Experimental observations of water‐like behavior of initially fluidized, dam break granular flows and their relevance for the propagation of ash‐rich pyroclastic flows

<jats:p>The physics of ash‐rich pyroclastic flows were investigated through laboratory dam break experiments using both granular material and water. Flows of glass beads of 60–90 <jats:italic>μ</jats:italic>m in diameter generated from the release of initially fluidized, slightly expanded (2.5–4.5%) columns behave as their inertial water counterparts for most of their emplacement. For a range of initial column height to length ratios of 0.5–3, both types of flows propagate in three stages, controlled by the time scale of column free fall ∼(<jats:italic>h</jats:italic><jats:sub>0</jats:sub>/<jats:italic>g</jats:italic>)<jats:sup>1/2</jats:sup>, where <jats:italic>h</jats:italic><jats:sub>0</jats:sub> denotes column height and <jats:italic>g</jats:italic> denotes gravitational acceleration. Flows first accelerate as the column collapses. Transition to a second, constant velocity phase occurs at a time <jats:italic>t</jats:italic>/(<jats:italic>h</jats:italic><jats:sub>0</jats:sub>/<jats:italic>g</jats:italic>)<jats:sup>1/2</jats:sup> ∼ 1.5. The flow velocity is then <jats:italic>U</jats:italic> ∼ <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrb15706-math-0001.gif" xlink:title="equation image"/>(<jats:italic>gh</jats:italic><jats:sub>0</jats:sub>)<jats:sup>1/2</jats:sup>, larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at <jats:italic>t</jats:italic>/(<jats:italic>h</jats:italic><jats:sub>0</jats:sub>/<jats:italic>g</jats:italic>)<jats:sup>1/2</jats:sup> ∼ 4. Granular flow behavior then departs from that of water flows as the former steadily decelerates and the front position varies as <jats:italic>t</jats:italic><jats:sup>1/3</jats:sup>, as in dry flows. Motion ceases at <jats:italic>t</jats:italic>/(<jats:italic>h</jats:italic><jats:sub>0</jats:sub>/<jats:italic>g</jats:italic>)<jats:sup>1/2</jats:sup> ∼ 6.5 with normalized runout <jats:italic>x</jats:italic>/<jats:italic>h</jats:italic><jats:sub>0</jats:sub> ∼ 5.5–6. The equivalent behavior of water and highly concentrated granular flows up to the end of the second phase indicates a similar overall bulk resistance, although mechanisms of energy dissipation in both cases would be different. Interstitial air‐particle viscous interactions can be dominant and generate pore fluid pressure sufficient to confer a fluid‐inertial behavior to the dense granular flows before they enter a granular‐frictional regime at late stages. Efficient gas‐particle interactions in dense, ash‐rich pyroclastic flows may promote a water‐like behavior during most of their propagation.</jats:p>