On the ability of pyroclastic flows to scale topographic obstacles

We present a simple analysis of momentum dissipation in a pyroclastic flow with high initial velocity and show that the deceleration is inversely proportional to the volumetric flow rate. Consequently, for a given mass flow rate, dense flows slow down more rapidly than dilute flows. The analysis ignores density stratification and flow unsteadiness, and assumes that deceleration is due to turbulent dissipation alone, but the results have implications for a wide range of flow regimes. We apply our analysis to the distribution of the 1800-year-old deposits of the Taupo pyroclastic flow, the velocity of which is constrained by the height of topographic obstacles that it scaled. The volumetric flow rate required to maintain velocities high enough to overtop the rugged topography is consistent with a dilute flow, but strongly argues against the dense flow hypothesis. The same conclusion applies to other pyroclastic flow deposits distributed over rugged terrain.