Lahar Risk Assessment: the Case Study of Vulcano Island, Italy

Abstract

Lahars are rapid flows composed of water and volcaniclastic sediments, which have the potential to impact residential buildings and critical infrastructure as well as to disrupt critical services, especially in absence of a hazard-based land-use planning. Their destructive power is mostly associated with their velocity (related to flow rheology and surrounding topography) and to their ability to bury buildings and structures (related to the deposit thickness). The distance reached by lahars depends on their volume, on sediments/water ratio, as well as on the overall characteristics of the path where they propagate. Here we present a novel strategy for the assessment of risk associated with lahar inundation related both to flow velocity and deposit thickness using Vulcano island (Italy) as a case study. First, a range of hazard scenarios has been identified that are related to the mobilization by intense rain events of tephra fallout deposited on the slopes of the La Fossa cone by a future Vulcanian eruption. Second, a numerical model has been used to identify the potential lahar impact areas on the northern sector of Vulcano, where both residential and touristic facilities are present. In this specific case we have used the Smoothed Particle Hydrodynamic (SPH) model that provides information on both flow velocity and deposit thickness. Finally, exposure and vulnerability surveys were carried out in order to compile risk maps for both lahar-flow velocity and final lahar-deposit thickness. Our analyses show the importance of carrying out accurate and detailed risk assessments exploring a variety of initial conditions in order to best quantify the potential damage and identify suitable mitigation strategies.