The effect of coastal defences on cliff top retreat along the Holderness coastline

This study investigates variations in soft cliff retreat and the impact of coastal defences along 55 km of the Holderness coast between 1845 and 2005. The analysis of three approximately 50-year periods (1854-1905, 1905-1952, 1952-2005) gives a retreat rate of between 0.8 +/- 0.4 m/yr and 2.1 +/- 0.4 m/yr. Retreat varies spatially and temporally due to natural causes and human activity, especially 19th century beach mining and coastal defence construction. Defences reduce sediment input and modify the sediment budget, usually resulting in a sediment deficit down-drift, leading to the development of a set-back. Using data from historical and modern Ordnance Survey maps, aerial photographs, and Differential Geographical Positioning System (DGPS) surveys, the cliff top was selected as a reference feature. Set-back due to defence construction at Barmston, Hornsea (two periods), Mappleton and Withernsea (two periods) was analysed, giving six different time periods. Short time periods are unreliable due to large mapping uncertainties and natural variability. Two cases showed excess retreat for tens to thousands of metres down-drift over a long time period since defence construction. Two cases were indicative of excess retreat, but require more time to elapse after defence construction (in these cases >20 years) to reach a definitive conclusion. Two cases were inconclusive, but possibly maintained a constant retreat rate post-defence construction. Hence, at the scale of Holderness, defences have changed the pattern of erosion rather than stopping it. Accelerated retreat down-drift of defences can threaten human infrastructure and buildings and reduce the efficiency of defences. This should be recognized in shoreline management planning.