Debris flow at Luib on the A87 strategic road: Isle of Skye, Scotland

Abstract

A rainfall-induced debris flow occurred on 30 September 2022, closing the A87 trunk (strategic) road on the Isle of Skye for around 2 h. Ground-based and aerial drone inspections were used to describe the event and a comprehensive rainfall analysis was undertaken. It is concluded that the event was triggered by overland water flow from the slopes above the source zone as a result of high rainfall. The resulting translational slide transitioned into a debris flow that reached a pre-existing channel where entrained superficial deposits were deposited, adding to pre-existing levee structures before reaching the road. The rainfall that led to the debris flow was high intensity–low duration; the rainfall analysis encompasses annual, monthly, daily, hourly and 15 min data, each aiding understanding of the climate of the area and/or the event rainfall. Comparison is made with a rainfall event the following month, which was of a greater magnitude but of longer duration and lower intensity. The hazard at this location is a concern but the risk is emphasized by the proximity of the road to the steep slopes above and the lack of a viable detour.