Can dredging prevent flooding?
In times of prolonged extreme wet weather over large river catchments the flow capacity of rivers may be exceeded, leading to fluvial flooding as streams and rivers cannot cope with these large amounts of water. After such events there are often calls for rivers to be dredged to allow them to carry more water and potentially lessen the effects of flooding, the advantages and disadvantages of which are still surrounded in controversy.
What is dredging?
Dredging may describe the straightening or deepening of substantial lengths of a watercourse, or the extraction of sediments that have settled on the river bed. Types of dredging include;-
- Excavation – boat or riverbank mounted excavators are used to dig out and remove material from the banks or bed of the watercourse.
- Suction – the material is sucked up from the river bed and may be recycled for other projects.
- Ploughing – this involves agitating the sediment into suspension, enabling it to be carried downstream.
Is dredging the right option?
Dredging can significantly impact upon the wildlife that lives in and around a watercourse. For example spawning grounds for fish, and riverbanks where plants, mammals and birds live may be adversely affected as their habitat is disturbed.
In high energy river systems, straightening or increasing the depth of a river to increase its capacity to carry water can also speed up the flow rate, and whilst this initially benefits the area that is dredged, it can actually lead to greater erosion, deposition of gravel and increased flood risk further downstream.
Furthermore, by allowing a river to carry a larger volume of water, structures such as bridge supports will reduce flow. This can cause afflux, where the river level upstream of the structure becomes higher than that downstream, potentially increasing the risk of flooding upstream.
Dredging may be effective on low energy watercourses “choked up” with fine sediments to enable them to hold more water and in turn, reducing the risk of flooding. In areas such as the Somerset Levels where river energy is low, fine sediments can accumulate over time and reduce the depth of the channel so dredging may be necessary to maintain channel depth/capacity. Programmes of work such as these would require ongoing maintenance and are costly to undertake.
Gravel removal is a more localised approach to improve the flow of rivers around;
- Meanders where material has been deposited.
Gravel is deposited by slower moving water on the inside of a bend in the river, this causes greater erosion on the outside of the bend. Targeted gravel removal in these areas can reduce the erosion of the river bank.
- Structures in the channel such as bridges or weirs which act as pinch points.
Bridges can act as pinch points and gravel bars can form as the bridge supports interrupt the flow of the water causing material to be deposited there. This would decrease the amount of water that is able to pass under the structure potentially causing problems upstream.
To reduce the amount of water that makes its way into our rivers initially, it would be beneficial to use different approaches such as natural flood management (NFM) which has the added benefit of enhancing habitats for wildlife, with the targeted use of gravel removal and dredging where appropriate.