Bank Protection
Bank Erosion
High levels of bank erosion drastically change the appearance of rivers, and bank erosion is expensive to repair. Water flow and waves caused by wind or boat traffic can all scour the riverbanks, washing the earth away. The Environment Agency is responsible for protecting flood defences from erosion and the Broads Authority encourages landowners to manage and protect riverbanks to reduce erosion, for instance with a river wall - an earth bank built up to protect farmland. In the past, protecting flood banks was achieved through the use of metal piling (long, interlinked metal sheets driven deep below the river bed level); this was often the only option available when the rond (area of vegetation between the riverbank and the river wall) had been lost and the bank was being undermined. This is an effective engineering solution but an expensive one, and it has a number of drawbacks. The sharp edges of a piled bank prevent the growth of riverside plants and prevent animals, such as water voles, from building their nests and burrows, and a piled bank also changes the character of the bank giving it a monotonous, artificial appearance.
Now flood protection work aims to set back many flood banks to allow more space for the rivers as they flow towards the sea. This not only helps increase the water storage capacity in times of flooding, but creates new habitat for wildlife, see the Broadland Flood Alleviation Project for more details.
Solutions to bank erosion
Work to restore water quality will also help to reduce the problems of bank erosion. Achieving good water quality will allow submerged plants to flourish, which will help to prevent futher loss of bank-side plants and so protect the river edge.
Research undertaken by the Broads Authority in the mid 1980s showed that only a small reduction in speed was needed to produce a significant reduction in boat wash, one of the main causes of bank erosion. As a result all speed limits were cut by 1 mph, with the highest speed limit on the Broads set at 6 mph. These lower speed limits have been in place since 1992 and their impact on rates of bank erosion is monitored.
Sediment from bank erosion affects not only the ecology of the rivers but also increases the amount and frequency of dredging required to maintain adequate water depths for navigation and conservation purposes. To try and resolve the problem of sedimentation, the Broads Authority has been working on a Sediment Management Strategy which, through partnership working, has a long-term aim to reduce inputs of sediments from all sources and take a holistic view on managing the sediment that currently exists in the waterways.
Bank protection: alternative methods
The Broads Authority, Broadland Environmental Services Ltd and the Environment Agency have been using more environmentally-friendly forms of bank protection where approriate. After several trials, the most promising 'soft' engineering options have been installed. The wide variation in depth, width, boating activity, tidal ranges and bank construction on different river sections means that no one solution will be sufficient. Techniques currently used include Profix matting, which is made up of tubes of geo-textile material filled with sand; and bitumen matting (see pictures below). Both these types of matting prevent erosion and help stabilise the re-profiled banks. Reeds are planted to grow through the matting, or are allowed to regenerate on their own, eventually giving a more natural appearance to the riverside.
Guidance on the most appropriate forms of bank protection can be found on our planning publications page.
