The influence of groin fields and other hard defences on the shoreline configuration of the soft cliff coastlines

Abstract

Building defences, such as groynes, on eroding soft cliff coastlines alters the sediment budget, changing the shoreline configuration adjacent to defences. On the down-drift side, the coastline is set-back. This is often believed to be caused by increased erosion via the ‘terminal groyne effect’, resulting in rapid land loss. This paper examines whether the terminal groyne effect always occurs down-drift post defence construction (i.e. whether or not the retreat rate increases down-drift) through case study analysis.

Nine cases were analysed at Holderness and Christchurch Bay, England. Seven out of nine sites experienced an increase in down-drift retreat rates. For the two remaining sites, retreat rates remained constant after construction, probably as a sediment deficit already existed prior to construction or as sediment movement was restricted further down-drift. For these two sites, a set-back still evolved, leading to the erroneous perception that a terminal groyne effect had developed. Additionally, seven of the nine sites developed a set back up-drift of the initial groyne, leading to the defended sections of coast acting as hard headland, inhabiting long-shore drift. Set-backs can also develop if defences are selectively removed along a heavily defended coastline, as found in an additional study site at Happisburgh, Norfolk.

Four possible post defence coastal configurations are presented, designed to support strategic shoreline management. Defences might not always be responsible for increased down-drift erosion, which has potential implications for shoreline management and litigation. Selective defence removal leading to changes in coastal configuration may become more common in the future.

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Identifiers

ORCID for Sally Brown: orcid.org/0000-0003-1185-1962

ORCID for Robert Nicholls: orcid.org/0000-0002-9715-1109

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