Spatial Variability in River Sinuosity across the Backwater Regime

Abstract:

Previous field observations indicate that sinuosity of large coastal rivers typically decreases with proximity to the outlet. Concomitantly, mud content of the bank material increases downstream, creating a more cohesive and erosion-resistant river bank. These field observations contradict conventional theory, which states that enhanced bank strength promotes greater channel sinuosity. To reconcile this contradiction between theory and field observation, we hypothesize that the time scale for a river to evolve to high sinuosity near the coast is much longer than the avulsion time scale, so that channel avulsions occur prior to the development of high sinuosity near the outlet. Development of high sinuosity usually accompanies high lateral migration rates. However, lateral migration rate may be reduced downstream due to (1) backwater hydrodynamics (i.e., downstream decrease in bed shear stress and reach-averaged flow velocity) or (2) increased bank strength as both bank and bed material size decrease downstream. To test this hypothesis, we couple a meandering river-planform model and backwater morphodynamic model for the Mississippi River. Preliminary results suggest that both downstream decreases in reach-averaged velocity and bank erodibility can independently drive a downstream decrease in the lateral migration rate, thus preventing the development of high sinuosity towards the outlet. Future work will include calibration of bank erodibility using historical migration rate data and modeled backwater hydrodynamics of the Mississippi River, development of empirical relations between bank erodibility and the composition of bank material via geotechnical borings, and quantification of the time scale for sinuosity development, which will be compared with the avulsion time scale for the Mississippi River.