On the Causes of Pulsing in Continuous Turbidity Currents

Velocity pulsing has previously been observed in continuous turbidity currents in lakes and reservoirs, even though the input flow is steady. Several different mechanisms have been ascribed to the generation of these fluctuations, including Rayleigh-Taylor (RT) instabilities that are related to surface lobes along the plunge line where the river enters the receiving water body and interfacial waves such as Kelvin-Helmholtz instabilities. However, the understanding of velocity pulsing in turbidity currents remains limited. Herein we undertake a stability analysis for inclined flows and compare it against laboratory experiments, direct numerical simulations, and field data from Lillooet Lake, Canada, and Xiaolangdi Reservoir, China, thus enabling an improved understanding of the formative mechanisms for velocity pulsing. Both RT and Kelvin-Helmholtz instabilities are shown to be prevalent in turbidity currents depending on initial conditions and topography, with plunge line lobes and higher bulk Richardson numbers favoring RT instabilities. Other interfacial wave instabilities (Holmboe and Taylor-Caulfield) may also be present. While this is the most detailed analysis of velocity pulsing conducted to date, the differences in spatial scales between field, direct numerical simulations, and experiments and the potential complexity of multiple processes acting in field examples indicate that further work is required. In particular, there is a need for simultaneous field measurements at multiple locations within a given system to quantify the spatiotemporal evolution of such pulsing.