The Entrainment Rate of Non-Boussinesq Hazardous Geophysical Gas-Particle Flows: An Experimental Model With Application to Pyroclastic Density Currents

Bernd Zimanowski and Ralf Büttner shared with the authors the experiment design and development. They are gratefully thanked. The paper greatly benefited from the revision of Gert Lube and Kristen Fauria. Published with permission of the Executive Director of British Geological Survey (UKRI). Data can be found at the Zenodo repository with the following link https://zenodo.org/search?page=1&size=20&q=dellino, with DOI 10.5281/zenodo.3408690.The entrainment rate of pyroclastic density currents is investigated by large‐scale experiments. The ground flows are initiated by the impact on the terrain of a dense gas‐particle fountain issuing from a cylindrical conduit, similarly to natural volcanic events. On impact, the excess density with respect to the surrounding atmosphere was up to 11.6 kg/m3, making the currents non‐Boussinesq. A power law model of the entrainment rate is developed, which is similar to that proposed for snow avalanches by Ancey (2004, https://doi.org/10.1029/2003JF000052) and is verified for the Richardson's number range between 0.25 and 5.95. Rapid changes of the entrainment are caused by (i) strong accelerations at the fountain impact on the ground; (ii) break in slope; and (iii) topographic obstacles. Such changes, together with the sedimentation rate, influence flow mobility. The use of the power law is suggested for modeling the motion of unsteady hazardous geophysical mass flows such as pyroclastic density currents and snow avalanches.Peer reviewe