Geo-archaeological analysis of the impact on the conservation of historic sites in South East Britain due to coastal erosion and landsliding dynamics

Mitigation strategies aimed at the preservation of historic sites exposed to natural hazards should always include a preliminary investigation phase, to provide an updated knowledge basis to clarify the impact of past instability and the current dynamics of deterioration processes. Such a scientific approach to conservation issues can be extremely effective in case of cultural heritage built in areas affected by past environmental changes and highly susceptible to further modification in the near future. This work presents the results from multidisciplinary studies on the ‘Roman Shore Forts’ and modern military installations in South East Britain, carried out in the framework of the scientific co-operation between University of Firenze and Kingston University. Along the stretch of coastline from Hampshire to North Kent, included the Isle of Wight, several fortifications and garrisons were built between the Roman period and the II World War, representing an historical record of many centuries of defence against invasions from the sea. Due to their geographic location and the geological features of the foundation substratum, most of the surveyed sites have being affected by instability processes – marine erosion, cliff retreat and coastal landslipping – which have produced different impacts on the historic structures, causing the partial or even total destruction of the ancient sites (e.g. Walton Castle). Succession of marine transgression/regression cycles, sequences of erosion/siltation and human interference in landscape change have also contributed to the modification of the original environmental context, with the main effect of creating a different perception of the archaeological sites, formerly coastal and currently inland (e.g. Pevensey Castle and Lympne). Starting from a literature search, a ‘site specific’ analysis was performed combining on site inspections with historical documentation, attempting a reliable reconstruction of the local environmental evolution in relation to past and recent coastal erosion and landsliding dynamics. Detailed surveys of deterioration patterns were performed both on entire sites and isolated monuments, recognizing, among others, masonry falls and collapses at Pevensey Castle, Reculver and Portchester Castle, as well as sliding down the coastal slopes for some 20th century ‘pillboxes’ at the Isle of Sheppey in North Kent. In particular, the availability of Ordnance Survey maps dating back to the 1840s allowed the temporal and spatial analysis of the direct impacts of coastal erosion and storms on the conservation of the masonry structures at Sandgate Castle, South Kent, showing in this case the different stages of deterioration in relation to water levels and tides. As final product, a ‘site specific’ mapping was obtained and the current condition of each site was evaluated with reference to the regional geologic and geomorphologic context. For those sites where interventions were executed (e.g. Reculver), the updated mapping was also exploited for discussion about the suitability of past mitigation measures and stabilization works, both in terms of effectiveness and sustainability. On the other hand, the analysis highlighted the persistence of hazard factors in other sites, confirming a continual activity of coastal erosion/landsliding processes and emphasizing the need of developing appropriate conservation strategies to preserve the exposed historic heritage.