Flooding and dredging - a reality check

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I’ve been the RSPB’s Conservation Director since May 2011. As I settle into the job, I’ll be blogging on all the big conservation topics and providing an inside view of our conservation projects. I hope you enjoy reading it and feel inspired to join in t

Flooding and dredging - a reality check

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The RSPB has joined forces with angling and nature conservation organisations to support a new report by flood management experts (CIWEM) critiquing the contribution that dredging can make in preventing future floods.  You can read the report here.  

We have made this intervention to counter some of the misinformation being bandied around about dredging and to influence the public debate about how we should adapt to and prepare for future flood events.

The crisis is affecting thousands of people and is having a noticeable impact on the natural environment (although the scale of this may not be known for some time).  And today, the country braces itself for another battering with those already suffering in the West Country in the frontline once again.  For a first-hand account of what life is like living in the Somerset Levels at the moment, read this blog (here) from our Area Manager, Jane Brookhouse.

The extreme weather is set to dominate the politcal debate for weeks to come.  And media interest, like the floods themselves, shows no sign of abating.  The 24-hour news culture means there is a lot of airtime and newspaper columns to fill.  Some of the commentary has been excellent while some of it has been simplistic or even disingenuous.  In particular, it is frustrating to hear those that continue to present dredging of our rivers as the golden bullet to solve all our future flooding problems. 

Today's report provides a reality check on the contribution that dredging can make.  It begins by describing what dredging is, highlights its advantages and risks and considers when it might form part of a flood risk management strategy.

We have repeatedly said that dredging can, in some places like the Somerset Levels, make a contribution to reducing the impacts of floods but we have also said that on its own it will be insufficient.  The report highlights studies on the Levels that show that the proposed dredge would not have prevented the floods but would significantly reduce the length of time water remains on the land.

The report ends with an overview of the range of approaches to flood risk management - from hard engineering to natural solutions.  We shall be sharing it with decision-makers and hope that it helps shape their strategy for building resilience to future extreme weather events.

Have a read and let me know what you think.

It would be great to hear your views.

Comments
  • Martin, this is spot on. It isn't dredging vs alternatives, its heavy engineering pure, simple and pricey (and unlikely to work) vs dredging as part of a sophisticated approach to land use, and natural events where we aren't always in control. Spreading the water is the obvious solution in the Levels - lets plan it better and pay the landowners who hold the water away from people's homes a fair rate - which may well be much less than the cost of an engineered solution which risks catastrophic failure which the experts predict is probable rather than just likely.

  • Martin, spot on. The big battle - and its going to be difficult - is that its not whether to dredge or not to dredge, it is whether a single, high cost simplistic political gesture on the one hand or a more sophisticated, integrated approach on the other is right. If the first wins the chances are everyone on the levels will be back where they are today within a decade at the longest. With an approach which uses all the weapons we have - slowing the flow upstream, spreading water and selectively clearing blockages in the river/drain system there's a chance we may be able to protect more homes against what could well be increasingly violent attacks.

  • The Report seems a well balance document. While saying that dredging can in some cases make a contribution to flood alleviation, it also says that specific knowledge of the area in question is important. In other words one cannot generalise in these matters, and flood alleviation plans have to be site specific.

    Broadly, the velocity of water flow (e.g. meters per second) will depend on the difference in levels between the up stream measuring point and the downstream point. The rate of flow (cubic meters per unit time) will depend on the velocity of flow (as above)and the cross sectional area of the channel. With the Somerset Levels having very very small differences in levels, it is unlikely the natural flow velocity can be increased significantly. The cross sectional area can be increased by dredging and hence the volume flow increased according to the increased cross section but given the huge amount of excess water to be drained, it seems to me dredging is not likely to have that much effect.

    It therefore seems to me that priority should be given to finding natural solutions to slow and absorb flows into the Levels thus giving the existing channels time to drain the water out of the Levels. There are no doubt other ways of tackling the problem by working with nature, but as mentioned, one needs detailed knowledge of the area.