Effect of large woody debris configuration on three-dimensional flow structure in two low-energy meander bends at varying stages

Abstract

Few process-based studies have investigated the effects of large woody debris (LWD) on the fluvial dynamics of obstructed meander bends. This field-based study examines the influence of LWD on spatial patterns of three-dimensional (3-D) flow in two meander bends of a low-energy stream in northern Illinois. The results of this study indicate that LWD substantially modifies 3-D flow structure through low-gradient meander bends. However, the detailed nature of the flow modification is highly dependent on the particular arrangement, density, and mobility of LWD in a bend. In one of the study bends the LWD primarily functions as roughness elements that reduce the velocity of flow and constrain the high-velocity core and helical motion to the center of the channel. In the other study bend, LWD obstructions act to strongly deflect the flow away from the outer bank rather than simply increasing the resistance to flow along this bank. While results indicate that LWD influences patterns of three-dimensional flow at all stages, the degree of influence appears to change with stage; the overall pattern of three-dimensional flow remains constant, but the spatial extent of the pattern increases with increasing stage.