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Title: A simplified 2D model for meander migration with physically-based bank evolution

Author
item Motta, Davide
item Abad, Jorge
item Langendoen, Eddy
item Garcia, Marcelo

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2011
Publication Date: 8/15/2012
Citation: Motta, D., Abad, J.D., Langendoen, E.J., Garcia, M.H. 2012. A simplified 2D model for meander migration with physically-based bank evolution. Geomorphology. 163-164:10-25. doi:10.1016/j.geomorph.2011.06.036.

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. Current two-dimensional computer models developed specifically to simulate the migration of meandering streams, however, employ simplified empirical relationships to determine channel migration and therefore stream bank erosion. To model the mechanics of meander migration correctly the stream bank erosion component of the CONCEPTS model was combined with the meander model RVR MEANDER developed by the University of Illinois. The performance of the proposed approach is compared to that of the more simple classic method through the application to several test cases for both idealized and natural planform geometry. The applications show that the improved physically-based method of bank retreat is required to capture the complex long-term migration patterns of natural channels. The presented methodology improves the accuracy and reduces the uncertainty in model outcome for streams with a meandering planform. One-dimensional channel evolution models are widely 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 design stream-channel conservation measures and assess their long-term stability and benefits.

Technical Abstract: The migration rate calculated by numerical models of river meandering is commonly based on a method that relates migration rate to near-bank excess velocity multiplied by a dimensionless coefficient. Notwithstanding its simplicity, since the early 1980s this method has provided important insight into the long-term evolution of meander planforms through theoretical exercises. Its use in practice has not been as successful, which is largely due to the complexity of the physical processes responsible for bank retreat, the heterogeneity in floodplain soils, and the presence of vegetation. As a result, calibration of the dimensionless coefficient is difficult, and it cannot be expected that this simple method will accurately simulate the evolution of meandering streams over engineering time scales. This paper presents a new approach that calculates meander migration rates using physically-based streambank erosion formulations. The University of Illinois RVR Meander model, which simulates meandering-river flow and bed morphodynamics, is integrated with the streambank erosion algorithms of the US Department of Agriculture channel evolution computer model CONCEPTS. The performance of the proposed approach is compared to that of the more simple classic method through the application to several test cases for both idealized and natural planform geometry. The advantages and limitations of the approach are discussed, focusing on simulated planform pattern, the impact of soil spatial heterogeneity, the relative importance of the different processes controlling bank erosion (hydraulic erosion, cantilever, and planar failure), the requirements for obtaining stable migration patterns (centerline filtering and interpolation of bank physical properties), and the capability of predicting the planform evolution of natural rivers over engineering time scales (i.e., 50 to 100 years). The applications show that the improved physically-based method of bank retreat is required to capture the complex long-term migration patterns of natural channels, which cannot be merely predicted from hydrodynamics only.