The meteoric event of 30th April 2006 at Ischia Island, Italy

Abstract

At 6.30 (solar hours) of 30th of April 2006, the northern slope of Monte di Vezzi, at Barano d’Ischia, was strongly affected by a succession of landslide events causing four victims. Barely 14 mm of rain fell on 29th and 30th April, immediately before the landslide event, as recorded by the meteorological station at Forio d’Ischia (contrada Calitto, at 150 m asl) 6 km away from the landslide. Some hours after the landslide, the station also monitored rainfall of 16 mm. However, the many reliable testimonies of both residents and tourists as well as clear signs of sliding and stagnation of water suggest that the effective rain falling around Monte di Vezzi was very intense and not recorded simply due to the lack of monitoring stations. Indeed, analysis of 500 hPa geopotential maps and of infrared and visible satellite images show the Southern Tyrrhenian sea broadly influenced by strong convective phenomena, able to generate intense but localized storms. The island of Ischia has been frequently devastated by landslides caused by sudden movements of its pyroclastic mantle that interacts with its alcalitrachitic and sedimentary bed in a complex way depending on its water content, broadly related to rainfall immediately prior to the event and to rain falling over a sufficiently long interval before the event. The complex orography of the island is responsible for a different rainfall regime, with the north-eastern sector more rainy overall than others. Thus the only way to mitigate the high hydrogeological risk at Ischia lies in the installation of a dense network of strategically distributed rain monitoring stations.