Turbulent boundary layer shear flows as an approximation of base surges at Campi Flegrei (Southern Italy)

Abstract

Small-volume base-surge deposits of Astroni and Agnano–Monte Spina eruptions at Campi Flegrei consist of sets of beds formed by the passage of multiple pyroclastic density currents. Each set is made by a fining-upward sequence consisting, from base to top, of: (1) an inversely graded layer made of bombs and lapilli; (2) a finely laminated layer with wavy bedforms made of coarse ash and fine lapilli; (3) a structureless fine-ash bed. Lack of discontinuities at layer contacts allows us to hypothesize that each set of beds represents the passage of one pyroclastic density current. The sequence formed as a function of the decreasing particle transportation capability of the base surge during the waxing–sustained–waning phases of flowage. We make the simplifying assumptions that the flow, on passing from the deposition of the inversely graded layer to the deposition of the finely laminated layer, had low particle concentration, was quasi-steady and incompressible. By such assumptions, some of the equations that regulate sediment mechanics are applicable to the base surges of Campi Flegrei to reconstruct flow density and shear velocity in between the deposition of the two layers. Structural and textural features of deposits stress the importance of shear and turbulence during transportation and suggest that the turbulent boundary layer shear flow is a good approximation to such pyroclastic density currents. We used the Coles formula to reconstruct velocity as a function of flow height during the formation of the finely laminated layer of Astroni and Agnano–Monte Spina base surges. The methodology we used allows the reconstruction of flow parameters just at a particular point in time and space of base-surge evolution, and does not provide the complete picture of the flow history of pyroclastic density currents. Nevertheless, flow velocity and density data, as obtained with the proposed approach, point to dynamic pressure values that are compatible with those reported for similar currents on other volcanoes. We conclude therefore that the results obtained give approximate but helpful information on the nature of such hazardous events at Campi Flegrei.