Hydro-meteorological reconstruction and geomorphological impact assessment of the October, 2018 catastrophic flash flood at Sant Llorenç, Mallorca (Spain)

Abstract. An extraordinary convective rainfall event, unforeseen by most prediction models, generated a devastating flash flood in Sant Llorenç des Cardassar, Mallorca on October 9, 2018. Four people died in the town (13 people died in the entire affected area). The event was reconstructed, implementing an integrated flash flood modelling approach in this small (23.4 km2) and semi-arid ungauged Mediterranean catchment, with a highly nonlinear hydrological response to heavy precipitation, based on three components: (i) generation of radar-derived precipitation estimates during the event; (ii) modelling of an accurate discharge hydrograph yielded by the basin; and (iii) hydraulic simulation of the event and mapping of affected areas. Radar-derived rainfall estimates showed very high agreement with rain gauge data (R2 = 0.98). Modelled flooding extent is in close agreement with the observed extension by Copernicus Emergency Management Service, based on Sentinel-1 imagery, and both far exceed the extension for a 500-year return period flood. Hydraulic simulation showed that water reached a depth of 3 m at some points, and modelled water depths highly correlate (R2 = 0.91) with in-situ after-event measurements. The flash flood eroded and transported woody and abundant sediment debris, changing channel geomorphology. Water velocity greatly increased at bridge locations crossing the river channel, especially at those closer to Sant Llorenç town centre. This study shows how the very low predictability of this type of extreme convective rainfall events and the very short hydrological response times typical of small Mediterranean catchments continue to challenge the implementation of early warning systems, which effectively reduce people's exposure to flash flood risk in the region.