Monitoring Changes in Channel Morphology on the Orljava River Based on UAV and GNSS Surveys (Požega-Slavonia County, Croatia)

This study investigates morphological changes of distinct meander locations along the Orljava River, using multi-temporal imagery acquired by an unmanned aerial vehicle (UAV) and GNSS surveys since March 2022. The selected sites in the river’s middle and lower course without bank protections show high channel dynamics. High-resolution digital surface models (DSMs) with a pixel size of 5 cm and orthomosaics with a pixel size of 1 cm were generated using Agisoh Metashape sohware. Ground control points and channel cross-section measurements were obtained using real-time kinematic (RTK)-GNSS. Various morphological features were analysed, including bank retreat, channel migration, fluvial sediment bars, and changes in bed level. Automated extraction of riverbank lines covered by low, sparse vegetation was performed using DSMs and convolution functions in ArcGIS Pro, while riverbanks covered by trees were assessed based on orthomosaics and digitised manually. The results showed that the most significant channel changes occurred following a flood event in December 2022. On certain meanders with riverbanks covered by sparse vegetation, the annual bank retreat rate amounted to 10–20 m. In addition, cross-sectional comparisons with data from 2012 revealed an increase in depth of approximately 1 m in the location upstream of the breached mill weir, indicating local bed incision. Although lateral channel migration presents a natural behaviour of actively meandering rivers, recently increased bank erosion is a pressing issue for local farmers due to the proximity of agricultural lands to the riverbanks.