Three‐dimensional flow structure and bed morphology in large elongate meander loops with different outer bank roughness characteristics

Abstract Few studies have examined the three‐dimensional flow structure and bed morphology within elongate loops of large meandering channels. The present study focuses on the spatial patterns of three‐dimensional flow structure and bed morphology within two elongate meander loops and examines how differences in outer bank roughness influence near‐bank flow characteristics. Three‐dimensional velocities were measured during two different events—a near‐bankfull flow and an overbank event. Detailed data on channel bathymetry and bed form geometry were obtained during a near‐bankfull event. Flow structure within the loops is characterized by strong topographic steering by the point bar, by the development of helical motion associated with flow curvature, and by acceleration of flow where bedrock is exposed along the outer bank. Near‐bank velocities during the overbank event are less than those for the near‐bankfull flow, highlighting the strong influence of the point bar on redistribution of mass and momentum of the flow at subbankfull stages. Multiple outer bank pools are evident within the elongate meander loop with low outer bank roughness, but are not present in the loop with high outer bank roughness, which may reflect the influence of abundant large woody debris on near‐bank velocity characteristics. The positions of pools within both loops can be linked to spatial variations in planform curvature. The findings indicate that flow structure and bed morphology in these large elongate loops is similar to that in small elongate loops, but differs somewhat from flow structure and bed morphology reported for experimental elongate loops.