THE EXCEPTIONAL SEDIMENT LOAD OF A FINE-GRAIN MEANDERING RIVER AND RELATION TO BEDFORM GEOMETRY: AN APPEALING EXAMPLE FROM THE LOWER YELLOW RIVER, CHINA (Invited Presentation)

Fine-grain (mud and very-fine sand) transport systems are ubiquitous on Earth’s surface, examples including rivers, subglacial settings, and near-shore to deep-ocean marine realms. For these environments sediment transport shapes surface morphology; however the physics behind mass movement of the fine fraction remains inadequately constrained. This is in part due to the fact that this study necessitates fluid equilibrium for simplification, whereby sediment transport achieves capacity for steady and uniform flow. Such conditions, however, are rarely achieved in nature or flume experiments, because the spatiotemporal scales necessary for fine-grain sediment equilibrium are uncommon. One natural exception is the lower Yellow River, China, where this silt- and sand-bed river achieves sediment transport capacity during flood hydrodynamics. For this talk, recent scientific investigations from the Yellow River are used to refine the physics of fine sediment transport. A generalized form of the Engelund-Hansen (GEH) transport theory, corroborated with data collected from six gauging stations, demonstrates that the sediment load in the lower Yellow River is 15-20 times larger than is found in coarser sand-bed rivers. A detailed analysis of the GEH theory, specifically investigating the resistance relation, shows that the lower Yellow River tends toward plane bed. If present, bedforms will possess very large aspect ratios (wavelength to height). This minimal bed topography profile reduces form drag to the point that essentially most of boundary shear stress is utilized for sediment transport. The prediction of dune aspect ratio is validated with field surveys that include multibeam bathymetry data to measure bedform profiles. Indeed, these field measurements confirm the theoretical analysis: bedforms possess extremely low-relief, and the aspect ratios are approximately one order of magnitude larger than those found in traditional sand-bed rivers. These finds could be potentially used for better understanding the morphodynamics and stratigraphy of the fine-grain subaqueous settings.