Observations of self-accelerating turbidity currents in the Xiaolangdi reservoir, Yellow River, China

Turbidity currents are an important mechanism for delivering sediment to depositional basins, whether the basin be a lake, reservoir, or ocean. However, since turbidity currents move along the bed and initiate with unpredictable timing, it is difficult to produce detailed observations of these flows. For example, despite acceleration over space (self-acceleration) being a critical process at the initial development stage of turbidity currents, this phenomenon has been rarely measured in the field. Here, we provide observations of self-accelerating turbidity currents in the Xiaolangdi reservoir, situated on the Yellow River, China. Multibeam bathymetric surveys provide a 3D depiction of the bed bathymetry, which was measured before and after a series of turbidity current events associated with the summer flood season. A parametric echo sounder is used to link bathymetry and stratigraphy of the sedimentary deposit. Velocity and sediment concentration profiles, and bed material and suspended sediment grain size measurements, were collected along the turbidity current pathway. These data shed insight onto the dynamics of the flow events. We highlight some striking features: (1) the spatiotemporal span of self-acceleration is markedly variable, and controlled by multiple factors, including input sediment concentration and flow discharge, local geometry, and downstream boundary conditions; (2) two types of self-acceleration have been observed: one close to the plunging point, where the mixed fluvial sediment-laden flow collapses into a stratified turbidity current layer, thus focusing momentum from the whole water column into a thin layer and resulting in acceleration, and the other one where the turbidity current flow encounters a local enhancement in bed slope; (3) instead of producing denser sediment concentration, the acceleration process of the turbidity current tend to induce a thicker layer of the flow along the path, indicating spontaneous entrainment of sediment and ambient fluid. These observations provide first-hand assessment of turbidity current flows in a natural, field-scale setting, and provide new insights into the impact of turbidity currents on bed morphology.