Hydrometeorological Analysis of an Extreme Flash-Flood: The 28 September 2012 Event in Murcia, South-Eastern Spain

Heavy precipitation following a prolonged summer drought led to widespread flash flooding across Andalucía, Murcia and Valencia in south-eastern Spain on 27, 28 and 29 September 2012. On September 28, an extreme flash-flood developed when 214 mm of rain fell in 8 h over the semi-arid and medium-sized Guadalentín River basin up to Paretón (~2800 km²). Six fatalities were reported, hundreds of homes were evacuated and a bridge spanning an ephemeral channel was undermined as the flood bore routed through normally dry river beds. Current estimates of flood damage are of €64 million, including extensive losses in livestock and agriculture. The last event of this magnitude over the Guadalentín occurred on 19 October 1973. Availability of high-resolution rainfall estimates from dense rain-gauge networks and radar observations, together with flood response observations derived from stream-gauge data and post-event surveys, provides the opportunity to study the hydrometeorological mechanisms associated with the responsible convective systems and the associated flash-flood. Results show that the basin faced a very rare rainfall event with extreme intensities and accumulations which, in combination with the catchment properties, led to extreme runoff. The distinct soil substrates and basin morphology led to varied runoff responses that required a multisite calibration of a hydrological model so as to successfully reproduce this flash-flood. Heavy precipitations resulted from deep convection triggered by local orography as well as the subsequent passage of a slow-moving mesoscale convective system (MCS). The motion of the MCS was crucial for exacerbating peak discharges, whereas times to peaks were modulated by the river network geometry and the temporal distribution of the rainfall rates. The roles of the different anthropogenic activities on the mitigation or intensification of the hazardous effects of this extreme event are also highlighted. Finally, some recommendations are proposed in order to mitigate future impacts of such catastrophic floods in a changing climate era.