Kinematic analysis of sea cliff stability using UAV photogrammetry

The coastal systems of East Sussex historically have been exposed to coastline retreat and sea cliff instability processes. Under this scenario, it is important to understand, quantify and model potential modes of slope failure, as it is a required stage in mitigating cliff instability hazards. Small unmanned aerial vehicles (UAVs) or drones are new types of aerial platforms from which high-resolution remote sensing measurements of landforms can be obtained. This research uses close range digital photogrammetry from an UAV to perform a kinematic slope stability analysis of chalk sea cliffs located at Telscombe, United Kingdom. The overall technique for data collection involved installation of a megapixel full frame digital camera on board an octocopter. Ground control for the survey was conducted using dGPS and total station surveying. The digital photogrammetry processing was undertaken in 3DM Analyst Mine Mapping Suite software. 210 individual Digital Terrain Models with a computed image accuracy of 0.25 pixels and a standard error around the control network of 0.13m were obtained. Rock mass discontinuities such as joints, faults and bedding planes were then manually mapped on the DTMs. These data were then used to assess differing modes of slope failure using stereographic projections for kinematic analysis. The results show that wedge failure is by far the most likely mode of slope instability, since 39% of the discontinuity intersections are favourable to wedge collapse occurring. Planar sliding is the second probable mode of slope failure, comprising 8% of all mapped joints. These types of cliff collapses are consistent with the dominant types of failures determined from other studies carried out on the Chalk cliffs between Brighton and Newhaven. The findings provide further understanding and numerical data about potential modes of cliff failure for the Newhaven Chalk of Sussex, and demonstrated the usefulness of using UAV photogrammetry for examining coastal geo-hazards.