Man made coastal defences could raise flood risks

Man made coastal defences could in fact be raising the risk of flooding in some areas by making the coastline steeper, a study by Halcrow has found.

It suggests that holding all defence structures in their current positions may not be sustainable in the long term as they will not be able to offer the same levels of protection.

Coastal steepening is the process by which the low water and high water marks move closer together on a shore. The landward retreat of the low water mark relative to the high leads to a narrowing and steepening of shorelines which can make waves much more damaging to the coast as the energy contained in them hits full strength at sea defences rather than losing energy on the journey up a beach.

The study found that 61% of the coastline had experienced steepening over the last 113 years, with the south of England showing the most dramatic changes. In some instances, as beach widths decrease and water depths increase in front of structures, they will no longer be able to offer the same levels of protection and the strength of wave energy will lead to structural damage and overtopping.

In addition, the narrowing of our coasts has serious implications for conservation of important natural habitats, such as beaches, mudflats and saltmarshes. In some locations, the area of such habitats is decreasing as the low water mark advances landward while the high water mark remains fixed.

Dr Nigel Pontee, Halcrow’s senior coastal scientist and one of the paper’s authors, believes that man made infrastructure on the upper parts of the shore, such as sea walls and promenades, prevents the natural functioning of coastal environments. In natural conditions, unrestrained beaches and mudflats tend to move inland under rising sea levels and therefore avoid steepening.

The interruption of the natural move inland is particularly marked on open coasts, where promenades and seawalls have been built across formerly wide sandy beaches, or in estuaries where low lying areas have been reclaimed for agriculture behind sea walls.

“These results have considerable implications for deciding on future coastal management options around not only the UK but also the rest of the world. If we are not to spend increasingly large amounts of money on sea defences we need to allow more room for coastlines to function as nature intended them to do,” said Dr Pontee.

This is the first study on UK coastal steepening and used data from Defra’s ‘Futurecoast Project’ which analysed past coastal changes and present day coastal processes to make predictions about future changes in the position of the coast around England and Wales.

The study was first published in the Royal Geographical Society’s Geographic Journal.

By David Hopkins

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