Using UAV‐derived point clouds to measure high resolution cliff dynamics in soft lithologies: Demons bluff, Victoria, Australia

Abstract Unoccupied Aerial Vehicles (UAVs) and Structure from Motion (SfM) photogrammetry have revolutionised data capture on the coast. In remote or hazardous areas, they enable the collection of morphological data from areas that are unable to be physically accessed, such as on vertical cliffs where active collapse is occurring. The digital surface models that are produced from vertical photogrammetry can, however, be limited in areas where cliff faces are uncut at their base, or the cliff top overhangs its face. To solve this issue, this study tests the applicability of UAV‐SfM‐derived point clouds for assessing cliff morphological change. A four‐year‐long timeseries, with a bi‐monthly sampling resolution (n = 29), was analysed for a 1.5 km stretch of vertical and overhanging sea cliffs, formed in soft clay of Tertiary age on the open coast of Victoria, Australia. The retreat rate for the upper half of the cliff face was 0.67 m/year (0.60 m 3 /m/yr), with nine high magnitude cliff‐top collapses, exceeding 1,000 m 3 (up to 9,500 m 3 ), occurring in the study period. Pre‐collapse deformation, namely seaward tilting of the face, was detected prior to 75% of collapses > 300 m 3 . Deformation was observed to occur in two ways. The first, preceding most large collapses (> 500 m 3 ), was caused by the expansion of tension cracks behind the cliff top. The second, associated with smaller collapse volumes (100–500 m 3 ), was initiated by rock slabs fracturing and cleaving away from the cliff face. An additional 14 instances of seaward displacement of cliff face have been identified that have not yet resulted in collapse. This study highlights the benefits and potential for using UAV‐SfM‐derived point clouds for the monitoring of hazardous cliff environments. Benefits extend from ease of data capture and generating extensive time series to the analytical insights it can provide. UAV SfM point clouds offer a promising low‐cost alternative to cliff monitoring compared to commonly used techniques such as Terrestrial Laser Scanning (TLS), especially in difficult‐to‐access areas.