Floodplain Vegetation Density Effects on Meandering Channel Flow Patterns and Channel-Floodplain Hydraulic Exchange

Channel-floodplain flow interactions are strongly influenced by riparian vegetation characteristics. However, due to the complicated relationships among floods, vegetation, and channel morphodynamics, these interactions are poorly understood. In this study, we investigate the influence of the density of rigid floodplain vegetation on flow in meandering compound channels through a series of laboratory flume experiments. We observed the flow field in a scaled meandering compound channel with three floodplain vegetation densities (0, 3, and 12.1 stems/m²) and three floodplain relative depths. We measured the 3D components of velocity using an ADV at 10 cross sections through one half-meander and across the water surface using LSPIV. We found that increased floodplain vegetation density promotes flow steering and enhanced secondary currents within the channel. High-density vegetation attenuates channel-floodplain exchange by over 60% compared to the bare floodplain condition at high floodplain relative depth. We describe three mechanisms that drive complicated flow structure within the channel: (a) helical flow arising from channel planform and bed topography, (b) a horizontal shallow mixing layer at bankfull height over the channel, and (c) a vertical shallow mixing layer at bankfull height along the channel-floodplain interface. We find that increased vegetation density enhances mechanisms 1 and 3 and mechanism 2 leads to less coherent secondary flow with an unvegetated floodplain. These findings suggest that riparian vegetation is a strong driver of meandering channel flow dynamics, which should be considered for best practices in riverscape management.