Modeling Large Wood Structures in Sand-Bed Streams

In-stream large wood structures (LWS) are becoming increasingly popular throughout the world. The LWS improve aquatic habitat quality and protect banks from erosion. While most reports describe the LWS in the Northwest as successful, LWS in one Mississippi sand-bed stream had an unacceptable failure rate of 33% two years after installation. Although LWS in the Northwest are often ballasted with coarse bed material, LWS in sand-bed streams must be anchored to the bed and banks. The problem of anchor loading as well as optimal structure orientation and configuration was examined in order to increase the success of the structures. Physical model tests were performed to evaluate the magnitude of forces on the anchors and to determine the effect of yaw angle on the anchors and on the effectiveness of the structure. A series of 1:8.7 scale models were constructed of fresh cut hardwood saplings. Clear-water tests were run in a 6 ft (1.83 m) wide fixed-bed concrete flume at the USDA-ARS Hydraulic Laboratory in Stillwater, Oklahoma. Yaw angle, structure design, flow depth, and flow velocity were varied over several runs. Tie-down cable loadings were measured to analyze forces applied to the anchors. Flow velocities and depths near the structure were recorded to examine hydrodynamic forces, and flow visualization was performed.