A two-layer model for the evolution and propagation of dense and dilute regions of pyroclastic currents

We present calculations of a simplified two-layer pyroclastic flow model, which incorporates different physics to represent the dense and dilute regions of column collapse pyroclastic density currents. The two layers evolve separately, but are coupled through mass exchange as suspended ash in the dilute cloud settles into the underlying dense basal layer. The runout distance of the upper dilute current and the associated runout time are found to increase with increasing initial column height and decreasing particle size. Likewise steeper slopes increase the propagation velocity and thus the runout distance. The independent runout distance of the basal flow exhibits opposite behaviour to that of the parent dilute current, increasing with decreasing initial column height and increasing particle size. Observed runout distances can be calculated for basal flows using a Coulomb friction law, but deposit morphology is not well reproduced and is more realistic when an empirical slope-dependent sedimentation rate is included. Dominant flow behaviour is controlled by the rate of mass transfer from the parent suspension current into the dense underflow. Tall column collapses, which contain fine-grained particles, transfer their mass slowly to the dense basal flow and are well described by dilute cloud assumptions. However, for short columns containing coarse grains, the particulate mass is rapidly transferred from the collapsing dilute current into the basal flow. The bulk of the pyroclastic current material propagates as a concentrated suspension for the majority of its travel distance and is better described by the physics of granular avalanches.