Pyroclastic density currents (PDCs) are composed of hot gas and glass like 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 to 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 a “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 show that overspills remain hot and extremely hazardous throughout the area they inundate. Energetic flows can splash up and out of the channel and when the splash collapses it causes more ash clouds to rise. This work illustrates the danger of overspilled currents and points out situations when they may occur.