Location: Watershed Physical Processes Research2013 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 13 an analysis of streambank material loadings along Skunk Creek using the Bank Stability and Toe Erosion Model (BSTEM) was performed and CONCEPTS simulations to evaluate the effects of bank protection on the long-term geomorphology of the Big Sioux River was developed. Field data was collected jointly with the South Dakota Department of Environmental and Natural Resources at 10 sites on Skunk Creek, South Dakota. These data were processed and analyzed to characterize the resistance to erosion of the channel boundary, that is erodibility, shear strength, bulk density, and grain size distribution. Using these data a BSTEM model was developed to assess streambank material loadings along Skunk Creek. A Conservational Channel Evolution and Pollutant Transport System (CONCEPTS) model of the Big Sioux River was developed jointly with the University of Pittsburgh using: remotely-sensed topography, aerial imagery, and bathymetry to build river geometry; and gaged flow data to derive discharge scenarios of different return intervals. The simulated near-bank boundary shear stress distributions using RVR Meander/CONCEPTS in FY 12 was used to modify the applied shear stresses calculated by CONCEPTS in meander bends. Model results show that bank protection has limited effects on bed scour.