Woodland dam trials to stem the floods

Deep among the dark conifers of Cropton Forest, at first sight it looks as though a tree has been blown down over a stream. But as you get closer a timber stake is seen to be holding the log firmly in place, and then you notice the watchful eye of a digital camera high on a nearby tree trunk.

The camera, it turns out, is taking time-lapse photographs of the stream. These will later be synced with local rainfall records to show exactly how the log and the build-up of twigs and branches behind it is affecting the flow of water.

There are more than 180 of these so-called woody debris dams constructed over the network of small streams which drain the huge forestry plantations on the southern slopes of the North York Moors. As the water carries more and more debris into the dams, the rate at which the current permeates down through the catchment area of Pickering Beck and the River Seven will be slowed.

By how much isn’t yet known but the early signs are promising, according to Jeremy Walker, one of the prime movers in the ‘Slowing the Flow at Pickering’ project. He says there is some observational evidence that the work helped to prevent a flood in Pickering in December 2012.

If this pilot scheme to stop bouts of intensive rainfall from washing straight off the moors and forests is judged a success, it may provide answers for other flood-prone areas like Calderdale in West Yorkshire. Since most rivers have their sources in upland areas, the scheme could be applied more widely in the UK to reduce the risk to thousands of homes on flood plains many miles downstream.

The woody debris dams are part of a suite of measures costing around £2m, the centrepiece being a new 35-acre storage reservoir a few miles upstream from Pickering largely funded by Ryedale and North Yorkshire County Councils. This is designed to fill only when there is a torrential rainfall event, thus saving the town from floods.

Pickering’s location at the foot of the Moors has made it prone to frequent flooding. As long ago as 1892 benches had to be put up in the Market Place for people to walk from one side to the other, but perhaps because of climate change floods have been more frequent in the modern era. In 1999 the waters spread from the Market Place to eight adjacent streets, and there was an identical flood after a week of torrential rain in June 2007.

At that time, Jeremy Walker, a former chief executive of North Yorkshire County Council, was chairing the Yorkshire regional flood defence committee, and he was asked to bid for funds to build flood defences in Pickering. But the design on the table then didn’t get off the drawing board because of its £7m price tage. “That was considered too expensive,” he says. “It would have cost £80,000 per property, and there was limited money available. The rule of thumb, I’m afraid, is that the Government grant for flood schemes must be around £5,000 to defend each property.”

Salvation came in the form of an approach from the Department for Environment, Food and Rural Affairs (Defra), which wanted to test new ideas for reducing flood risk by working with nature rather than simply trying to contain it.

A plan for slowing the run-off from heavy rainfall higher up in the catchment of Pickering Beck and also the River Seven, which had flooded the village of Sinnington in 2007, was produced with help from academics at Durham University. It is one of three pilot projects in England to trial woody debris dams, flood storage bunds to hold back moorland run-off and other measures to stop downstream flooding events. Similar schemes are underway in Exmoor and the Peak District.

The debris dams were not designed by computer, Jeremy says. “It’s just an idea that if you allow material to collect in a channel it will eventually hold back the water. If you put them in the right places and space them out along a water course you get an enhanced effect. The beauty of them is that they pretty much maintain themselves.”

Most of the dams have been built by apprentices working for the North York Moors National Park. Others were provided by the Forestry Commission.

Higher up in the North York Moors, so-called “grips” or drainage channels have been being blocked by bales of heather to achieve the same water-slowing effect. Other measures involve limiting the practice of heather burning, which creates habitats for red grouse, because heather reduces the rate of rainfall run-off into streams. Also, upland farmers are rethinking how they manage their pastures to stop livestock from creating erosion and run-off channels.

Another part of the work is the planting of more than 100 acres of woodland, with more planned. Tree cover helps absorb rainfall and slows the rate at which water runs drains off the land.