Is dredging a sustainable solution for managing UK floods?

In a recent report by the Chartered Institution for Water and Environmental Management, the UK Government’s recent proposals for increased dredging activity to take place in order to reduce future flooding impacts have been labelled as ‘a cruel offer of false hope to those living in flood prone communities’. Whilst dredging may have its role to play, the report warns that it should only be used as part of a wider more integrated management approach.

Dredging involves the removal of sediment from the bottom and sides of river channels and can also include straightening of channels and deepening of the bottom. These dredged channels require long term maintenance and continued work as new silt will accumulate within the river bed over time. The overall aim of most dredging activities is to reduce the extent of flooding and act as a flood management tool. However, understanding the extent to which dredging can mitigate against flood risk and flooding events can be difficult because of the differences between location and river system hydrology and ecology, which can influence the resulting outcomes.

From an ecological perspective, dredging can have many significant impacts upon river ecosystems and their wildlife. Plant communities can be affected through physical disturbances, alterations in river flow affecting their ecology and non-native invasive species such as Himalayan balsam can also increase in their occurrence. Invertebrates and fish can be affected via sediment disturbances and loss of habitat, which can have knock on effects for those higher up the food chain. Those living on river banks, such as water voles, or those on the floodplains themselves, such as wading birds, can also be affected.

To better understand the role of dredging in flood management, the advantages and disadvantages of dredging have previously been assessed by the Environment Agency using six pilot study sites within the UK. In some cases dredging did reduce the water levels of rivers, but this did not lead to reductions in flooding risk during extreme flows because other features such as infrastructure upon the floodplain increasingly affect water levels rather than the capacity or shape of the river channel itself. In some cases, dredging actually increased the flood risk further downstream.

Using the Somerset Levels case study as an example, the report demonstrates that dredging alone is unlikely to be the answer when there are rarer, more extreme flooding events taking place, such as the recent UK floods.  Whilst dredging can in some cases reduce flooding duration, it cannot prevent flooding during these extreme events and should therefore not be considered as the winning solution to flooding events. Instead, the report suggests that dredging needs to be considered on a case by case basis and used in conjunction with other flood management measures. As our own Ecological Issues suggested, using natural flood management techniques should be a core focus of flood mitigation measures; dredging alone is not a sustainable solution for future flood management in the UK.