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

Abstract

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.

Mediterranean cyclones: Current knowledge and open questions on dynamics, prediction, climatology and impacts

“…Indeed, a wide range of scales is typically relevant to forecast accurately the timing, location, mode and intensity of the damaging impacts (e.g. Delrieu et al 2005;Lorenzo-Lacruz et al 2019). Forecast strategies accounting for uncertainties linked to moisture sources and evolution, such as surface fluxes, boundary layer flows and microphysical processes must be investigated to face those challenges.…”

More exposed but also more vulnerable? Climate change, high intensity precipitation events and flooding in Mediterranean Spain

“…The hydrological response to this amount of precipitation was also quite common. Most of the precipitation fell on a small basin dominated by a 500-meter high mountain with a wide reception area and a narrow course downstream (Lorenzo-Lacruz et al, 2019). It is also known how in Mediterranean basins most rainfall turns into runoff with extremely rapid concentration times.…”

The heavy precipitation event of 14–15 October 2018 in the Aude catchment: A meteorological study based on operational numerical weather prediction systems and standard and personal observations

“…During the week of 10-15 October 2018, several coastal regions neighbouring the northwestern Mediterranean basin were concerned by intense precipitating episodes. The Balearic Islands were the first impacted by very heavy downpour, in the night of 9 October 2018, that have ravaged the eastern coast of Mallorca, killing 13 people and causing considerable material damage (Lorenzo-Lacruz et al, 2019). A few days later, the southern regions of France (especially the Aude department) were concerned, in turn, by heavy precipitation.…”