Comparing heuristic landslide hazard assessment techniques using GIS in the Tirajana basin, Gran Canaria Island, Spain

Abstract

As part of the EU Environment and Climate Programme's RUNOUT project dealing with the modelling of large-volume landslides, a GIS database was compiled and used to generate mass movement hazard maps at a medium scale (1:25,000) in a high-relief area in central Gran Canaria Island, Spain. The Barranco de Tirajana study area is a 49 km2 large depression that is semi-oval in plan, 11 km long and 6.5 km wide. Its base presents a very irregular topography and it is almost completely enclosed by large rock scarps, up to 350 m high, with total altitude differences reaching 1600 m from the lowest part of the Barranco de Tirajana river to the upper scarps. The Barranco de Tirajana depression is composed of a series of large landslide masses, derived from gravitational sliding of lava flow and volcanic breccia sequences. The landslides are believed to have originated during intensive erosive periods during the Quaternary, as a consequence of the rapid deepening of the central ravine. These primary large landslide bodies have undergone a number of reactivation episodes, from the Middle Pleistocene to the present, as well as retrogressive enlargement of the depression. Currently the most active processes are rockfalls, and reactivation of the landslide toe areas, due to further undercutting by the streams. In order to evaluate the present mass movement hazard, a GIS-based study was carried out using two different types of knowledge-driven approaches: a direct method and an indirect method. In the direct method very detailed geomorphological mapping was carried out, using uniquely coded polygons, which were evaluated one-by-one by an expert to assess the type and degree of hazard. The indirect method followed an indexing approach. Parameters including slope angle, landslide activity, landslide phases, material, proximity to drainage channels and reservoirs, and land use change were combined using multi-criteria evaluation techniques.