Question to the Department for Environment, Food and Rural Affairs:

To ask the Secretary of State for Environment, Food and Rural Affairs, what recent assessment he has made of the effect of dredging rivers on flood prevention.

The Environment Agency (EA) prioritises those activities which achieve the greatest benefit in terms of better protecting people and property from flooding. Dredging and clearing channels are important parts of the EA’s maintenance regime, when it improves the channel’s ability to carry increased river flows and manage flood risk.

In 2010, the EA carried out a comprehensive series of trials to review and update understanding of the benefits and effectiveness of dredging as one method for maintaining channel conveyance. The trials showed that dredging can reduce flood risk, but its effectiveness and value for money varies significantly depending on the location. Since then, further studies have been carried out, validating the results of this trial, including the Thames bathymetry review, which reached a similar conclusion. In many cases, rivers naturally return to their pre-dredged state very quickly, and therefore any flood risk benefits are so short lived that the work cannot be economically justified. The implications for downstream communities also need to be considered.

The Somerset Rivers Authority and internal drainage boards have trialled alternative methods for dredging. These included small-scale trials of both cutter suction and water injection dredging, and a full-scale 5km trial using water injection dredging on the River Parrett. The trials demonstrated some potential to reduce costs of dredging in tidal rivers.

In 2014, the Chartered Institution of Water and Environmental Management produced an independent report entitled ‘Floods and dredging, A reality check’. This explored the effectiveness of dredging and is available at:

https://www.ciwem.org/assets/pdf/Policy/Reports/Floods-and-Dredging-a-reality-check.pdf

The EA uses the results of such trials and studies to decide where and how dredging will be effective, on a case-by-case basis.

Typically over each of the past three years the EA has spent between £45 million and £55 million a year on channel maintenance of which between £5 million and £11 million is for dredging. Channel maintenance includes a range of activities to maintain conveyance such as dredging, weed cutting and removing blockages.