Woody Debris Stream Bank Stabilization
Joe Paul Edwards, Roberto Espinoza, Dave Mercer, Ryan Woolbright
Scientists and engineers have gained a greater appreciation of the importance of large wood in fluvial systems in recent years. Wood can control channel form and migration rates as well as provide cover and a diversity of hydraulic conditions for all types of biota. The USDA-ARS-National Sedimentation Laboratory has tested Large Woody Debris Structures (LWDS) in the Little Topashaw Creek located in Oxford, Mississippi. These man made structures have proven to be an efficient method for channel erosion control and habitat rehabilitation. Major advantages of these structures over existing stream rehabilitation methods include low cost and a natural, aesthetically pleasing design. Three years after construction, thirty-six percent of structures failed during high flows. The loss of these structures has created the need for the establishment of a more durable design.
The Little Topashaw structures slowed, stopped or in some cases reversed bank erosion. However, a large portion of these structures did not survive significant flow events. The USDA-ARS Sedimentation Lab has asked RAW Engineering to investigate the failures and develop a new design for LWDS to be used in stream bank rehabilitation. The LWDS were designed to withstand a 10 yr flow event with a minimum life of five years. Causes of failure were determined to be increased buoyant force due to drying of structure members, loss of branches and upper members of LWDS, and inadequate anchoring. The natural buoyancy and gradual decay of the LWD are aspects of the design which can not realistically be altered. Therefore, our engineers will focus on the performance of the anchoring systems. Using scale models our engineers will determine forces acting on the anchors and compare them with theoretical forces the structure was subjected to. RAW then plans to analyze the structure geometry and develop a more durable design. Another goal is to increase sediment deposition by altering the hydraulic conditions imposed by the geometry of the structure.
Scope of Work
Testing will take place at the USDA-ARS Hydraulics Laboratory in an outdoor flume. The flume is a six foot wide concrete flume capable of reproducing a wide range of flow conditions. First RAW engineers will recreate the structures constructed at Little Topashaw Creek, using dimensional analysis and similarity to determine the scale of the model and hydraulic conditions. RAW will make an approximately 1/8 scale model of the LWDS designed by Dr. Doug Shields. Froude number calculations will be used to determine discharge velocity and depth. Models will be made of Eastern Red Cedar or another suitable, locally available wood. RAW engineers will perform tests with scale models of the LWDS and determine the forces acting on the anchoring system. The results of these tests will be analyzed and compared to theoretical lift and drag forces calculated for the structures. Statics and Strength of Materials concepts will then be applied to determine a more resilient design. The new design will likely include alternate structure geometry, changes in anchoring positioning and cable orientation in order to minimize the forces acting on the anchors. The USDA-ARS Hydraulics Lab uses siphons to draw water out of Lake Carl Blackwell. These siphons can not be used in subzero temperatures. Due to this fact, there will be a limited availability for testing from November through March. The lab does have some smaller indoor demonstration flumes. Kinematic similarity is difficult to achieve in these flumes, but they can provide an excellent visual representation of flow variations through and around a structure. RAW engineers will use these flumes to evaluate alternate designs during the winter months. Large scale testing on revised designs will resume in the spring.