Coastal changes in historic times – linking offshore bathymetry changes and cliff recession in Suffolk

Executive summary: The coastline of Suffolk, UK has the fastest rate of contemporary recession in the UK, reaching 5 m a-1 locally. Along this coastline historical recession has continued for as long as there is archival data (old maps, Admiralty Charts, anecdotes, diaries, parish records, OS maps, aerial photographs, elevation/bathymetric surveying). There has been much interest focussed on Dunwich (the site of lost churches and historic buildings – once a thriving port) while other cliffs appear to have attracted fewer detailed studies. However, this whole coastal region has undergone considerable morphological change involving both the coastline position as well as the associated nearshore bathymetry. The cliffs between Benacre Ness and Southwold, 7-15 km to the north of Dunwich, are currently characterised by the fastest recorded recession rates in the UK. This study reports the results of a detailed historical investigation of coastline retreat for both Dunwich-Minsmere and Benacre-Southwold. The cliffs of Benacre-Southwold have elevations of between 10 and 15 m above sea level, and stretch for several kilometers alongshore. The geology comprises soft, sandy sediments of the pre-glacial Norwich Crag Formation. Hence recession in these cliffs is particularly important for sediment release into the southern North Sea. Since sources of sediment are highly dynamic, shifting as the foci of cliff retreat changes, continual reassessment of cliff retreat is required and reliance should not be placed on previous studies which fall out-of-date very rapidly in such dynamic settings. In particular, cliff sediments are important for the maintenance of nearshore banks and shoreline features (eg: growth in Sizewell-Dunwich Bank system; movement of Benacre Ness), with feedbacks into continued coastal erosion through regional changes in wave heights and local currents. This study has quantified contemporary and historical retreat rates using the recentlydeveloped Digital Shoreline Analysis System (Thieler et al., 2005), an extension of ArcMap. A new methodology has been derived, combining DSAS analysis of recession rate with Surface Spot data on cliff elevation and extent (using NextMap Digital Terrain Models), to quantify contemporary volumes of sediment released. This has revealed considerable differences in the sediment sources from previously published estimates. The new methodology for rapid assessment of changing location and quantity of sediment sources from retreating cliffs has been used further to assess future sediment sources for the cliffs of Suffolk, where coastal management presents particular challenges. Finally the nearshore bathymetry has been evaluated for the present day and for the past 100 years. Links have been established between the changing bathymetric configuration of the nearshore region and the recent acceleration in cliff retreat, particularly notable in the cliffs of Covehithe and Benacre