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

Gravity current experiments, consisting of aqueous glucose flows and turbid ity suspensions, were performed to investigate mixing and sedimentation pro cesses, the evolution from high-density to low-density flows, and the effec ts of variable grain density. The initial solution or suspension was releas ed from a lucite box into a water-filled flume 5.5 m long. The aqueous gluc ose 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 11 96 kg m(-3). The initial bulk densities of the turbidity suspensions were 1 040 and 1062 kg m(-3). Both the aqueous glucose flows and the turbidity cur rents became density stratified with distance, with the densest flows exhib iting the largest vertical density gradients. For a given initial bulk dens ity, 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 st able density stratification in the flows. Flow transformation was more pron ounced in the denser flow due to its larger vertical density gradient. Thes e experiments are relevant for both subaqueous and subaerial pyroclastic de nsity currents generated from explosive volcanic eruptions. Many subaqueous pyroclastic flow deposits exhibit a twofold division corresponding to a ma ssive, coarse-grained, poorly sorted lower part overlain by a finer-grained , better-sorted, laminated sequence. The experiments suggest that these com ponents develop, in part, as a result of density stratification and transfo rmation of the flow during sedimentation. Subaerial pyroclastic flows commo nly show transitions to lower-density surges. The experiments indicate that transformation to surge-type behavior as well as the subsequent behavior o f the surge are partly controlled by the timing and rate of sedimentation f rom the parent pyroclastic flow.