Human interference in soft coastal cliff retreat causes problems worldwide. Building defences alters the sediment budget, frequently causing a sediment deficit down-drift resulting in increased retreat rates. Subsequently, undefended shorelines become set back from protection works, often causing excessive and unexpected land and infrastructure loss, prompting defence extensions. Down-drift of groynes, this is known as the terminal groyne effect. From case studies, this paper determines the effects of human interference on soft cliffs, and investigates excessive land loss and likely future coastal response.
Using 10.5 km of soft cliffs in Christchurch Bay, UK as a study region, a historical shoreline analysis from the mid 19th century to the present day was undertaken. Detailed analysis was conducted at three case study sites to determine whether retreat rates had increased down-drift after the construction of protection works. After defence construction, increased retreat occurred at all three sites, albeit for only a few hundred metres down-drift, as propagation was limited owing to a headland or large sediment volumes. Set-backs can lead to artificial headland formation, making the coast more challenging and costly to defend. Shoreline management plans advocating protection or realignment should take account of natural features to enhance engineering design and reduce excess land loss.