Title: Floodplain heterogeneity and meander migration Authors
|Motta, Davide -|
|Abad, Jorge -|
|Garcia, Marcelo -|
Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: June 30, 2011
Publication Date: September 6, 2011
Citation: Motta, D., Abad, J.D., Langendoen, E.J., and Garcia, M.H. (2011). Floodplain heterogeneity and meander migration. In: Proc. 7th IAHR Symposium on River, Coastal and Estuarine Morphodynamics, X.-J. Shao, Z.-Y. Wang and G.-Q. Wang, eds., Tsinghua University Press, Beijing, China, pp. 1971-1986. 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. Most research therefore has focused on the complexity of the equations describing the hydrodynamics, whereas simple, empirical relationships were used to describe river migration. Using a physically-based model of river bank retreat and a fairly simple model of hydrodynamics, scientists at the USDA-ARS National Sedimentation Laboratory in collaboration with researchers of the University of Illinois have shown that the resistance-to-erosion of floodplain soils and its distribution may exert a greater control on meander planform shape than river hydrodynamics. The new model has shown to capture the complex long-term migration patterns of natural channels more realistically. 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: The impact of horizontal heterogeneity of floodplain soils on rates and patterns of meander migration is analyzed with a Ikeda et al. (1981)-type model for hydrodynamics and bed morphodynamics, coupled with a physically-based bank erosion model according to the approach developed by Motta et al. (2011). We assume that rates of migration are determined by the resistance to hydraulic erosion of the soils, which is described by an excess shear stress relation. This relation uses two parameters characterizing the resistance to erosion: critical shear stress and erodibility coefficient. The spatial distribution of critical shear stress in the floodplain is generated on a regular grid with varying degree of randomness to mimic natural settings and the corresponding erodibility coefficient is computed with a relation derived from field-measured pairs of critical shear stress and erodibility. Centerline migration and associated statistics for randomly-disturbed distribution based on the distance from the valley axis are compared for sine-generated centerline using the Monte Carlo method. Two relevant parameters are identified: (i) the standard deviation of the critical shear stress distribution, which is an indicator of the local soil heterogeneity, determines centerline skewness and its variability, and can lead to development of downstream skewness and complex planform features, while not significantly affecting lateral migration; (ii) the cross-valley increase in soil resistance, which mainly constrains rates of lateral migration also affecting skewness. The Monte Carlo approach, applied this time to the case of a natural river alignment and purely random floodplain-soil distribution, shows that migrated centerlines present larger variability the coarser is the scale of the floodplain heterogeneity (third key parameter for describing the effect of floodplain heterogeneity on migration) and the increase in variability is less then linear. Finally, when the approach for meander migration based on hydraulic erosion is compared to the “classic” approach based on excess velocity at the outer bank, the former approach produces more variability and shape complexity for equal stochastic variability of the corresponding governing parameters, because of the presence of a threshold for bank erosion missing in the case of classic approach for migration.