CIWEM / Blueprint for Water ‘Executive summary: The term dredging is routinely used to refer to the systematic removal of accumulated material from river or other watercourse channels. In its most extreme form dredging may be used to re-align river channels creating linear, canalised watercourses.
It would be infeasible to dredge channels with the capacity to carry flood flows of the kind
witnessed this winter (2013/14). However, there is significant evidence that dredging can
increase channel conveyance, reducing water levels and small floods.
This is borne out by studies of the Somerset Level and Moors system which suggest that the
proposed dredge would have not prevented flooding but could significantly reduce the length
of time water stands on the land.
Numerous studies have pointed to significant unintended consequences of dredging, namely:
Increasing flood risk for communities downstream by speeding up the movement of flood
water through the river and drainage network.
Destabilising river banks, causing erosion and so risking damage to infrastructure.
Loss of wildlife and habitats both within the river and across the wider floodplain. These
impacts can be significant and permanent.
It is also important to note that dredging can be a conservation tool, particularly in heavily
modified environments where natural processes that maintain ecosystem function are
constrained.
Flood risk management strategies should look to a range of interventions, and include action
to reduce runoff by working with natural practices to slow water, and increase infiltration and
storage throughout the catchment.
Strategies will also need to manage the use of naturally flood prone land through a
combination of behavioural and engineering options, including flood zoning, warning, changes
in land use practices, as well as flood defence structures and operations.
Land management lies at the heart of these strategies, so the design of farm subsidies and
engagement with stakeholders, especially landowners, land managers and farmers is critical
to flood risk management. The Catchment Based Approach provides a platform for this
engagement.
We conclude that dredging can play an important role in flood risk management in some cases, but is not a standalone solution. It should be considered in the context of a range of tools and the origins of different sources of flood water, and comes with significant risks that must be understood at a local and catchment scale.