Design of Large Wood Structures in Sand-Bed Streams

Abstract

Large wood structures (LWS) are potentially an efficient and cost effective way to protect streambanks from erosion while enhancing aquatic habitat. While LWS have been successful in some cases in the Pacific Northwest when ballasted with rock, the failure rate in sand-bed streams typical of the mid-continent is a concern. Recently built structures in Mississippi experienced a 33% failure rate two years following installation. From earlier reports, it is known that a large portion of the failures were due to overloading the anchors and not having the optimal structure orientation or configuration. Model LWS constructed using hardwood saplings on a 1:8.7 scale were run in a 1.83 m (6 ft) wide concrete flume at the USDA-ARS Hydraulic Laboratory in Stillwater, Oklahoma to determine the magnitude of the forces on the LWS anchors and to study the effectiveness of the structure in reducing near the bank velocity. The yaw angle, structure configuration, flow depth, and flow velocity were varied to analyze effects on tie-down cable loadings. Flow velocity profiles were recorded, and flow visualization was performed to further study the effects of the different structure configurations and orientations on the flow. The study showed that a yaw angle of 15 degrees produced the highest drag force, while the 180 degree structure had the greatest reduction in near-bank velocity. Tests indicated that a prototype anchor loading of 38 kN (6800 lbs) was necessary to allow successful LWS installation in sand-bed streams, without the need for rock ballast. An analysis of soil anchors that are suited for stabilizing the LWS was also done. It showed that a variety of anchor types could be used in the sand-bed streams.