Creating a national scale debris flow susceptibility model for Great Britain: a GIS-based heuristic approach

Abstract. Debris flows in Great Britain have caused damage to transport infrastructure, buildings, and disruption to businesses and communities. This study describes a GIS-based heuristic model developed by the British Geological Survey (BGS) to produce a national scale spatial assessment of debris flow susceptibility for Great Britain. The model provides information on the potential for debris flow occurrence using properties and characteristics of geological materials (permeability, material availability and characteristics when weathered), slope angle and proximity to stream channels as indicators of susceptibility. Building on existing knowledge, the model takes into account the presence or absence of glacial scouring. As determined by the team of geologists and geomorphologists, the model ranks the availability of debris material and slope as the two dominant factors important for potential debris flow initiation, however it also considers other factors such as geological controls on infiltration. The resultant model shows that over 90 % of the mapped debris flows in the BGS inventory occurred in areas with the highest potential for instability and approximately 6 % were attributed to areas where the model suggested that debris flows are unlikely or not thought to occur. Model validation in the Cairngorm Mountains indicated a better performance, with 93.50 % in the former and less than 3 % in the latter category. Although the quality of the input datasets and selected methodological approach bear limitations and introduce a number of uncertainties, overall, the proposed susceptibility model performs better than previous attempts, representing a useful tool in the hands of policy-makers, developers and engineers to support regional or national scale development action plans and disaster risk reduction strategies.