Assessment of flow forces on large wood in rivers

Authors: Shields Jr, Fletcher; ALONSO, CARLOS - Retired ARS Employee

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2012
Publication Date: 4/17/2012
Citation: Shields Jr, F.D., Alonso, C.V. 2012. Assessment of flow forces on large wood in rivers. Water Resources Research. 48(4):1-16.

Interpretive Summary: Large wood is an important component of river ecosystems, providing habitat value and exerting significant influence on hydraulics, erosion and sedimentation. The physical stability of a given piece of large wood is governed by fluid forces of buoyancy, lift and drag but computation of lift and drag forces is dependent on selection of appropriate coefficients that are unknown and must be estimated based on limited data from small scale laboratory tests. Measurements of flow forces on actual large wood were made in an outdoor channel that showed that the coefficients from small scale tests may be extended to field conditions for simple logs. Coefficients for branching logs tend to vary widely with log geometry and flow properties, but as more branches are added, coefficients converge toward those for simpler geometries. These findings are valuable to engineers and scientists studying the transport and deposition of large wood in channels and designing large wood structures for habitat and erosion control.

Technical Abstract: Large wood (LW) exerts an important influence on the geomorphology and ecology of streams and rivers. LW management activities are diverse, including placement in streams for restoring habitats or controlling bank erosion and mitigation of LW-related hazards to bridges and other structures. Flow forces on LW are typically computed using time mean lift and drag coefficients determined in laboratory flume studies using metal, plastic or smooth wood cylinders, and buoyant forces are computed based on assumed LW geometry and specific gravity. A few studies have included laboratory measurements of forces on scale model LW or small actual logs. Herein we report measurements of lift, drag and buoyant forces on LW of varying complexity (simple cylinder, branching, and complex rootwad) and surface (bark) roughness made in an outdoor grassed channel under steady and unsteady flows. Steady flow depths were approximately 0.8 m, and incident flow velocities were either 0.74 or 1.20 m s-1. 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. Measured drag coefficients for cylindrical (unbranched) LW, corrected for blockage effects, ranged from 0.25 to 1.70 and lift coefficients 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 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 as great as the corresponding steady flow temporal mean values.