Assessment of flow forces on large wood in rivers

Abstract

Large wood (LW) exerts an important influence on the geomorphology and ecology of streams and rivers. The magnitudes of flow forces on LW are needed to support stream management activities and are typically computed using time mean lift and drag coefficients determined in laboratory flumes using small, smooth cylinders. Herein we report measurements of forces on LW of varying complexity (simple cylinder, branching, and complex root wad) and surface (bark) roughness made in an outdoor grassed channel under steady and unsteady flows. LW orientation relative to the primary flow direction and LW relative submergence were varied. Drag and lift coefficients for cylindrical (unbranched) LW followed patterns reported by others for metal cylinders in wind tunnels. Drag coefficients for cylindrical (unbranched) LW, corrected for blockage effects, ranged from −0.05 to 1.29, and lift coefficients ranged from −0.88 to 0.52, varying systematically with LW position relative to the channel bed and incident flow direction. Measured drag coefficients for the noncylindrical LW, corrected for blockage effects, ranged from 0.22 to 6.27, while lift coefficients varied from −3.65 to 30.84. Systematic relationships between the relative submergence and orientation of branching LW and the drag and lift coefficients were not observed, but coefficients were greatest for LW with few branches and converged on smaller values typical of blunt bodies as LW complexity increased. For both simple and complex LW, maximum lift and drag forces during the rising limb of unsteady flows were about 2–3 times greater than steady flow temporal mean values.