Do Meandering Rivers Reach a Stable Long-Term Geometry if Channel Width can Vary?

Abstract:

Numerical modeling of the long term evolution of river meanders consistently show a tendency towards a statistical equilibrium when averaged over repeated episodes of channel extension by migration and channel shortening by cutoff. This statistical steady state is typically defined using reach averaged channel wavelength and sinuosity, which oscillate about a stable mean value for channels of constant width. Meander geometry has long been shown to correlate closely with channel width and this paper builds on previous modeling work by using a meander simulation model that allows channel width to vary. Independent submodels are specified for bank erosion and deposition and channel width is free to vary locally as a result of differential bank migration. In addition to variable channel width, the numerical model captures mean bed elevation changes in response to sediment exchange with both banks. This new modeling framework presents an opportunity to look at long-term statistical behavior of evolving meanders not only in terms of wavelength and sinuosity, but also in terms of channel width. In this paper, we evaluate whether the stable equilibrium sinuosity/wavelength observed in constant-width models also occurs when channel narrowing and widening also occur during cycles of channel elongation and shortening. In addition to evaluating the interaction between width, sinuosity, and wavelength over many meander cycles, the paper explores how the long term evolution statistics are controlled by other important drivers, especially the rate and grain size of upstream sediment feed.