Generation of Overspill Pyroclastic Density Currents in Sinuous Channels

Due to their mobility, high velocities, and common occurrence, small‐volume pyroclastic density currents (PDCs) represent a major hazard around volcanoes. Small‐volume events are particularly sensitive to topography and channelization into drainage basins. Understanding the flow transition initiated by avulsion or overspill from valley confined PDC to unconfined PDC is necessary to mitigate their damage. We present three‐dimensional multiphase models of channelized PDCs and link the generation of overspill currents to channel geometry (width, depth, and curvature). Our simulation geometries span the range of natural channels commonly found in stratovolcanic landforms. Channels help inhibit ambient air entrainment into the underflow and can maintain and deliver material with minimal cooling during transport. We show that the main avulsion mechanism can include significant portions of the insulated underflow layer from the channel leading to a dangerously hot overspilled current. In all sinuous channel simulations, the underflow of the current becomes superelevated when it encounters bends and is able to overwhelm channel walls. The amount of mass overspilled is a function of channel curvature. Superelevation of the current increases with channel curvature and decreasing channel width but is underestimated using traditional estimates of superelevation due to the lack of the free surface in these flows. We also identify the occurrence of buoyant plume and secondary overspill events due to cross stream flow within the channel which would produce complex deposits associated with overspill events. Plain Language Summary Pyroclastic density currents (PDCs) are composed of hot gas and ash or rocks. They can be very mobile, flowing over obstacles with ease but their pathway is often controlled by the topography they flow over. When a PDC is confined in a valley it increases the distance it can travel but the PDC can overflow from the valley unexpectedly increasing the hazards of the volcanic events. We apply 3D computer modeling to model how PDCs flow in simplified valley channels. We find that highly curved bends in an “S” shaped channels combined with thin widths increase the amount of overspilling. The overspill is caused by the flow rising above the walls of the valley and pouring out onto the channel banks. Modeling the temperature of these flows shows that overspills remain hot and extremely hazardous throughout the area they inundate. Energetic flows can spl