January 29, 2014

expert reaction to Somerset flooding

Residents of Somerset have experience weeks of flooding, and debate has sprung up over whether dredging could have mitigated the extent of the damage.

Prof Roger Falconer, CH2M HILL Professor of Water Management at Cardiff University, said:

“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 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.”

Dr Hannah Cloke, a flooding expert from the University of Reading, said:

“Dredging would not have prevented the flooding in Somerset. The Prime Minister’s assertion that dredging will provide a long-term solution to flooding is just not backed up by the evidence.

“Dredging increases the carrying capacity of river channels, helping more water to flow downstream. But carrying capacity of rivers is just one small part of an area’s drainage pattern and its susceptibility to flooding. Land use, topography, underlying geology, and above all, rainfall levels are also relevant. Given the amount of rain that has fallen, you could have doubled the carrying capacity of every drainage channel in Somerset, at huge cost, and large parts would still have flooded.

“It’s understandable that those affected by flooding are calling for more action. But you cannot use human memory to predict floods – you have to look at the hydrological evidence.

“Dredging is not a panacea. There needs to be a much wider programme of flood prevention to protect people’s homes in the future, and other measures, such as capturing water upstream in lower-risk areas, are likely to be more effective than dredging.”

Ola Holmstrom, UK Head of Water at consultancy firm WSP, said:

“No one could fail to empathise with the communities in and around the Somerset Levels after seeing the images of inundated land and residents being ferried across the floodwaters.  Flooding is devastating for all involved and has both short and long term effects to property, land and people.  There is much anger and frustration towards the bodies that are perceived to have let communities down and a rising political pressure to reinstate dredging, and as soon as possible.  I don’t work for the Environment Agency directly or indirectly and am not here to comment on their responses to these floods; however, as a hydrologist I must ask does dredging really work?

“Dredging is hotly debated as a flood risk mitigation measure within the hydrological community.  The river channel itself only has a small storage volume for floodwaters compared to the floodplain.  The removal of bed material from the river channel does increase storage and conveyance potential, but this is relatively insignificant compared to the amount of water stored on the floodplain.

“Dredging certainly does increase conveyance (i.e. the rate of flow) in the river meaning – in theory – that waters will travel downstream faster towards the largest pond of them all, the ocean. This idea of conveying waters away to the ocean as soon as possible was for a long time the simple mantra of flood risk management. The problem is, after a while it doesn’t work anymore. It leads to passing the problem on from one community to another, concrete channels and walls, a reduction in habitat and a loss of connectivity between floodplain and channel. Improve the conveyance in Muchelney and you will more than likely increase flood risk in Bridgwater – is this acceptable?

“Flood risk management should be based less on focussing on rivers and rather the catchment as a whole.  I was present at last week’s Somerset Water Management Partnership and there was much discussion over the ‘catchment-based approach’.  However, politically this is seen as inaction and that the dredge would solve immediate issues and answer the communities’ cries.  However, given the huge cost of ‘the big dredge’ there must be quantitative proof that this will benefit both the upstream and downstream communities.  Catchment based schemes would be significantly cheaper and more sustainable than dredging; they must be given a chance even if the political pressure is on, for the good of the environment and communities throughout Somerset in the future.

“We need to consider why the rivers are silting up in the first place and to devise ways of slowing down the movement of water both overland and under the ground and finally accept that we will need to flood some areas to save others.  Unfortunately there simply is not enough space to protect everything.  A National Trust experiment near Minehead has involved farmers allowing their land to be flooded once river levels rise to reduce the amount of water passing downstream.  Now, to ask farmers to deliberately set aside land to flood is both controversial and unfair without Government subsidies to cover loss of earnings.  However, in areas such as the Levels where there is so much water, is it a debate to be had?

“Surely, there is now a need to seriously consider allowing land that hasn’t previously flooded to act as flood storage areas for the benefit of the wider community.  In this setting dredging may be appropriate to ensure that water reaches its intended locations but not as a tool for passing flows downstream.  Dredging will virtually never be the silver bullet, and the economic and ecological cost coupled with its short-term nature make it unpalatable to many.

“Unfortunately with increasing urbanisation and climate change the question of flood risk management in Somerset is not going to go away.  Rising sea levels may eventually mean that draining water from the system naturally becomes almost impossible and so someone will need to make some tough decisions in the future.  For the benefit of those currently flooded and those whose livelihood depends on the land, it would be best if this happens sooner, rather than later.”