Secondary Flow Characteristics in Confluent Streams with Vegetation

Vegetation commonly exists in river confluences and alters flow structures, affecting the strength and direction of secondary flows. This study investigated the impact of vegetation on the secondary flow structures of a confluence by measuring three-dimensional instantaneous velocities under different vegetative conditions. By utilizing downstream and horizontal flow velocity characteristics, a secondary flow coefficient concept was introduced to describe secondary flow vorticity directions and strengths. The influence of vegetation was assessed in the context of Reynolds stress, the relative strength of secondary flows, and the direction of secondary flow vorticities to clarify the mechanisms by which vegetation affects secondary flows. The findings reveal opposing vertical forces in the confluent section due to incoming tributary flows, resulting in secondary flow generation. Vegetation redistributes cross-sectional water energy, increasing energy convergence within vegetated areas. This amplifies secondary flow strengths and alters vorticity directions. The impact of vegetation on secondary flows is evident in the increase in rotational intensity. Considering various factors, such as the confluence ratio, flow velocity, and distance from the confluence, is crucial, as the influence of vegetation on flows is multifaceted.