|MOTTA, DAVIDE - University Of Illinois|
|ABAD, JORGE - University Of Pittsburgh|
|GARCIA, MARCELO - University Of Illinois|
Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2012
Publication Date: 9/13/2012
Citation: Motta, D., Abad, J.D., Langendoen, E.J., Garcia, M.H. 2012. The effects of floodplain soil heterogeneity on meander planform shape. Water Resources Research. 48(9):W09518. doi:10.1029/2011WR011601.
Interpretive Summary: Application of one-dimensional channel evolution computer models, such as the ARS computer model CONCEPTS, is problematic in meandering streams because of the strong three-dimensional nature of the stream flow. The topography and flow dynamics in meander bends increase the forces exerted by the flowing water on the stream bank at the outside of a meander bend compared to those exerted on stream banks in straight channels. Scientists at the USDA-ARS National Sedimentation Laboratory in collaboration with researchers of the University of Illinois have developed an improved meander model that incorporates a physically-based method of bank retreat, which has shown to capture the complex long-term migration patterns of natural channels more realistically. Applying the model to study the effects of floodplain soil heterogeneity has shown, that the spatial distribution of floodplain soils greatly controls meander planform development. These findings challenge the analyses found in literature, which state that meander planform shape is mainly controlled by river hydrodynamics. 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 rates and patterns are a function of the forces exerted by the flowing water on the streambanks and the resistance to erosion of the bank soils. Past analytical studies of planform development have mostly focused on the complexity of the governing equations, i.e. hydrodynamics, and less so on the resistance of the stream banks. Meander migration results in highly heterogeneous distributions of floodplain soils, which are difficult to describe deterministically. This motivated the use of a Monte Carlo approach to examine the effects of floodplain soils and their distribution on planform development. Meander migration was computed using the classic Ikeda et al. ’s model for hydrodynamics and bed morphodynamics, but coupled, instead of a simple bank migration coefficient, with a physically-based bank erosion model. Simulated bank erosion rates are controlled by the resistance to hydraulic erosion of the bank soils, which is represented by the critical shear stress and erodibility coefficient parameters in an excess shear stress relation. The spatial distribution of critical shear stress across the floodplain is delineated on a rectangular, equidistant grid with varying degree of variability to mimic natural settings. The corresponding erodibility coefficient is computed using a relation derived from in situ measurements of critical shear stress and erodibility. Two spatial distributions were considered for the resistance to erosion properties: (1) a purely random (PR) distribution, and (2) a randomly-disturbed (RD) distribution in which the mean resistance to erosion exponentially increased away from the valley centerline. For the RD distribution two relevant parameters are identified. First, the standard deviation of the critical shear stress distribution, which is an indicator of the local soil heterogeneity, controls skewness and variability of the channel centerline. It can produce downstream-skewed bends and complex planform features, while not significantly affecting lateral migration. Second, the cross-valley increase in soil resistance primarily constrains rates of lateral migration though bend skewness is also affected. For the PR distribution, the simulated migration of channel centerline exhibited larger variability for increasing spatial scales of the floodplain-soil heterogeneity, though their relation appeared to be less than linear. Finally, relating meander migration to hydraulic erosion of the bank soils produced more variability and shape complexity than the “classic” approach relating migration rate to excess velocity at the outer bank for equal stochastic variability of the corresponding governing parameters. This is most likely caused by the absence of a threshold for bank erosion in the classic Ikeda et al. ’s approach.