in rivers and flood peaks downstream. Within a short period of time, this huge increase in water volume can pose a serious flood threat to property and people".

Tudeley Woods Leaky dams 1To this end, the steep-sided streams in Tudeley Woods – or ghylls, as they are known locally – now have logs jammed across them. Over time, these will catch sticks and leaves to become what are known in the conservation world as "leaky wooden structures" or simply leaky dams. The project has been carried out in December 2019 and January 2020.

The streams in Tudeley Woods stem from natural springs within the woods and feed into the Alder Stream, a tributary of the River Medway with a history of flooding. The project aims to help protect over 50 residential properties at risk from the Alder Stream in the villages of Capel and Five Oak Green.

The dams were put to the test in February, just a few weeks after the project was completed. Although some of the properties were flooded after sudden torrential rain on February 9th, during Storm Ciara, the cause was hillside run-off, rather than flooding from the Alder Stream. "Whilst it's too early to claim that the number of leaky woody structures we've put in have had a significant impact yet, we checked on them during the storms and they're functioning as they're designed to," explains Dean.

Tudeley Woods hadlow estate"It's all the more important that we work with nature in our water management, given the changing climate in recent years. We have seen very wet winters, this last one being a case in point. There is a lot we can do in a low-tech, nature-friendly way to mitigate this change in terms of flooding."

The benefits of the project will also be felt within the woods themselves, says Harry's wife Kate, who has been working on the scheme for some time. "The idea of using our upland woodland for natural flood management has been under consideration for a couple of years. We first commissioned a survey of the woods, including the streams and a large central pond, to see exactly what was happening. We discovered that despite the wet winters, the warmer summers have meant that the woods are drying out year-on-year, which can have serious consequences for biodiversity," Kate says.

Tudeley Woods butterflies"Tudeley Woods are a remnant of ancient woodland that once covered most of South East England. The woods are home to nearly 1200 varieties of fungi as well as rare birds and plants. There's the nationally-rare Marsh Valerian plant as well as many butterflies and orchids. And in terms of birds you could see tree pipits, nightjars, woodlarks and Britain's rarest woodpecker, the Lesser Spotted Woodpecker."

The retention of water in the woods, preventing it from rushing away downstream, is very important for preserving habitats, says Harry. "We are using natural methods to increase resilience to climate change. Leaky dams are an effective way of preventing flash flooding further downstream but we are also working to prevent the woods from drying out, preserving the biodiversity of the flora and fauna here. It's a win-win situation."

To this end, the large pond, known as the Decoy Pond, in the centre of the woods will be restored. "It has dried out quite significantly," says Harry, "So we will be coppicing the invading willow and repairing the dam which holds water in the pond. If we don't take action it is likely that Decoy Pond could dry out altogether, causing a loss of habitat. The pond will also act as a store for flood waters."

Tudeley Woods rspb signThe Natural Flood Management project is the first of its kind in Kent and was inspired by similar schemes including Pickering, Hebden Bridge and Stroud.

In time, the dams will blend into the landscape as natural debris such as sticks and leaves becomes caught up in the logs. In Tudeley Woods, much of the timber used for the dams had fallen naturally within the woods, with other logs being felled as part of routine tree management. "Each of the dams has been built to a slightly different design, depending on the depth of water and the shape of the bank," says Harry. "We are watching to see which structure works best and to learn from how they perform."

Pioneering as this project seems today, one could argue that the scheme marks a return to traditional land management practices, says Dean Morrison: "What we are doing here is encouraging the sort of behaviour of waterways that, historically, helped to prevent floods. Rivers and streams naturally tend to have blockages and to meander. And the more that the water can be slowed down, the less likely it is that sudden downpours of rain will lead to flooding."

This low-tech solution to the dangers of flooding is all part of a wider plan to tackle the issue in the River Medway's catchment area, Dean explains: "We are increasingly realising that working with nature in the small streams that constitute the head waters of a river can really help what happens further downriver."

Until now, the image of flood prevention has been one of expensive engineering work, pumps and interventions near the site of a flood risk. Now, thanks to this timely project in Tudeley Woods, it seems that fallen trees, sticks and leaves can also make a contribution to protecting property. "It's a scheme that, if successful, could have implications for woodland management both nationally and further afield," says Harry Teacher. "We look forward to monitoring the project in the months and years ahead."