The run-out distance of large-scale pyroclastic density currents: A two-layer depth-averaged model

Abstract

We have developed a new two-layer pyroclastic density current (PDC) model that considers the effects of the entrainment and thermal expansion of ambient air in the upper dilute layer in order to investigate the dominant factors controlling the run-out distance of large-scale PDCs. Numerical simulations show a dilute current spreading radially from the collapsing eruption column edge and forming a thin, dense, basal current through particle settling; i.e., a two-layer PDC forms. Forward propagation of the two-layer PDC stops owing to lift-off of the dilute current and/or deposition of the dense current, and eventually the two-layer PDC converges to a steady state. Our parametric study classifies the flow patterns of steady-state two-layer PDCs into one of two flow regimes according to the relative magnitude of the run-out distances of the dilute and dense currents. This relative magnitude critically depends on the ratio of the deposition speed at the base of the dense current (D) to the speed of particles settling from the dilute current to the dense current (Ws). The run-out distance of the whole two-layer PDC is determined by that of the dilute current when D/Ws is large (≳4 × 10−3). Otherwise, when D/Ws is small (≲4 × 10−3), the run-out distance of the dense basal current exceeds that of the dilute current; the former increases as D/Ws decreases. The run-out distance of the dilute current is too short to explain some of the long run-out distances of PDCs observed in the field (e.g., the 1991 Pinatubo and 2014 Kelud eruptions). It is suggested that the dense current traveling beyond the lift-off point of the parent dilute current plays a significant role in the emplacement of PDCs with such long run-out distances.