The nature, thickness, and location of deposits from a particulate gravity current is strongly influenced by whether the flow is concentrated or dilute, whether it is laminar or turbulent, and whether it is supercritical or subcritical. These transitions are causally linked, because there is a clear contrast between a concentrated, laminar flow-type (e.g., pyroclastic flows and debris flows) and a dilute, turbulent flow-type (e.g., pyroclastic surges and turbidity currents). In this paper it is shown that the primary transition is from a dense to a less dense current, leading, in turn, to a transition from laminar to turbulent flow. This density transition can be caused by interface instability or by vigorous entrainment of ambient fluid at a hydraulic jump. It is also shown in this paper that hydraulic jumps occur at Froude numbers significantly different from unity. These concepts are confirmed by previously published data from a gravity current in the San Dimas Reservoir.

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