The application of UAV Digital Photogrammetry for the investigation and mapping of slow-moving landslides

Research on lateral spreads and block slides requires long-term investigations due to their extremely slow speeds. Their investigation and mapping need extensive field surveys, which are time consuming and expensive tasks. The aim of this study is to demonstrate the advantages of using commercial UAVs weighting less than 1 kg as support for mapping landforms produced by lateral spreads and block slides, which are abundant along the North sector of Malta. In particular, attention was given to the inventory and analysis of persistent joints and megaclast deposits at four sites due to the presence of spectacular examples of lateral spreads that evolve into block slides. Lateral spreads produce persistent joints that partially isolate large blocks along the edge of limestone plateau. Thousands of these blocks are detached and moved downslope from block slides producing peculiar landscapes named Rdum by Maltese. The research was performed by means of Google Earth (GE) image interpretation and UAV-Digital Photogrammetry technique. UAV-DP outputs permitted the identification of tens of persistent joints that populate the external portions of karst plateau, and the size categorization of blocks detached and moved from block slides. With reference to gravity-induced joints, a favorable agreement was found between existing datasets (results of field surveys combined with GE anlysis) and UAV-DP outputs in terms of the identification of joints and their persistence values. Regarding the extensive megaclast deposits, UAV-DP enabled the identification of 8943 individuals which, compared with the 5059 individuals identified by GE analysis, showed an increase in the total population of 76%. This is related to the high accuracy of DP-derived orthomosaics and 3D models, which are particulary useful for identification of detached blocks. The inexpensive technique (approximately 2k USD for drones plus 3k USD for software) used in his study shows the potential for being extended to other rocky coastal sites affected by these types of slow-moving landslides.