Estimating gully volumes and erosion rates using UAV photogrammetry and lidar in the Salagou Valley, France

Abstract

Gully erosion is a significant form of land degradation globally, with profound on-site impacts such as soil losses and off-site impacts like increased flood risk. Accurate assessment of gully erosion rates and volumes is crucial for understanding landscape evolution and the development of mitigation strategies. This study assesses erosion rates, gully volumes and DEM (Digital Elevation Models) accuracies of four gully systems in the Salagou Valley, France, using both UAV photogrammetry and lidar (Light Detection and Ranging) data. Gully volumes were estimated by reconstructing the pre-erosion footslope using the Spline Interpolation Method and generating DEMs of Difference (DoDs) with the 2024 SfM (Structure from Motion) and 2022 lidar DTMs. From this, erosion rates since the Last Glacial Maximum (LGM) were determined, and short-term erosion rates based on DoDs for the periods 2013–2024, 2013–2022, and 2022–2024 were compared. Results showed that gully erosion rates since the LGM range from 0.05–0.27 cm/yr, while calculated short-term erosion rates (0.5–8 cm/yr) were an order of magnitude higher, likely due to DEM alignment issues and resolution. Volumes ranged from 10,047 m³ to 1,515,815 m³ across the four gullies due to topographic differences influencing erosion rates and development. The Vailhes gully displayed lower erosion rates and volumes due to its gentler slopes and smaller drainage area compared to the other gullies. A comparison of UAV SfM and lidar volume estimates showed minimal differences (0.7–3.67%), which were likely caused by small DoD errors at the gully edges. Absolute DEM XY accuracies were found to be 0.153 m and 0.117 m, while absolute Z accuracies ranged from 0.074 m to 0.352 m, which were all below the desired 0.02 m. The results gave useful insights into current and past gully development speeds and imply that the gully systems will continue to grow in size and volume in the future. This will be accelerated by more extreme events and drier and hotter summers in the future, as more rainfall and drier soils lead to enhanced surface runoff. As a result, negative impacts such as floods and damages to infrastructure can occur more frequent, which even more so underlines the importance of monitoring gully erosion.