Quantifying geomorphic change in Andean river valleys using UAV-PPK-SfM techniques: An example from the western Peruvian Andes

Abstract

The western Peruvian region is prone to erosion and geomorphic change. Extreme precipitation events lead to rapid change in river channel and floodplain morphology due to bank erosion and debris flows delivering detrital material to the fluvial system. Monitoring geomorphic events and their associated topographic changes at high spatial and temporal resolutions remains a challenge. Here, we used an Uncrewed Aerial Vehicle - Post-Processing Kinematic - Structure from Motion (UAV-PPK-SfM) approach that includes co-registration of point clouds by using relative Ground Control Points (GCPs). This workflow adjusts each elevation model to a reference model using invariant features that did not change their position or form over time. We applied this technique to monitor landscape change (2019–2021) in an area of 0.3 km2 located in the Cañete River basin. Our results showed that a minimum observable elevation change of 0.56 m (95 % confidence interval) can be achieved using this workflow, beyond which an actual elevation change can be separated from systematic error. Using object-based classification techniques on the aerial images, we separated geomorphic dynamics from land cover changes. This allowed us to isolate the effect of geomorphic processes, and quantify rates related to gully erosion, river scouring, bank erosion, and sediment deposition. Within the study area, a hotspot of geomorphic change corresponded to an ephemeral tributary channel. The gully channel incising an alluvial fan is highly dynamic, showing bank erosion of 0.75 to 3.2 m and net export of 37 m3 of sediment in the 25-month study period. Given that the monitoring period did not include high intensity rainfall events, the study illustrates how geomorphic activity in ungauged Andean river basins, such as the Cañete valley, may be considerably underestimated in literature.