Shipwrecks and man-made coastal structures as indicators of historical shoreline position. An interdisciplinary study in the Sancti Petri sand spit (Bay of Cádiz, SW Spain)

Abstract

Shoreline evolution studies are mostly performed by comparison of vertical images (e.g. aerial photographs and satellite imagery), being limited to the last decades. However, phenomena such as longer term climatic fluctuations may cause important secular coastal changes which are mostly unknown. The use of archaeological remains as shoreline proxies could overcome this limitation by allowing to expand back in time the historical shoreline record. The present study endorses the effectiveness of the use of shipwrecks and man-made structures as proxy data for analyzing historical shoreline evolution. This was confirmed by the evaluation of nearshore evolution (historical shorelines from 1787 to 1823 and recent changes from 1956 to 2013) and the reconstruction of the palaeobathymetry along Sancti Petri sand spit (SW Spain) by means of the analysis of historical documents, historical maps, modern aerial photographs and a recent bathymetry (2011), along with the archaeological prospection supported by the numerical modelling of the dispersion of archaeological remains. The analysis of historical maps has revealed a significant shoreline retreat over the medium term scale in Sancti Petri sand spit, averaging −1.2 m·year−1. Results from historical maps and archaeological remains show slight differences with respect to the most recent retreat rates (−0.9 m·year−1) obtained from aerial photographs for the period 1956–2013. Spatial variations in coastal retreat along the sandspit are closely linked to local patterns of wave height and wave-induced currents. Significant temporal variation in rates of shoreline change was observed, probably related to storminess variability in the study area, as indicated by the relation between the rates of coastal change for each analyzed period and the corresponding rate of change of winter NAO index.