Flow Structure and Turbulence Characteristics in Ecological Rivers: Effects of Submerged Vegetation Patch Quasi‐Static Lodging Posture

ABSTRACT In river systems, vegetation undergoes varying degrees of a quasi‐static lodging posture under flow conditions. Under steady flow conditions, after reaching dynamic equilibrium with the ambient flow, the vegetation maintains a relatively stable quasi‐static lodging posture. This process significantly alters the flow–vegetation interaction mechanisms. To investigate the impact of submerged vegetation patch lodging morphology on the flow characteristics of engineered rivers with ecological objectives, taking the quasi‐static lodging posture as a conceptual basis, this study conducted numerical simulations of rectangular vegetation patches parameterized by rigid cylinders with four different prescribed lodging morphologies (0°, 20°, 40° and 60°) under submerged conditions based on the three‐dimensional Reynolds Stress Model (RSM). The reliability of the model was verified through flume experiments, and the effects of lodging morphology variations on velocity distribution, wake characteristics and turbulent kinetic energy were systematically analysed. The results indicate the following: (1) For upright vegetation patches, the flow velocity distribution at different submerged positions of the vegetation layer varies significantly, and the vegetation lodging effect plays an important role in effectively weakening this difference in flow velocity distribution. (2) Increasing lodging morphologies substantially reduces patch flow resistance, leading to enhanced internal velocity, weakened lateral flow divergence and shortened wake scale and velocity recovery length, thereby systematically regulating spatial heterogeneity in the local flow velocity field. (3) As the lodging morphology of vegetation increases, the turbulent kinetic energy of water flow within and behind it decreases significantly, which is not conducive to the construction of a highly heterogeneous spatial turbulence field. This study clarifies the quantitative relationship between vegetation lodging morphology and hydraulic factors and provides basic hydraulic parameters and preliminary theoretical references.