Flow evolution of experimental gravity currents: Implications for pyroclastic flows at volcanoes

Gravity current experiments, consisting of aqueous glucose flows and turbidity suspensions, were performed to investigate mixing and sedimentation processes, the evolution from high-density to low-density flows, and the effects of variable grain density. The initial solution or suspension was released from a lucite box into a water-filled flume 5.5 m long. The aqueous glucose solutions had initial densities ranging from 1014 to 1206 kg m(-3). The turbidity suspensions consisted of spherical acrylic resins with a narrow grain size of 0.21-0.30 mm (2.25-1.75 Phi) and two densities of 1143 and 1196 kg m(-3). The initial bulk densities of the turbidity suspensions were 1040 and 1062 kg m(-3). Both the aqueous glucose flows and the turbidity currents became density stratified with distance, with the densest flows exhibiting the largest vertical density gradients. For a given initial bulk density, the turbidity flows had lower velocities, densities, viscosities, and Reynolds numbers due to the effects of sedimentation. Flow transformations occured in the turbidity flows after most sedimentation had taken place and were thus a result and response to sedimentation and the development of stable density stratification in the flows. Flow transformation was more pronounced in the denser flow due to its larger vertical density gradient. These experiments are relevant for both subaqueous and subaerial pyroclastic density currents generated from explosive volcanic eruptions. Many subaqueous pyroclastic flow deposits exhibit a twofold division corresponding to a massive, coarse-grained, poorly sorted lower part overlain by a finer-grained, better-sorted, laminated sequence. The experiments suggest that these components develop, in part, as a result of density stratification and transformation of the flow during sedimentation. Subaerial pyroclastic flows commonly show transitions to lower-density surges. The experiments indicate that transformation to surge-type behavior as well as the subsequent behavior of the surge are partly controlled by the timing and rate of sedimentation from the parent pyroclastic flow.