Location: Watershed Physical Processes ResearchTitle: Modification of meander migration by bank failures Author
Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2014
Publication Date: 5/9/2014
Publication URL: http://handle.nal.usda.gov/10113/59413
Citation: Motta, D., Langendoen, E.J., Abad, J.D., Garcia, M.H. 2014. Modification of meander migration by bank failures. Journal of Geophysical Research: Earth Surface. 119(5):1026-1042. doi:10.1002/2013JF002952. 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 and the horizontal heterogeneity of floodplain soils control the rate of lateral migration. However, floodplain soils exhibit great vertical stratigraphy reflecting the meander planform evolution. Scientists at the USDA-ARS National Sedimentation Laboratory in collaboration with researchers of the University of Illinois developed a new conceptual model to represent the impact of differing volumes failed bank-material debris controlled by vertical stratigraphy on bank erosion rate. Combining this new model with a physically-based model of river bank retreat and a fairly simple model of hydrodynamics, we showed that the vertical heterogeneity of floodplain soils significantly impacts lateral migration rates and meander planform shape and increases their spatial and temporal variability. Continuous, smaller failures of the upper soil layer dominate when the upper soil layers have significant erosion resistance, and its impact is controlled by the thickness of the less erodible upper layers. For less cohesive banks, episodic mass failures of the entire bank profile become increasingly important, which can lead to preferential migration patterns. The presented methodology improves the accuracy and reduces the uncertainty in model outcome for streams with a meandering planform. It can be used by federal and state agencies, such as the US Geological Survey, the US Bureau of Reclamation, the US Corps of Engineers, the Natural Resources Conservation Service and the US Environmental Protection Agency, to assess the long-term stability of re-meandered streams.
Technical Abstract: Meander migration and planform evolution depend on the resistance to erosion of the floodplain soils. To date, research to quantify meandering river adjustment has largely focused on resistance to erosion properties that vary horizontally. This paper evaluates the combined effect of horizontal and vertical floodplain soil heterogeneity on meander migration by simulating fluvial erosion and cantilever and planar bank mass failure processes responsible for bank retreat. The impact of streambank failures on meander migration is conceptualized through a bank armoring factor associated with the dynamics of slump blocks produced by cantilever and planar failures. Simulation periods smaller than the time to cutoff are considered, such that all planform complexity is caused by bank erosion processes and floodplain heterogeneity and not by cutoff dynamics. Cantilever failure continuously affects meander migration, because it is primarily controlled by the fluvial erosion at the bank toe. Hence, it impacts migration rates and meander shapes through the horizontal and vertical distribution of soil erodibility. Planar failures are more episodic. However, in floodplain areas characterized by less cohesive soils, it can affect meander evolution in a sustained way and produce preferential migration patterns. Model results show that besides the hydrodynamics, bed morphology and horizontal floodplain heterogeneity, the stratigraphy of the floodplain soils can significantly affect meander evolution, both in terms of migration rates and planform shapes. Specifically, downstream meander migration can either increase or decrease with respect to the case of homogeneous floodplain soil; lateral migration generally decreases as result of bank protection due to slump blocks; and the effect on bend skewness depends on the location and volumes of failed bank material caused by cantilever and planar failures along the bends, with possible achievement of downstream bend skewness under certain conditions. Therefore, soil stratigraphy must be accounted for when estimating meander migration within floodplains.