Estimating the mass of tephra accumulated on roads to best manage the impact of volcanic eruptions: the example of Mt Etna, Italy

Abstract. Explosive eruptions release significant quantities of tephra, which can spread and settle on the ground, potentially leading to various types of damage and disruption to public infrastructure, including road networks. The quantification of the tephra load is, therefore, of significant interest to evaluate and reduce environmental and socio-economic impact, as well as for managing crises. Tephra dispersal and deposition is a function of multiple factors, including the mass eruption rate (MER), tephra characteristics (size, shape, density), top plume height (HTP), grain size distribution (GSD), and local wind field. In this work, we quantified the tephra mass deposited on the main road network on the east-southeast flanks of Mt Etna (Italy) during lava fountains that occurred in 2021, which reached heights of hundreds of metres. We focused on road connections of municipalities significantly affected by these events such as Milo, Santa Venerina, and Zafferana Etnea. First, we analysed a sequence of 39 short-lasting and intense lava fountains detected by the X-band weather radar, applying a volcanic ash radar retrieval approach that permits us to compute the main eruption source parameters (ESPs), such MER, HTP, and GSD. When radar measurements were unavailable for a specific event, we analysed images acquired both by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) radiometer and by the visible and/or thermal infrared camera of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (Catania), to derive the main ESPs. Second, we used the computed ESPs as inputs to run two different numerical models, Tephra2 and Fall3D, and to reproduce tephra dispersal and accumulation on the road network. Finally, we produce, for the first time, georeferenced estimates of tephra mass deposited on the whole road network of three municipalities, allowing us to identify the main roads which have been mostly impacted by tephra accumulation, as well as to estimate the total mass of primary tephra that has been removed from roads. Such information represents a valuable input for planning and quick management of the short-term tephra load hazard for future explosive events on Mt Etna.