Large Scale Rockfall Reach Susceptibility Maps in La Cabrera Sierra (Madrid) performed with GIS and Dynamic Analysis at 1:5,000

Abstract

La Cabrera Sierra is a granitic paleozoiczone homogeneous in lithology andmorphology 5 km long and 60 km from Madridwith rockfall problems. Mostof the maps used in common practice, aremainly maps of susceptibility toinstability or failure. When rockfall takesplace, the main problem for theexposed elements at risk is the block reachdue to high mobility of rock-blocks,thus the susceptibility to take into accountmust be the reach susceptibility, beingthe run-out, the key parameter for mapping.Several kinds of reach susceptibilitymaps have been developed for land-use planningin this case according with thesusceptibility philosophy, an approach to thecomplex problem of slope movementsfrom field observations. The Map of MaximumRun-out represents the susceptiblezone as the one between the top of scarp andthe maximum reach achieved by mostmobile fallen blocks. The Map of MaximumAbsolute Run-out represents the susceptiblezone as the one between the top of scar andthe maximum run-out achieved by any blockextended to all the area. The Map of MinimumReach Angle, an index used for mobilitycharacterization of slope movements, representsthe absolute minimum value achievedby a block extended to the entire zone. TheMap of Reach Susceptibility by DynamicAnalysis is a geotechnical map, representingreach susceptibility for the maximum blocksize (maximum severity) computed with dynamicsoftware. All maps have been performedwith the aid of a GIS, taking into accountthe discussions about the optimal use of thesesystems. The legends have been carefullydesigned for the exploitation of possibilities,due to working with large scales for saferland use planning. Comparisons among thesetypes of maps from the point of view offeasibility-risk analysis-economy are performed,and also reflections about the limits of GIS inthese cases as well as conceptual problems.