Hydraulics of subaqueous ash flows as deduced from their deposits: 2. Water entrainment, sedimentation, and deposition, with implications on pyroclastic density current deposit emplacement

Abstract

Turbidity currents are the most common flows of water, and suspended and bed load sediment occurring in the relatively deep sea and lakes, and act as large systems of sediment distribution. Due to the turbid nature of the flow, they may impact subaqueous infrastructures, as well as ecosystems, during motion, so quantifying the turbidity degree is important to predict the current impact. A particular type of these currents is the so-called volcaniclastic turbidity currents or subaqueous ash flows, which are mostly composed of fine-grained volcanic particles, and are used here as synonymous (sensu hydraulic) with the meaning of secondary current.In this paper, a method to estimate the water entrainment (column condition model), as well as the sedimentation and deposition rates (conveyer model), in volcaniclastic turbidity currents is proposed, by starting from the physical features of the deposits or inverse procedure. Some criteria of sediment mechanics are used to approximate the flow hydraulic parameters needed to quantify the water entrainment, as well as the shear velocity in volcaniclastic turbidity currents.The deposits used as case study are the impressive, meters thick, well-sorted rhyolitic ash turbidites of Late Pliocene cropping out in Southern Italy, particularly in the Craco area, Matera. The water entrainment coefficient of the currents is calculated in a range of particle concentration and slope angle, whereas the slope angle giving the sedimentation rate is calculated in a range of flow shear velocity, which in turns gives the deposition rate.The results have a general validity for depositional turbidity currents laden with well-sorted sediment, and they show that the water entrainment is low for relatively dense, slow-moving, subcritical flows, but it increases as the Richardson number decreases for relatively dilute, fast, supercritical flows. Moreover, the sedimentation and deposition rates are high for relatively intense flows moving over gentle slopes, but they decrease for relatively weak flows moving over steep slopes. The deposit sedimentary structures thus infer sedimentation, and particularly depositional processes.Finally, the conveyer model is extended conceptually to pyroclastic density currents, when approximating, but not exclusively, the column condition model in the flow above the flow boundary zone.