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Abstract:
Dilute pyroclastic density currents (dilute PDCs) are frequent and hazardous volcanic phenomena. Better mitigating against PDC hazards requires understanding of the vertical velocity and density structures inside flows and how these are modified during flow runout, particle sedimentation, air entrainment and buoyancy reversal. Direct measurements of the velocity and density structure of PDCs are currently absent. We present measurements from dynamically and kinematically scaled, hot large-scale experiments using natural volcanic material and air. In these experiments we systematically vary the roughness of the lower flow boundary from hydraulically smooth to naturally scaled terrain and investigate their influence on the vertical flow structure.We show that vertical velocity profiles are mathematically characterized by a time variant power-Gaussian, including variation in basal slip. We demonstrate that boundary slip and its time-variance is related to erosion- and deposition behaviour at the lower flow boundary and should be included in analytical and numerical models of dilute PDCs. A strong correlation between vertical and downstream velocities is shown to occur in the currents body, showing the interdependence of velocity components and providing a physical model for the vertical velocity component. Turbidity current models of the vertical density structure agree well with our experiments. However, deviations are caused by rapid sedimentation of mesoscale clusters in proximal to medial reaches and buoyant rising of ash in distal reaches. These results have implications for PDC models that aim at predicting flow velocity, reach, distances of buoyancy reversal and destructive power.
