Amponsah W., Borga M., Marchi L., Nikolopoulos E.I., Marra F., Crema S., M. Pirastru, Zoccatelli D, Niedda M., Marrosu R., 2014, The flash-flood of November 2013 in NE Sardinia (Italy): Post-event documentation and hydrological modelling, 8th HyMeX Workshop, La Valletta, Malta, 15-18.08.2014,
URL: http://www.cnr.it/prodotto/i/297195

On November 18, 2013, the North-Eastern part of Sardinia was impacted by extreme flash flooding, which killed 19 people and caused up to 1 billion Euro damage. The flooding was associated with an extratropical cyclone in the western Mediterranean Basin, named Cyclone Cleopatra, which developed slow-moving embedded thunderstorm complexes, as cold air flowing from the north entered the Mediterranean and interacted with warm moist air to the east. The impacted area is up to 1500 km2, and includes a number of basins, including the Posada ( 690 km2) and Cedrino (625 km2) basins. The cyclone brought extremely heavy rain to the area, with up to 450 mm of rain in 12 hours (from 07_00 to 19_00 local time), resulting in severe flooding. A high concentration of fatalities and damages occurred in the area around the northeastern city of Olbia, also due to high vulnerability resulting from inappropriate urbanization. Post-flood documentation, focused on the Cedrino and Posada river systems, included the reconstruction of peak discharge by means of topographic surveys, observations on the geomorphic effects of the event both in the channel network and on hillslopes and assessment of the timing of the flood based on the accounts of eyewitnesses. Technical services of regional authorities and local administrations contributed to the documentation of the flood by providing hydrometeorological data, accounts of volunteers of the civil protection and photos taken during and immediately after the flood. The assessment of unit peak discharges confirmed the severity of the flood, with values up to approximately 10 m3s-1km-2 in catchments up to 500 km2. The strong spatial gradients of the precipitation had a major influence on flood response, with large difference in peak discharge between neighbouring catchments. The availability of high-resolution rainfall estimates from radar observations and a rain gauge network, together with indirect peak discharge estimates from the post-event survey, provided the opportunity to study in detail the hydrometeorological and hydrological mechanisms associated with this extreme storm and the ensuing flood. Observations and model results are combined to examine two main questions, (i) assessment of the distribution of the runoff ratio and how it is controlled by the heterogeneous geological structure; and (ii) analysis of how the spatial and temporal distribution of the extreme rainfall, and more specifically storm motion, controls the flood response.

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