Andrew Moodie, Jeffrey Nittrouer, Hongbo Ma, Brandee Carlson, Yuanjian Wang, Michael P. Lamb, Gary Parker 

Despite a multitude of models predicting sediment transport dynamics in an open-channel flow, the interaction between fluid and sediment, so to produce self-organized vertical density stratification, has not been robustly investigated and as such is poorly understood. This two-phase phenomenon develops in channels that possess low channel-bed slope and high sediment concentration. As the Yellow River, China, maintains one of the highest sediment loads in the world for a low sloping system, this location is ideal for documenting particle and fluid interactions that give rise to density stratification. Herein, we present analyses from a study conducted over a range of discharge conditions (e.g., low flow, rising limb, and flood peak) from a lower reach of the Yellow River, whereby isokinetic water samples were collected at targeted depths to measure sediment concentration and, simultaneously, velocity measurements were collected throughout the flow depth. Importantly, sediment concentration varied by an order of magnitude between base and flood flows. By comparing measured concentration and velocity profiles to predictive models, we show that the magnitude of density stratification increases with sediment concentration. Furthermore, a steady-state simulation of sediment suspension is used to determine that sediment diffusivity increases with grain size. Finally, we use this model to simulate concentration and velocity profiles over a spectrum of conditions that represent typical global river conditions during bankfull flow. We determine that the magnitude of density stratification can be predicted by a function that considers an entertainment parameter, sediment concentration, and bed slope.