Rockfall hazard and risk along the coast of Ibiza (Balearic Islands, Spain)

The coast of Ibiza is characterized by a large number of small coves and pocket beaches, impended by cliffs carved in relatively weak rocks such as calcarenites and marls. Due to its structural, geomorphological and lithological characteristics these cliffs are subjected by the widespread occurrence of rockfalls. Despite their moderate magnitude, these represent a major threat to the safety of tourists during the long touristic season of the island. This threat has increased in the last decades, as the island of Ibiza has become one of the major tourist destination in Europe. The management of rockfall risk is particularly complex, since risk varies locally according to both the hazard at the sites and the number of tourist attending the different beaches.

In this perspective, we performed an island-wide high-resolution 3D rockfall simulations, exploiting the capabilities of the Hy-Stone rockfall runout model along 210 km of the Ibiza coastline, in order to characterise rockfall risk at regional scale. Rockfall source areas have been identified using a morphometric approach with a slope threshold value of 50° obtained by a 2x2 m Lidar, further refined by mapping rockfall evidence and additional unstable spots. In order to characterize the slope surface and its interaction to rockfalls, as a basis for model parametrization, we combined lithology and land use. The first was classified with a geotechnical approach based on the average value of resistance to simple uniaxial compression into thrre classes: “weak rocks”, “moderately – hard rocks” and “hard rocks”, along with the Quaternary deposits. The land use has been classified into 8 classes, including forested, non-forested, and urban areas. The calibration of the restitution and friction surface parameters was obtained by back analysis of the 2017 rockfall in Es Cubells, for which field-based evidence was collected. The results of the rockfall simulations have been used within a multicriteria risk assessment by adopting the AHP weighting methodology. In addition to the results of the models, the multicriteria analysis includes indicators related to number of tourist and the presence of buildings, both obtained by a dataset provided by the Emergencies General Management and Interior of the Government of the Balearic Islands. The multicriteria analysis made it possible to rank the different beaches according to their rockfall risk, thus contributing to the risk management and mitigation plan strategies of the sites.

Two of the most at risk sites, Es Cubells and Cala d’Hort, were further simulated at the local scale, based on high-resolution data collected thought UAV survey and field activity.

In conclusion, this research combined robust 3D simulations and detailed field data to characterize rockfall hazard both at regional and local scale for the Ibiza coastal cliffs. Moreover, through the multicriteria analysis it provides a qualitative risk estimation that allows the optimization of the risk management and planning for the beaches of the island.