LSPIV analysis of large wood effect on the direction and velocity of surface flow in meandering river

Large in-channel wood, typically present in meandering rivers, serves as an obstacle in the water course, affecting hydraulics of the water flow. It is an important but so far overlooked agent in river morphodynamics. One of the challenges in predicting the hydraulic impact of large wood is the scarcity of field data; this issue is typically explored through laboratory flume experiments. To analyse the effect of large wood on the distribution and velocity of water flow we conducted LSPIV analysis on selected channel segments of meandering Odra River (Czechia). Large Scale Particle Image Velocimetry (LSPIV) is a remote image-based technique, enabling monitoring of the direction and velocity of surface flow in a river channel segment. LSPIV employs a method of tracking surface particles visible in sequential images extracted from video recordings of river water levels. These recordings are captured from an aerial perspective, either oblique or vertical. For our study, we utilized UAV to record 30-second vertical video segments of river sections during periods of both low and high discharge. Additionally, we implemented ground control points and reference scales along the river banks to enhance the accuracy and scale of our measurements. For the LSPIV analysis, we used free Fudaa-LSPIV software (INRAe). As we anticipated, our findings indicate that the impact of large wood on surface flow is contingent on two primary factors: the size of the wood and its position within the channel. However, we observed that this effect significantly varies across different flow stages. We observed that large wood effectively redirects water flow. According to its position in the channel cross-section, it is either preventing the erosional banks from lateral erosion, or accelerating the flow towards the bank and supporting lateral erosion. As discharge increases and large wood becomes submerged, its effect diminishes. During low discharges, stabilization effect of large wood is more important, creating calm water areas and supporting sediment accumulation. Our research offers a comprehensive framework for advancing the qualitative and quantitative evaluation of the hydraulic and morphodynamic effects of large wood in meandering rivers. Such insights are crucial for guiding sustainable river management and informing river restoration projects.