Landslide hazard and risk level assessment of quarried slopes in Lebanon using drone imagery

Abstract

Human activities are continuously altering the geometry and steepness of natural slopes. When left exposed and unprotected, these slopes become susceptible to slide due to natural triggering factors like earthquakes and rainfall. Quarrying activities, in particular, contribute largely to slope failures worldwide, especially when unorganized and chaotic. In Lebanon, quarries are scattered randomly across the country and lack proper urban planning and management. Regional scale maps have been recently generated in a Geographic Information System (GIS) platform to identify hazard and risk areas for co-seismic and rainfall induced landslides; however, their applicability to altered “quarried” slopes has not been tested. In this research, we present a thorough methodology to assess, at a site-specific level, the hazard and risk levels of quarried slopes under three conditions: dry condition, heavy rainfall condition, and seismic condition. The aim is to ascertain the degree of accuracy of the regional scale maps in predicting landslides in quarried areas. A jointed limestone quarry in Bafliye, South of Lebanon, was mapped with a DJI Phantom 4 V2.0 drone, recreated as a 3D scene, assessed kinematically, and then analyzed using limit equilibrium and numerical modelling techniques. At failure, the maximum runout distance and the corresponding angle of reach were determined. The studied slope showed signs of failure under seismic events having a 10% chance of exceedance in 50 years, and under rainfall events having a 10-year return period with a relatively small runout distance of 7.76 m, yielding a low risk failure. The output of safety factors matched between the site specific and the regional scale analyses, while the failing mass volume and the corresponding risk levels did not. This indicated that the wedge failure analysis used in the regional scale analysis is adequate in predicting only hazard levels at quarried sites.