On the joint impact of marine (storms) and terrestrial (flash floods) extreme events along the Catalan coast (NW Mediterranean)

The Mediterranean coastal zone is a hotspot to the impact of extreme events due to the combination of high values at exposure (concentration of population, large urban areas, infrastructures), large vulnerability (natural protection provided by beaches decreasing due to coastal erosion) and presence of extreme hydro-meteorological events of marine (storms) and terrestrial (flash floods) origins. The Catalan coast (NE Spain) can be considered a paradigm of Med hotspots. On one hand, due to its climatic conditions, orography and land use, flash floods are one of the main causes of inundation risks in the coastal fringe, inducing numerous damages and even casualties (e.g. Llasat et al. 2013). On the other hand, coastal damage associated to the impact of marine storms have been increasing during the last decades along this coast (Jiménez et al. 2012). However, existing studies have not analysed their joint impact to assess the most hazardous conditions, when the coastal zone would be subjected to the combined action of both types of extreme events.

Within this context, this works analyses the combined presentation of extreme events of terrestrial (flash floods) and marine (storms) origin in the Catalan coast. First, extreme events causing significant damage (based on reported damages, insurance costs and casualties) along the coast were identified for the period 1981-2014. 69 events were identified and classified according their origin (marine and/or terrestrial). Each event was characterized in terms of their marine (wave height, period, direction, storm duration) and rainfall characteristics. Since the coastline length is about 600 km, these events verify at specific locations. To cover this spatial variability, storms were locally characterized by using data from existing rain gauges and radar stations along the territory as well as hindcasted wave conditions along the entire coastal fringe. To fully characterize these events, synoptic conditions were also recorded.

From this, first, we directly obtained the corresponding marginal probabilities of each event. Then, compound frequencies were assessed and compared to the marginal ones. Finally, we identified synoptic situations with higher probability of associated compound hazards and bound the range of corresponding wave and rain conditions. By jointly considering the location where they verified, we identify coastal areas (and corresponding geomorphologic conditions) with higher probabilities of suffering damages due to impact of compound extreme events.

This work was carried out within the framework of the M-CostAdapt (CTM2017-83655-C2-1-R) research project, funded by the Spanish Ministry of Economy and Competitiveness (MINECO/AEI/FEDER, UE).

Jiménez et al. 2012. Storm-induced damages along the Catalan coast (NW Mediterranean) during the period 1958–2008. Geomorphology 143, 24-33.

Llasat et al. 2013.  Towards a database on societal impact of Mediterranean floods within the framework of the HYMEX project. NHESS, 13, 1337-1350.