Flood Control

(asked on 8th June 2020) - View Source

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 (a) river dredging and (b) de-silting of minor waterways on reducing the incidence of flooding; and if he will make a statement.


Answered by
Rebecca Pow Portrait
Rebecca Pow
Parliamentary Under-Secretary (Department for Environment, Food and Rural Affairs)
This question was answered on 11th June 2020

The Environment Agency (EA) refers to dredging as the removal of accumulated material in a river. It is often done to maintain channel conveyance and ensure water can flow freely. Dredging also includes ‘de-silting’ which removes material like gravel or soil washed into rivers, as well as rocks and plant life.

To protect people and properties from flooding the EA prioritises activities that achieve the greatest benefit. Dredging and clearing channels are important parts of the EA’s maintenance regime when they improve the channel’s ability to carry increased river flows and manage flood risk. Over each of the past three years the EA has spent between £45 million and £55 million per year on channel maintenance, and between £5 million and £11 million of that is spent on dredging.

In 2010, the EA carried out a comprehensive series of trials to review and update the understanding of the benefits and effectiveness of dredging. It showed dredging can reduce flood risk, but its effectiveness and value-for-money varies significantly depending on location. Historic records and modelling also show that dredging in some locations can increase erosion and flood risk for communities downstream.

Since then, further studies such as the Thames bathymetry review have validated the results of the 2010 trials. In many cases, rivers naturally and quickly return to their pre-dredged state. Therefore any flood risk benefits are so short lived that the work cannot be economically justified.

The Somerset Rivers Authority and internal drainage boards (IDBs) have trialled alternative methods for dredging. These included both cutter suction and water injection dredging, and a full-scale 5 km trial using water injection dredging on the River Parrett. The trials demonstrated some potential to reduce costs of dredging in tidal rivers. The Environment Agency uses the results of such trials and studies to decide where and how dredging will be effective, on a case-by-case basis.

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