Assessing Coastal Heritage Sustainability: Crustal Deformation and Sea-Level Trends at the Qaitbay Citadel in Alexandria, Egypt

The UNESCO Agenda 2030 emphasizes the preservation of cultural heritage sites, focusing on coastal heritage preservation, which still poses substantial difficulties. While earlier studies have addressed the overall consequences of natural hazards along the Alexandria coastline, there is a gap in how they specifically affect coastal heritage sites, e.g., the Qaitbay citadel (our case study). This work seeks to bridge this gap by assessing the critical hazards faced by the Qaitbay citadel, including crustal deformation due to tectonic events, earthquakes, and Sea Level Rise (SLR) resulting from climate change. To comprehensively assess these challenges, a stack of Sentinel-1 SAR datasets (2017-2021) was processed using the Persistent Scatterer InSAR (PS-InSAR) technique to conduct spatial and temporal deformation variations of the citadel's site and its buildings. GPS measurements of Alexandria's (e.g., ALX2) station were correlated with InSAR results within the same period. Furthermore, satellite altimetry data from 1993–2021 covering the citadel and surroundings were processed to highlight long-term SLR trends. The findings indicate 1) A subsidence rate of $-$1 $\pm$ 0.2 mm/yr, associated with the citadel's foundational structures and identified through the PS-InSAR analysis, was caused by load-bearing effects and sand migration beneath the citadel, 2) a vertical displacement of $-$1.3$\pm$0.6 mm/yr obtained from ALX2-GPS station, consistent with the LOS velocity rate of PS-InSAR time series analysis at that specific location, 3) a SLR trend of +3.96 mm/yr, with notable peaks potentially related to episodes of Northern Ionian Gyre reversal, that could result in changes in water mass redistribution in the surrounding region.