It seems that after every major flood, there is a cry to ‘dredge the rivers’ with the apparently logical rationale that increasing river capacity and getting water out to sea quickly is the best way to avoid flooding. Dredging was commonly carried out in the post war years up to the late 1980’s, but since then flood engineers have realised that dredging is often not the answer and can exacerbate flooding as well as destroying precious wildlife habitats. With the scale of floods that we are now experiencing, no amount if dredging will create enough capacity to hold all the flood water in the channel.
Jeremy Purseglove’s seminal book Taming the Flood was published in 1989 and much of the content is still highly relevant today.
The Chartered Institute of Water and Environmental Management publication Floods and Dredging — A Reality Check is an excellent summary and includes an opinion price by Tony Juniper who is now Chair of Natural England.
The EA has produced a very useful document ‘Evidence; Impacts of Dredging’
Paul Gaskell has produced the video sequence below demonstrating the destabilising impacts dredging can have on a river channel.
Nature abhors a vacuum and removing accumulated material from a river channel creates a vacuum that can have far-reaching and unintended consequences.
Upstream of the dredged reach, the river will try to fill the hole created by losing sediment by increasing the demand for eroded river-bed and river-bank material from upstream. This can dramatically increase the rate of erosion in upstream reaches.
Downstream of the dredged reach, the river is ‘starved’ of sediment. The bed material is being re-accumulated in the dredged area to fill the space left by dredging, so there is far less material being supplied further downstream. Cutting off of the sediment supply causes the river to increase the amount of erosion downstream of the dredged reach to fill the sediment gap.
Rivers always behave to achieve an equilibrium that responds to changes — a dynamic equilibrium. If the equilibrium is disrupted by a major change like dredging, then the river will react to try to get back to a stable state.
How many people who undertake dredging understand these processes? Is the move to “fast track” UK-landowners’ ability to dredge their own streams with a much lower requirement for external assessment likely to create more problem than it solves?
Blueprint for Water briefing paper on dredging