Temporary dams could help solve Welsh flooding problems

Soft engineering in the hills of Wales could help to alleviate the problems of flooding caused by climate change. Every year, hundreds of thousands of pounds worth of property is damaged across Wales by flooding.

Latest research from Robinwood, the Welsh European project looking at how trees and woodlands can provide all kinds of new benefits, shows that low cost solutions could be a valuable aid to riverside towns.

The Forestry Commission Wales Interreg IIIC programme asked researchers to look at the case for the use of riparian and floodplain woodland in river tributaries as a sustainable method of flood alleviation.

And trials in woodlands near Abergavenny – one of the towns affected by flooding in 2007– point the way to cheap and effective ways of reducing the risks.

“Trees, undergrowth and woody debris increase the hydraulic 'roughness' of the floodplain, slowing down the passage of flood flows,” said Robinwood project manager Kim Burnham.

“Trees and woody debris can dissipate flows, forming multiple channels and backwater pools, enhancing flood storage. The net effect can delay and reduce the size of the flood peak. That means less risk of flooding downstream, potentially reducing the reliance on expensive ‘hard’ engineered defences.”

At the heart of the concept is the use of large woody dams (LWDs) – five of which were built along a 500 metre stretch of the River Y Fenni using locally sourced fallen trees and brash. These constructions can effectively hold back the river flow without stopping it completely.

Water level recorders were installed above and below the dams to monitor the watercourse and any flood events and initial trials appear to confirm that this low-cost method of containment can work.

“Model predictions show that the hydraulic roughness created by LWDs in the River Y Fenni catchment acted to slow down water flows, raising water levels upstream by up to 1.5m over a distance of 165m,” said Huw Thomas of Forest Research, who carried out the trials.

“Although the dams appeared to have little effect on the height of the flood peak, its passage was delayed by an average of 2-3 minutes per dam. Of particular note is the ability of LWDs to raise water levels above bank height, promoting out-of-bank flows and further increasing flood storage.

“While the lack of flood events during the monitoring period precluded testing of the model predictions, observations of the development of the dams suggest that they are functioning as expected and the work is now on-going.”

Now further research work is being carried out by Forest Research to confirm and build on those initial findings.

Improved management of existing woodlands and new planting will also have other beneficial effects in terms of increased biodiversity and the restoration of native ecosystems.

Sediment storage and erosion control will help improve water quality, and there would be increased economic outputs through local employment from woodland management.

For more details on Robinwood contact Kim Burnham - e-mail kim.burnham@forestry.gsi.gov.uk – tel: 08456 040845 or on the Robinwood web-site: www.robin-wood.it.

ENDS

Contact: Kim Burnham, Robinwood, Wales programme manager - Tel: 0845 6040845

Editor’s note

Robinwood has given Wales its first opportunity to be a partner in one of the European Interreg 111c RFO (Regional Framework Operation) projects aimed at bringing partner countries together, fostering cross border work between academics, private business and the public sector. The Welsh part of the three and a half year project is worth 1 million Euros to Wales.

Robinwood is a cross-border European project aimed at identifying how forests, trees and timber across Europe can play a vital role in the revitalisation of rural communities through new business opportunities, flood prevention and wood fuel energy. It is hoped that it will also change political perception of the role woodlands can play in the social and economic future of rural communities and build new links between the partner countries involved.

Robinwood promotes the sustainable development of rural areas and overcome obstacles to progress by creating: a woodland chain process - territorial promotion, marketing - new techniques in forestry development - certification, policy changes, silviculture - hydrogeological improvements - models to prevent instability and reduce risks promotion of wood as a fuel source for energy - new management/technologies.