Large Woody Debris Structures are a very enticing alternative for stream bank stabilization. Normally, stream banks are stabilized with rocks or concrete structures that are very costly and not aesthetically pleasing. LWDS have many positive attributes that make them more appealing than the traditional stream bank stabilizations techniques. The LWDS when installed properly can reduce channel erosion and induce sediment deposition within and around the structure. Also the structure can rehabilitate habitat when it performs properly.

The Large Woody Debris Structure project was introduced to RAW Engineering by the USDA-ARS National Sedimentation Laboratory in Oxford, MS. These Large Woody Debris Structures were originally implemented in the Upper Northwest where they were referred to as Engineered Log Jams. However, because of variations in bed material and different types of trees in Mississippi, many of the techniques used in the Northwest could not be used for the same type of applications in Oxford. The original test site in Mississippi for the implementation of the LWDS was the Little Toposhaw Creek. In this location 72 structures were put in place and within 3 years, 36% of the structures had failed. This brings about the need for further study and analysis of the forces acting on the structures within the streams as well as analysis of the failure modes.

For the LWDS to be considered successful, the following criteria must be met:

• Provide habitat for aquatic biota
• Reduce stream velocity and induce sediment deposition
• Stabilize bank toe
• Withstand 5-yr return period flows
• Cost less than traditional methods

The criteria that are clearly the most important are that the structure meet its design life and that the structure is less expensive that traditional methods. If the structure survives the specified return period flow, then the other criteria are complimentary if the structure behaves properly.

The project handed to RAW Engineering involves studying the original structure to obtain a drag coefficient as well as study the forces on the structure at different orientations. One of these orientations is achieved by rotating the structure at varying angles from the key members being perpendicular to the flow. This angle is called the YAW angle. Other structure orientations that were tested included rotating the structure 180 degrees from the original orientation as well as placing the structure parallel to the flow compared to the 15 degree shift of the original structure. Also velocity profiles upstream and downstream of the structure were recorded to study how the structure reduces local velocities.

During the course of the Fall 2005 and the Spring 2006 semesters, several tests were run at the USDA-ARS Hydraulics Lab at Lake Carl Blackwell in Stillwater, Ok. These tests were run in a 6-foot wide concrete flume. Using Froude number similarity and the width of the flume as our governing parameter, a 1:8.7 scale model of the LWDS was constructed. The tests were performed using this structure at the different orientations. During the fall semester a test was ran with the load cables arranged similarly to the anchoring system that was implemented on the Little Toposhaw Creek. Using this setup to resolve drag, lift and buoyant forces proved to be very difficult. During the Spring Semester, the force measuring cables were altered so that they measured purely drag and purely lift and buoyant forces. The results of these test were much more easily deciphered.

The results of our testing provided forces on the structures as well as detailed velocity profiles both upstream and downstream of the structure. From these measurements, RAW Engineering concludes that the structure in the reverse orientation of the original makes a more effective structure. This structure has a lower drag coefficient without sacrificing the velocity reducing potential of the structure. A lower drag coefficient is important because it means that the forces exerted on the structure are less therefore decreasing the failure or structures.