Risk of the Underwater Cultural Heritage in the Bay of Cadiz due to Waves: Impact of Climate Change

Abstract

The  underwater cultural heritage (UCH) is constantly subjected to the impact of marine agents capable of degrading its materials. Among these factors, waves stand out as one of the main elements affecting the preservation and evolution of UCH in shallow-water areas. Despite the relevance and high density of cultural heritage in littoral areas, there is a lack of comprehensive assessment of the long term evolution of risks induced by waves. This becomes particularly relevant because the frequency and intensity of coastal storm and waves could be altered by climate change. Therefore, this study aims to assess the impact of climate change in UCH preservation produced by modifications in the  wave conditions in the Bay of Cadiz. The three most influential hazards in the shallow UCH sites have been analyzed: decontextualization, scouring, and wear erosion. To achieve this, extreme wave conditions were analyzed for a historical period (1970-2005), a near future (2035-2060), and a distant future (2074-2099), considering the RCP 4.5 and RCP 8.5 scenarios, using five global wave models (ACCESS 1.0, CSIRO-Mk3.6.0, EC-EARTH, GFDL-ESM2G, and GFDL-ESM2M). The  choice to use five models aims to provide a more robust basis for decision-making and planning to preserve and conserve UCH. The global wave models output are calibrated using a wave buoy record and the Quantil-Mapping method.Subsequently, a hybrid downscaling methodology combining a numerical wave model with statistical tools is used to transfer wave climate from intermediate to shallow waters. In this process, the wave time series were reconstructed in each computational cell, taking into account the sea level rise for each period and for each model. This methodology was validated using SIMAR points located inside the Bay of Cadiz.  After obtaining the necessary variables for the characterization of wave climate and for calculating the risk, an ensemble is performed to reduce uncertainties in the projections. To assess and quantify the wave impact on UCH, a  the risk assessment is performed as a function of vulnerability (depending on archaeological materials, slope, and type of seabed) and the hazard generated by waves. This study reveals that, despite the general trend towards less intense waves in the North Atlantic, significant changes in risk indices and material degradation rates in UCH can occur. This is due, in part, to the increase in wave energy in shallow areas caused by the rise in sea level.   Although there are still significant uncertainties in extreme wave projections in coastal regions, these studies can facilitate   the identification of UCH sites at risk in the present and future conditions and the implementation of mitigation measures to protect UCH sites from the impact of climate change.