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The unintended consequences of dredging
Tuesday, 8th October 2013
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?