|Shields Jr, Fletcher|
Submitted to: Proceedings of the World Water and Environmental Resources Congress
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2007
Publication Date: 5/15/2007
Citation: Ward, R.A., Brown, G.O., Weckler, P.R., Temple, D.M., Shields Jr, F.D., Alonso, C.V. 2007. Modeling large wood structures in sand bed streams. Proceedings of the World Water and Environmental Resources Congress, 2007, American Society of Civil Engineers. CD-ROM. 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 was 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: 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.