|Shields Jr, Fletcher|
Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 6/20/2007
Publication Date: 7/1/2007
Citation: Ward, R.A., Weckler, P.R., Brown, G.O., Temple, D.M., Shields Jr, F.D., Alonso, C.V. 2007. Design of Large Wood Structures in Sand-Bed Streams. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). Paper number 072241, 2007 Annual Meeting, ASABE, St. Joseph, MI.
Interpretive Summary: Many streams in agricultural landscapes are rapidly eroding their banks, threatening nearby land and structures and degrading habitat and water quality. Corrective measures can be quite costly and contribute little to aquatic habitat rehabilitation. Erosion control structures made from felled trees (large wood) are attractive economically and as habitat elements, but an experimental project using this technique in a sand bed stream in Mississippi failed, largely due to pull-out of earth anchors. In order to improve large wood design criteria, forces on a physical model structure in a flume were measured, and the variation of forces with various design variables were noted. Findings showed that large wood structure anchors for structures similar to those that failed must be designed to withstand loads of about 10,000 lbs. These results will be useful to engineers designing large wood structures for stream rehabilitation.
Technical Abstract: Large woody 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.