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Dredging rivers won’t stop floods. It will make them worse

Flooding in Somerset, U.K.

‘You can increase the flow of a river by dredging, but that is likely to cause faster and more dangerous floods downstream.’ Photograph: Ben Stansall/AFP/Getty Images


Powered by article titled “Dredging rivers won’t stop floods. It will make them worse” was written by George Monbiot, for The Guardian on Thursday 30th January 2014 14.30 UTC

For a moment, that rarest of beasts – common sense – poked a nose out of its burrow and sniffed the air. Assailed by angry farmers demanding dredging in the Somerset Levels, the environment secretary, Owen Paterson, broke with time-honoured protocol and said something sensible: “Dredging is often not the best long-term or economic solution and increased dredging of rivers on the Somerset Levels would not have prevented the recent widespread flooding.”

He went on to suggest something I never thought I would hear from his lips: “Also, we need to do more to hold water back, way back in the hills.”

Coming from the man who insisted in November that he would do what he could to help farmers keep the hills bare, this was an astonishing and welcome turnaround. It reflects what his advisers in the Environment Agency have been trying to say for years, before being sat on by ministers wanting instant answers to complex problems and then – as the government still plans – being sacked in droves.

A presentation by the agency, called To Dredge or Not to Dredge?, spells out the problems in terms that even ministers can understand: “The river channel is not large enough to contain extreme floods, even after dredging. Dredging of river channels does not prevent flooding during extreme river flows … The concept of dredging to prevent extreme flooding is equivalent to trying to squeeze the volume of water held by a floodplain within the volume of water held in the river channel. Since the floodplain volume is usually many times larger than the channel volume, the concept becomes a major engineering project and a major environmental change.”

Is that not bleeding obvious? A river’s capacity is tiny by comparison to the catchment from which it draws its water. You can increase the flow of a river by dredging, but that is likely to cause faster and more dangerous floods downstream when the water hits the nearest urban bridge (something the residents of towns like Taunton and Bridgwater should be worried about). If you cut it off from its floodplain by turning it into a deep trench, you might raise its capacity from, say, 2% of the water moving through the catchment to 4%. You will have solved nothing while creating a host of new problems.

Among these problems, the Environment Agency points out, are:

1. Massive expense. Once you have started dredging, “it must be repeated after every extreme flood, as the river silts up again”.

2. More dangerous rivers: “Removing river bank vegetation such as trees and shrubs decreases bank stability and increases erosion and siltation.”

3. The destabilisation of bridges, weirs, culverts and river walls, whose foundations are undermined by deepening the channel: “If the river channels are dredged and structures are not realigned, ‘Pinch Points’ at structures would occur. This would increase the risk of flooding at the structure.” That means more expense and more danger.

4. Destruction of the natural world: “Removing gravel from river beds by dredging leads to the loss of spawning grounds for fish, and can cause loss of some species. Removing river bank soils disturbs the habitat of river bank fauna such as otters and water voles.”

As the agency says, dredging is primarily a tool for improving navigation and, in some places, land drainage. It has been mistaken by people who ought to know better, including ministers, as a means of dealing with a different problem: flooding.

If you want to stop rivers from ruining people’s lives, you should engage with the kind of issues that Paterson hinted at. That means, broadly speaking, the following:

• More trees and bogs in the uplands – reconnecting rivers with their floodplains in places where it is safe to flood (and paying farmers to store water on their fields while the danger passes);

• Making those floodplains rougher by planting trees and other deep vegetation to help hold back the water – lowering the banks and de-canalising the upper reaches, allowing rivers once more to create meanders and braids and oxbow lakes. These trap the load they carry and sap much of their destructive energy.

None of these produce instant results. But they are distinguished from dredging in one significant respect: they work.


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