Location: Watershed Physical Processes Research2012 Annual Report
1a. Objectives (from AD-416):
The primary objective of this research is to determine optimal application of engineered log jams or other bank-stabilizations measures to minimize associated bed erosion along the Big Sioux River and Skunk Creek, SD.
1b. Approach (from AD-416):
Streambank erosion can be an important contributor of sediment. Bank failures result in channel widening and the loss of adjacent lands. The USDA-ARS National Sedimentation Laboratory (NSL) has determined through an earlier study along the Big Sioux River that various types of bank-stabilization measures would be effective at reducing bank-erosion rates (Bankhead et al., 2010). The Bank-Stability and Toe-Erosion Model (BSTEM) was used to compare erosion rates under existing and mitigated conditions. What is unknown, however, is whether bed erosion (and then further bank erosion) will be initiated as a result of the reduction in sediment supply if successful bank-stabilization measures are undertaken at a large scale along the river. To determine this, a model that not only can dynamically adjust the bed and banks, but also routes flow and sediment needs to be applied. NSL’s channel evolution model CONCEPTS has been used successfully for this type of analysis in various settings across the country. CONCEPTS will be employed along about 100 miles (total) of the Big Sioux River and Skunk Creek to determine the optimal amount of bank stabilization to reduce sediment-loading rates and initiate bed incision. Cross-section data will be surveyed at about 80 cross-sections and combined with field measurements of the resistance of the channel bed and banks to be used as inputs into the CONCEPTS model. A representative flow series of at least 25 years will be used to provide hydraulic input. Simulation runs will be initially conducted for existing conditions and then, the maximum length of bank-stabilization planned/proposed by the state to determine if reductions in bank loads result in bed incision. If de-stabilization of the channel bed is indicated, additional runs will be conducted simulating a reduced length of bank stabilization until an optimal mitigation effort is indicated. The exact types and locations of stabilization measures to be simulated will be determined by NSL and the State. Engineered log jams (ELJs) will certainly be included in the simulation analysis. To support the investigation of performance characteristics of ELJs, physical experiments will be conducted. An existing flume at NSL will be modified to conduct experiments on the hydraulic properties and effects of ELJs on shear-stress distributions and the maximum shear stress that they can withstand. These experiments should help to provide design criteria for these structures. Some of the variables that will be examined include dimensions relative to channel size (height, width), spacing, angle exposed to flow etc. Results of these experiments will then be included in CONCEPTS simulations to determine the optimal conditions for ELJs along the channels.
3. Progress Report:
In FY 2012 we collected and processed field data along the Big Sioux and Skunk Creek, South Dakota. A flume was constructed at the State University of New York (SUNY) at Buffalo to study the impact of engineered log jams (ELJs). Simulations were performed with the River Meander/CONCEPTS model to calculate the boundary shear stress distributions at meander bends along the Big Sioux. We collected field data jointly with the South Dakota Department of Environmental and Natural Resources at 38 sites on the Big Sioux and 4 sites on Skunk Creek. We processed and analyzed these data to characterize the resistance to erosion of the channel boundary, that is erodibility, shear strength, bulk density, and grain size distribution. In collaboration with SUNY at Buffalo we constructed a flume (1.9 m wide x 7 m long) in their hydraulics laboratory. The large width minimizes the blockage factor of model bank protection structures that will be evaluated in this flume. We finalized the design of two model ELJs to be tested. We developed a River Meander/CONCEPTS model of the Big Sioux jointly with the University of Pittsburgh using: remotely-sensed topography, aerial imagery, and bathymetry to build river geometry; and intensity-duration-frequency analysis of gaged flow data to calculate bankful flow. The model accurately simulates the transverse bed slope in meander bends, which indicates that the predicted flow should agree well to that occurring at bankful flow conditions. We will use the simulated near-bank boundary shear stress distributions to modify the applied shear stresses in the planned one-dimensional model applications.