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Discussion papers
https://doi.org/10.5194/nhess-2019-226
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2019-226
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Jul 2019

Research article | 11 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

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

Jorge Lorenzo-Lacruz1, Celso Garcia1, Enrique Morán-Tejeda1, Arnau Amengual2, Víctor Homar2, Aina Maimó-Far2, Alejandro Hermoso2, Climent Ramis2, and Romualdo Romero2 Jorge Lorenzo-Lacruz et al.
  • 1Department of Geography, University of the Balearic Islands, Palma, 07122, Spain
  • 2Department of Physics, University of the Balearic Islands, Palma, 07122, 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.

Jorge Lorenzo-Lacruz et al.
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Jorge Lorenzo-Lacruz et al.
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Latest update: 18 Jul 2019
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Short summary
On October 9, 2018, an extreme convective storm (> 300 mm accumulated in 6 h.) generated a flash flood (307 m3/s) in the Ses Planes torrent that devastated the town of Sant Llorenç (Mallorca, Spain). Water reached a depth of 3 m in the most affected areas, and there was greatly increased flow velocity at bridges crossing the town. The floodwaters were very powerful and modified the channel morphology: more than 5000 metric tonnes of sediment were deposited in the 2 km reach upstream of the town.
On October 9, 2018, an extreme convective storm ( 300 mm accumulated in 6 h.) generated a flash...
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