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 numerical weather prediction models, generated a devastating flash flood (305 m3 s−1) in the town of Sant Llorenç des Cardassar, Mallorca, on 9 October 2018. Four people died inside this village, while casualties were up to 13 over the entire affected area. This extreme event has been reconstructed by implementing an integrated flash flood modelling approach in the Ses Planes catchment up to Sant Llorenç (23.4 km2), based on three components: (i) generation of radar-derived precipitation estimates, (ii) modelling of accurate discharge hydrographs yielded by the catchment (using FEST and KLEM models), and (iii) hydraulic simulation of the event and mapping of affected areas (using HEC-RAS). Radar-derived rainfall estimates show very high agreement with rain gauge data (R2=0.98). Modelled flooding extent is in close agreement with the observed extension by the Copernicus Emergency Management Service, based on Sentinel-1 imagery, and both far exceed the extension for a 500-year return period flood. Hydraulic simulation revealed 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 9 October 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 the Sant Llorenç town centre. This study highlights 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.

Seagrasses provide a novel ecosystem service by trapping marine plastics

“…1B). The occurrence of an extraordinary convective rainfall event in Mallorca Island in autumn 2017 44 may had loaded coastal waters with an extra amount of PP pellets (Supplementary Table 2). Drifting floating plastic items were flushed ashore during autumn storms, alongside with seagrass remains from natural massive seasonal leaf loss 45 .…”

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

“…Surface observations result from the combination of standard weather stations (SWSs), which are Météo-France operational weather stations sampling atmospheric parameters at a time step of 1 min on the one hand, and crowd-sourced personal weather stations (PWSs), on the other hand. The PWS time series of mean sea level pressure (MSLP), temperature, and relative humidity are processed following the method presented by Mandement and Caumont (2020a). Gridded analyses of surface pressure, mean sea level pressure, temperature, relative humidity, and virtual potential temperature derived from observations near the ground are built at a 5 min time step and 0.01 • resolution in latitude and longitude.…”

“…Virtual potential temperature fields are built from the previous fields. Details are given by Mandement and Caumont (2020a).…”

“…During the week of 10-15 October 2018, several coastal regions neighbouring the north-western Mediterranean basin were concerned by intense precipitating episodes. The Balearic Islands were the first impacted by very heavy downpour, on 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 affected, in turn, by heavy precipitation.…”

Hydrogeomorphological analysis and modelling for a comprehensive understanding of flash-flood damage processes: the 9 October 2018 event in northeastern Mallorca

“…This software obtains high-quality results easily from algorithms known as SfM. Further details on the implemented algorithms can be found in Lowe (2004) and Westoby et al (2012). For the proper acquisition of the imagery, flight altitude was set at 70 m, ensuring ground resolution close to 0.02 m px −1 , and the camera was programmed to shoot every 15 m, flying at an average speed of 5 m s −1 .…”