Suspended Sediment-Induced Stratification Inferred From Concentration and Velocity Profile Measurements in the Lower Yellow River, China

Abstract

Despite a multitude of models predicting sediment transport dynamics in open-channel flow, self-organized vertical density stratification that dampens flow turbulence due to the interaction between fluid and sediment has not been robustly validated with field observations from natural rivers. Turbulence-suppressing density stratification can develop in channels with 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 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 calculation of sediment suspension is used to determine that sediment diffusivity increases with grain size. Finally, we calculate concentration and velocity profiles, showing that steady-state sediment suspensions are reliably predicted over a range of stratification conditions larger than had been previously documented in natural river flows. We determine that the magnitude of density stratification can be predicted by a function considering an entrainment parameter, sediment concentration, and bed slope.