Comment on “Assessing gully erosion and rehabilitation using multi temporal LiDAR DEMs: Case study from the Great Barrier Reef catchments, Australia” by Khan et al., 2023

Abstract

Geomorphic change detection (GCD) using high resolution topographic data can provide important insights into geomorphological systems. However, considerations must first be given to the mechanisms and dynamics producing landscape change when considering an appropriate experimental design. Khan et al. (2023) investigate gully erosion rates and processes in different untreated and rehabilitated gullies using multi-temporal aerial lidar survey (ALS) data. However, an inappropriate time interval between sampling, a lack of uncertainty measures and lack of baseline monitoring survey data lead them to arrive at incorrect conclusions. Additional data is presented from the same field sites, which demonstrate gully sediment losses have been underestimated by at least 330% and potentially over an order of magnitude. A number of critical shortcomiongs of the paper are outlined. Insufficient time intervals between data collection have led to a lack of detection of some sediment transport processes. Earthworks associated with gully rehabilitation have been conflated with geomorphic change, as no post-construction baseline data was collected. A lack of post-construction baseline data for this analysis means ongoing erosion and deposition cannot be resolved in the rehabilitated gully landscape. Given these errors in approach, discussions of gully geomorphic processes, erosion mechanisms and evaluations of rehabilitation efforts are unsupported, overstated and inaccurate. This has important implications for land management efforts and planning as well ongoing research on alluvial gully erosion, which is largely overlooked by Khan et al. (2023).