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We provide a new perspective on the transition of hyperpycnal flows into saline turbidity currents, which permits longer runout lengths than might be otherwise expected. This mechanism relies on the differential turbulent diffusion of salt and sediment, and in contrast to ambient saltwater entrainment it enables the salinification of the freshwater current without diluting the sediment concentration field by a corresponding amount. The freshness-to-sediment ratio is introduced in order to quantify the transition process. The results of high-resolution simulations provide estimates for the transition distance of hyperpycnal model flows into saline turbidity currents.

Plain Language Summary In some near-coastal areas of the world's oceans, sediment-rich rivers trigger so-called hyperpycnal currents, which are currents composed of river-derived freshwater and sediment that propagate along the seafloor. In general, one may expect such currents to have a short runout length, since sediment deposition will reduce their density until they loft from the seafloor. There is evidence, however, that in some settings hyperpycnal currents can have much larger runout lengths than expected. Here we propose a novel perspective on such large runout lengths, by showing that differential diffusivities can enable the hyperpycnal flow to transition into a saline turbidity current. This mechanism relies on the fact that salt and sediment diffuse at different rates, so that the freshwater current can become salinified without diluting its sediment concentration field.