2011 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.
Sediment contributions by streambank erosion represent an important source subject to control to reach water-quality targets. Previous research determined the rates of bank erosion and potential reduction in streambank-derived sediment loads under various mitigation strategies. This research aims to quantify the cumulative effects of these potential mitigation measures (load reduction) on stream processes, particularly whether the reduction in loads from banks will cause incision on the channel bed. Field work on the Big Sioux River and Skunk Creek will be conducted to obtain all the necessary data to simulate river processes with CONCEPTS. The CONCEPTS modeling will be conducted by the University of Pittsburgh. Model runs will be undertaken under existing conditions as well as several combinations and extents of erosion-control measures to evaluate impacts to river processes and morphology. In addition, a series of laboratory experiments will be conducted by the University of Buffalo with support from Griffith University to determine the optimal design of engineered log jams (ELJs) to protect eroding streambanks. Results from the experiments will be incorporated into the CONCEPTS modeling runs to evaluate the impact of these types of mitigation measures on bank stabilization and fluvial processes. Field work has been delayed due to high water in South Dakota this spring and summer but is now scheduled for August 2011. Monitoring of progress will accomplished by quarterly conference calls, e-mail and bi-annual meetings.