Location: Watershed Physical Processes ResearchTitle: Spatial variability in bank resistance to erosion on a large meandering, mixed bedrock-alluvial river Author
|Ursic, Michael - Mick|
Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2015
Publication Date: 1/1/2016
Publication URL: https://handle.nal.usda.gov/10113/5625723
Citation: Konsoer, K.L., Rhoads, B.L., Langendoen, E.J., Best, J.L., Ursic, M.E., Abad, J.D., Garcia, M.H. 2016. Spatial variability in bank resistance to erosion on a large meandering, mixed bedrock-alluvial river. Geomorphology. 252: 80–97. Interpretive Summary: To date the development of computer models that simulate the evolution of meandering streams has been based on the premise that river hydrodynamics controls the rate of lateral migration and the floodplain properties are assumed to have uniform erosion-resistance properties. However, recent computer studies using randomly-generated spatially-varying floodplain-soil properties have shown that the soil heterogeneity can substantially influence river form. Given these findings, scientists at the USDA-ARS National Sedimentation Laboratory in collaboration with researchers of the Universities of Illinois and Pittsburgh investigated lateral and vertical heterogeneities in bank material properties and riparian vegetation between two meander bends along the Wabash River on the Illinois and Indiana state line. Results showed substantial differences in the grain size of the bank materials, soil cohesion, and critical shear stress necessary for sediment entrainment between the two bends, and are also highly variable within each bend, both laterally and vertically. Analysis of channel planform from long-term historical aerial imagery showed that river adjustment was strongly correlated to the spatial variability in resistance properties of the bank materials, riparian vegetation, and bedrock within the channel. The presented field data therefore support the earlier findings from the use of computer models, and may lead to improved prediction of the planform evolution of meandering streams and reduced uncertainty in model outcome.
Technical Abstract: Spatial heterogeneities of the erosion-resistance properties of the channel banks and floodplains, such as grain size characteristics and the presence of vegetation and bedrock, can have a substantial influence on river morphodynamics, resulting in complex planform geometries and highly variable rates of bank erosion. While the influence of these external forcings on planform dynamics have been documented for small and moderate sized rivers, the spatial variability of erosion-resistance properties for large rivers remains poorly understood. Furthermore, with the exception of recent numerical models apply stochastic variability within the floodplain, most models rely on uniform erodibility of the bank and floodplain materials. In this paper, the lateral and vertical heterogeneities in bank material properties and riparian vegetation between two elongate meander loops on a large mixed bedrock-alluvial river are investigated using various geotechnical field and laboratory methods. Additionally, a numerical model of bank retreat and repeat terrestrial LiDAR surveys are used to evaluate the capacity of the bank material properties to modify the rates and mechanisms of bank retreat. Results show substantial differences in the characteristic grain size of the bank materials, soil cohesion, and critical shear stress necessary for sediment entrainment between the two bends, and are also highly variable within each bend, both laterally and vertically. Riparian forests are shown to increase the resistance to erosion through added cohesion to the bank materials, and are capable of increasing bank stability along a large river. The spatial variability in resistance properties of the bank materials, riparian vegetation, and bedrock within the channel are dominant factors contributing to the variability in rates and mechanisms of erosion determined from short- and long-term changes in channel planform.