Prof Roger Falconer's expert opinion on dredging to prevent flooding

Published on 4 February 2014

Prof Roger Falconer, CH2M HILL Professor of Water Management at Cardiff University and leader of Cardiff School of Engineering's Hydro-environmental Research Centre, has contributed to the current debate about whether dredging rivers would prevent future flooding in areas like Somerset.

Somerset is currently experiencing severe flooding, and a number of individuals and organisations are discussing whether dredging the help the situation and prevent floods in the future. Drawing on decades of expertise and experience, Prof Falconer has voiced his concerns about this approach:

Flooding"I feel extremely sorry for the people living in the region and I cannot imagine the difficulties which they are experiencing.  It must be extremely frustrating for them to be encountering such stress and I can imagine their wish to conclude that a lack of dredging has considerably exacerbated their problems.

"However, I have lectured in hydraulic engineering (in civil engineering) at three universities for over 35 years and have been involved in many environmental impact assessment studies worldwide.  Furthermore I am currently President of the International Association of Hydro-environment Engineering and Research.  And regrettably I cannot see that dredging would make much impact in alleviating the problems in the Somerset Levels.

"To reduce significantly the peak water levels one needs to increase the hydraulic gradient, i.e. the water surface slope, and thereby increase the flow from the marshes to the sea. This will not be significantly achieved by dredging.  What dredging will do is to increase the area of flow, which will marginally increase the flow over the short term.  Furthermore, the dredged bed will rapidly readjust itself with time to the natural hydraulic conditions – over a relatively short time – and one is then back to square one, i.e. more flooding and more dredging.  Added to this one has climate change and rising sea levels, thereby reducing the hydraulic gradient even further and making the problem worse.

"In my view there are two effective solutions to address the real fundamental hydraulics problem: (i) raise the land, or (ii) lower the sea level and create a much larger hydraulic gradient. The first solution is not practical. The second is. There have been a number of proposals in recent years to build a Bridgwater Bay Lagoon to create renewable energy. Such a structure would involve separating the water level in Bridgwater Bay from that in the Bristol Channel.

"If such a project were to be built then one would have the opportunity of producing clean, green renewable energy, protecting the levels now against excessive flooding, and, in particular, mitigating against the effects of sea level rise and storm surges in the future. In particular, if the impoundment were to be built then it could operate normally under low-flood risk conditions, producing renewable energy. When excessive flooding occurs the electricity supply company could be paid to lower the water level in the Bay to low tide and hold it for much longer than normal. This would create an excessive hydraulic gradient and the Somerset Levels could be drained or, better still, never be allowed to flood in the first place. Hence a Bridgwater Bay lagoon provides a better solution than dredging and has the potential for two very positive benefits: green renewable energy and flood risk reduction and mitigation when needed.  That's two for the price of one."

Prof Falconer founded the Hydro-environmental Research Centre in Cardiff School of Engineering in 1997. With colleagues he has recently been awarded a major EPSRC Centre for Doctoral Training grant in Water Informatics: Science and Engineering. More information can be found here.

Photograph: Bob Embleton