Open Access

Juan David Mercado Leal

• This research aims at evaluating the use of drone (UAV) imagery in the monitoring of hydromorphological changes in a segment of Erft River in Neuss-Gnadental, which has undergone restoration. In four flight campaigns from February 2022 to November 2023, UAV images were processed with open-source photogrammetry and point clouds processing software (WebODM and CloudCompare) to create 2D orthophotos and 3D point clouds. These products supported delineation of the channel and calculation of geometric properties such as the length, sinuosity, and number of meanders which indicated good correspondence with restoration design objectives and continuous morphological changes like erosion, sediment bars, and woody debris. Hydromorphological quality at the segment scale was assessed by employing an adapted LAWA protocol using only the 2D orthomosaics and 3D point clouds. Analysis showed that restoration of the river segment produced improvements in characteristics such as flow variety, substrate type, and banks while the near “technical” parts of the segment experienced small alterations. The UAV flights were conducted frequently, enabling documentation of seasonal fluctuations that might not have been observed if the surveys had been carried out using satellite imagery. Certain advantages included centimetre-resolution imagery, flexibility in flight schedules, and visuals friendly to stakeholder impact. Nevertheless, there were specific workflow challenges. Utilising photogrammetry on open-source tools had high computational costs and required hands-on troubleshooting. Vegetation overshadows complicated semi-automated classification routines and requires manual refinement for reliable channel delineation. However, the approach proved effective for high-resolution, local-to-reach-scale monitoring, strengthening adaptive, data-driven management of river restoration sites. Subsequent campaigns should include data from multiple sensors (LiDAR, multispectral), the inclusion of classification algorithms, and longer, repeated surveys to evaluate the hydromorphological characteristics under different flow conditions. In summary, UAV imagery enhances hydromorphological monitoring and provides a powerful yet still-developing tool for science-based evaluation of restoration efforts.