Why dredging our rivers shouldn't be a first response to flooding

Catherine Morgan
Wednesday, 1st July 2015

While the decision to dredge the River Parrett in response to flooding a year ago was understandable and expedient, the relief that dredging provides is temporary. Relying on regular dredging to prevent future flooding of the Somerset Levels is not a viable way forward. The Levels have flooded for millennia – flooding and flood sediments initially created and continue to maintain them – hence flooding can be managed, but it cannot and indeed should not be prevented entirely. Physical geography limits our ability to prevent flooding, because the Somerset Levels are the lowest points in the drainage basins of the Rivers Parrett-Tone, Brue and Axe; near or below sea level; naturally very poorly drained; and in the long term, dependent on incoming water and sediment to maintain their elevation, fertility, agricultural productivity and biodiversity. We cannot change the position of the Levels within their river catchments, nor can we artificially raise their elevation.

Climate change means that sea level will continue to rise in the Bristol Channel, storms and coastal surges will probably become more frequent and intense, and the land will continue to subside in those areas protected from flooding. We can reduce the frequency, extent and duration of flooding by enhancing the internal drainage network and installing larger pumps, and we can and must protect population centres and key infrastructure. However, we cannot prevent flooding when extreme rainfall, exceptional river flows, and very high tides coincide – as they did during winter 2013/14. Dredging to increase the capacity of rivers seems like an obvious way of reducing flooding, but on the Levels it’s not that simple, because the drainage system is tidal. While an enlarged channel conveys more fresh water to the sea at low tide, it also allows more sea water to flow upstream during high tide. This can exacerbate flooding due to tidal and storm surges, especially when water from heavy rain and high river flows is trapped in the Levels – a process termed ‘tide-locking’.

The viability of regular dredging depends on the frequency with which it must be repeated. Taking the Parrett as a case in point, sedimentation in the lower, tidal reach of this river occurs because it receives a heavy load of silt from its catchment plus additional silt carried upriver from the Bristol Channel twice a day, on the tide. The UK Flood Risk Management Research Consortium estimated the average annual sediment flux in the lower River Parrett to be of the order of a quarter of a million tonnes. To put the dredging performed in 2014 in context, this major effort removed approximately 220,000tonnes of sediment (ie less than an average year’s sediment load), at a cost of £6m.In the short term, regular dredging reduces the symptoms of sedimentation, but it doesn’t treat the cause – that’s why it has to be done repeatedly. Dredging also has unintended consequences for the stability

of the channel, and especially its banks. Bank failures due to the recent dredging are already starting to occur along the lower Parrett and Tone. Bank erosion triggered by regular dredging not only requires further expensive engineering; it also contributes additional sediment that then has to be dredged. Experience shows that the frequency with which regular dredging is required tends to increase through time and this is why, eventually, the cost of dredging becomes unsupportable. Regular dredging was discontinued in the late 1990s for good reason, and that decision was made not only because dredging is known to damage the river’s physical, environmental, aesthetic and ecological functions but primarily because it is a poor use of the funding available to support flood risk management. None of this should be taken to suggest that the Somerset Levels should be abandoned or somehow ‘returned to nature’.

People have lived and worked on the Levels for centuries and they can, should and will continue to make good use of the area’s rich natural resources. The contribution dredging can make to managing future flood risk is modest, being limited to de-silting and shoal removal at sediment hotspots, but it is significant and should not be ignored. Dredging will only be effective, however, as part of Integrated Flood Risk Management (IFRM) planned and coordinated at the catchment scale. Delivering IFRM requires a strategic alliance among flood risk management stakeholders that brings all the key organisations and institutions together to negotiate viable solutions that balance the multiple functions the rivers draining the Somerset Levels fulfil, while meeting the needs of farmers, businesses and communities equitably and sustainably.

 

This artcile was first written by Professor Colin Thorne for The House Magazine as part of a debate article on dredging.  To read the alternative perspective presented from Ian Liddell-Grainger, refer to the January 2015 edition of House Magazine.  FCERM.net would like to extend our thanks to Professor Thorne and The House Magazine for permission to publish this work.