Hydrodynamics of Four Moments in the Life of a Floodplain Forest in Compound Channels

Natural rivers often function as compound channels, comprising main channel and floodplain. Flow dynamics in these systems, particularly during floods, are influenced by factors such as vegetation, and flood intensity. Floodplain vegetation significantly modifies flow structures, affecting hydraulic conveyance and ecological functions. As floodplain forests develop through succession, their composition and hydraulic characteristics evolve, altering the flow in compound channels. This study examines the impact of forest succession, forest management and flood intensity on compound channel hydrodynamics. The forest composition was based on a forest in the Upper Rhine, and was heterogeneously distributed through the experimental flume. Our results revealed that forest aging reduces, due to the reduction in the leaf area index (LAI), velocity differences and shear, narrowing the mixing layer. Consequently, lateral mass exchanges decreased and a hydraulically smoother and more uniform flow was created in the floodplain, with implications for hydraulic modeling. Forest management practices, such as selective vegetation removal, can significantly alter flow hydrodynamics, particularly water depth. Flood intensity influences the peak mean transverse velocity, backwater effect, and lateral discharge distribution. Solute dispersion remains mainly advective in the main channel, with flood intensity exerting limited influence. In contrast, vegetation enhances tracer dispersion within the floodplain and at the channel–floodplain interface. The study highlights the limitations of traditional analytical models and emphasizes the need for approaches that incorporate natural vegetation distribution, such as the one developed here. These findings underscore the importance of integrating forest succession into river management to maintain flood protection and navigability.