Suspended-sediment induced stratification inferred from concentration and velocity profile measurements in the flooding lower Yellow River, China

Suspended sediment in a river can result in density stratification that alters turbulent flow structure, thereby modulating water flow velocity and sediment transport capacity. However, measurements of density stratification in natural rivers are limited. Herein, we take advantage of engineered floods from the Xiaolangdi Reservoir on the Huanghe (Yellow River), China, to measure profiles of water velocity and sediment concentration during extreme changes in suspended sediment load. The engineered floods consist of a sediment-depleted clear-water discharge, by way of skimming from the reservoir surface (Phase 1), followed by sediment-enriched release by sluicing through gates embedded at the base of the reservoir dam (Phase 2). Hence, water discharge is directly controlled throughout the flood period and sediment concentration varies significantly. Detailed measurements of the vertical profile of velocity, sediment concentration and grain size were collected within a straight reach 80 km upstream the river mouth. During Phase 1, depth-averaged sediment concentration ranged from 2-10 kg/m³ and was driven by local suspension of sandy bed material. During Phase 2, depth-averaged sediment concentration increased to 15-20 kg/m³ reflecting the enhanced supply of fines from the dam. Preliminary analyses indicate the development of density stratification, based on recorded modulations of both the velocity and concentration profiles. The observed stratification is correlated with additional measured properties of the river system, including local bed grain-size and shear stress, washload fraction, and bed slope. These parameters are explored to develop a better understanding of the conditions leading to the development of density stratification.