Quantifying the Spatial Variability of Bedform Morphology and Bedload Fluxes within Sand-Bedded River Bends

Abstract

Dunes are the predominant bedform across fluvial systems and are the primary drivers of hydraulic roughness, sediment transport, and topographic variability along the riverbed. As they grow and decay with changing discharge, dunes reflect localized hydraulic and sediment transport conditions and, in turn, modulate near-bed flows. These relations highlight their importance in morphodynamic evolution, as dunes both influence and respond to changes in flow and sediment interactions. Furthermore, their migration is the primary mechanism of sediment movement through fluvial systems. Thus, quantifying their morphologies and migration rates provides insight into the spatial and temporal processes that govern riverbed evolution. In the context of larger channel dynamics, dunes record sediment pathways as erosion redistributes sediment and alters the river's shape and boundaries. Within sand-bedded rivers, eroded bank sediments are deposited along the riverbed, creating variable flow depths that drive hydraulic heterogeneity and velocity gradients across the channel. As these processes persist over time, rivers increase in curvature and develop complex river bend shapes, forcing the redirection of flow toward deeper channel regions and leading to channel migration. Current research quantifying bedform dynamics is largely limited to straight reaches, where dunes align with the flow and exhibit low morphologic variability across the width of the channel. However, the effects of hydraulic heterogeneity on river bend bathymetry have been widely oversimplified, leading to a gap in understanding of bedform morphodynamics within bends. This study aims to quantify the spatial variability of dune morphologies and bedload fluxes within river bends to contextualize these processes with our understanding of meandering mechanisms across a range of flow discharge. Using repeat multibeam echosounder surveys of the Sheepnose Bend, Missouri River, MO, we quantify heights, wavelengths, and leeside angles for each dune by deploying bedform analysis methods. We interrogate these morphologies further through spatial analysis, allowing us to observe cross-sectional and streamwise variabilities in addition to reach averages. We situate these field observations within transport scaling relations informed by experimental data and evaluate their efficacy for non-equilibrated bed configurations. By combining our bedform morphology results with calculated bedload transport rates, we compare spatial trends in dune morphologies and sediment fluxes to determine how morphologic variability drives localized differences in sediment transport rates. This information offers insight into the long-term evolution of the channel, thus improving our understanding of the links between bedform dynamics and meandering mechanisms.