Defining morphological units from UAV images to study the evolution of bars in an active meandering gravel bed river

In an active meandering river, gravel bars are in continuous evolution, allowing for the renewal of habitats that are extremely valuable for both faunistic and floristic biodiversity. The presence and characteristics of these bars are directly related to discharges variability in flow rates and bedload supply and to the lateral mobility of the bed. In low-energy rivers, the equilibrium on which these dynamics rely is fragile and is often disturbed by anthropogenic activities. To better understand these relationships, we studied five river reaches of the Armançon river, a low energy but still very dynamic gravel bed river of central France. Overflight campaigns using Phantom 4 RTK drones were carried out annually between 2021 and 2025 to collect high-resolution photogrammetric data on twelve bars and associated opposite banks. The surfaces of the bars were divided and classified into 10 different morphological units (MUBs) based on the local slope, tangential curvature and elevation from the water level. The evolution of these MUBs as well as the size and volume of the bars, the particle size on bars and the river sediment mobility were monitored over the 4 years of the study. These factors were then linked to the river hydrology and the vegetation evolution on the bars. Our results show a progressive evolution of the bars and their MUBs, well correlated with bank retreat during the first twoyearsofthestudy.This was followed by a massive transformation of the bars but relatively limited bank retreat in the third year, following a 20-year return period flood that occurred in May 2024. Through this study, we also demonstrated the benefits of defining Morphological Units on Bars (MUB) to better describe the evolution of the bars and understand their progressive stabilization and encroachment by vegetation.