The impact on the runout distance of pyroclastic density currents by density stratification

Pyroclastic density currents (PDCs) generated by volcanic eruptions initially are denser than the atmosphere, but then become buoyant through sedimentation and entrainment. This buoyancy reversal limits the distance PDCs travel. Sedimentation and entrainment are linked, however, through the density stratification within the current, because air entrainment is suppressed when a strong vertical density gradient exists. A density gradient develops from settling and resuspension of particles within the current. To investigate the feedback between density stratification and entrainment, we performed subaqueous, analog experiments by sending a slurry of plastic particles and water (concentration = 11.3 g L^-1) into a narrow flume submerged in a larger, water-filled pool. The slurry was heated to induce a buoyancy force in some experiments, and calcium chloride was added to some to enhance density stratification. The gradient in density varied with the amount of added salt and the relative temperature of the current. Currents with vertical density gradients ≥ 0.9 g L^-1 were found to not reverse buoyancy. In those strongly stratified currents, mixing appears to be suppressed, as these currents have greater downstream average velocity gradients than buoyant currents. Vertical velocity fluctuations were often negative, indicating fluid and particle momentum was carried toward the bed. In buoyant currents, the vertical velocity fluctuations were spread more evenly between positive and negative values, possibly indicating that the buoyancy force was counteracting downward momentum. In addition, turbulent kinetic energy near the bed of non-buoyant currents decreased, suggesting that large, turbulent eddies did not extend down through the entire current, and that entrainment and mixing were less vigorous than in buoyant currents. We suggest that strong density stratification within PDCs may inhibit buoyancy and increase runout distance compared to PDCs that are well mixed. This especially may be the case in high particle-concentration currents, in which sedimentation is not as impactful as in dilute currents and which have a greater dependency on entrainment to become buoyant.