Poorly known 2018 floods in Bosra UNESCO site and Sergiopolis in Syria unveiled from space using Sentinel-1/2 and COSMO-SkyMed.

The instability situation affecting the Middle East poses threats to preservation of cultural heritage. Mapping efforts based on satellite imagery currently concentrate more on recording human-induced damage than impacts of unforeseen natural events (e.g. floods). In 2018, the UNESCO World Heritage Site of Bosra and the landscape of Sergiopolis-Resafa in Syria were flooded after heavy rainfall. While the first incident was reported by heritage organisations (although information was limited to the main monument), the second event was completely unknown. Using optical and radar satellite images from the European Commission's Copernicus Sentinels fleet and the Italian Space Agency's COSMO-SkyMed constellation, we prove that these data are an enormous reservoir of information to assess comprehensively the duration, extent and severity of such natural events. In Bosra, several key assets were flooded besides the Roman Theatre, with waters taking from few days to several weeks to evacuate and dry out. In Sergiopolis, while the main ruins were sheltered by the fortification walls, the nearby floodplain was inundated. The floodwater-flow pattern resembled the simulations developed by archaeologists to prove the existence of an ancient system of embankments and dam. Our results suggest that many other events posing risk to heritage assets, otherwise unnoticed, may be unveiled if current satellite imagery archives, yet to analyse, are systematically screened.