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Design of Large Wood Structures in Sand-Bed Streams

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2007 ASAE Annual Meeting  072241.(doi:10.13031/2013.23035)
Authors:   Rebecca A Ward, Paul R Weckler, Glenn O Brown, Darrel M Temple, F Douglas Shields, Jr., Carlos V Alonso
Keywords:   Large Wood, Woody Debris, Streambank Protection, Flow Meter, Open Channel Flow, Hydrologic Modeling, Flow Visualization

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 flow on the different structure configurations and orientations. 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. Also, the required prototype anchor design loads to allow for successful LWS installation in sand-bed streams were determined for the structures tested

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