Tsunamis generated by pyroclastic flows: experimental insights into the effect of the bulk flow density

For a better assessment of hazards related to tsunamis triggered by pyroclastic flows entering water, it is crucial to know and quantify the contribution of the physical parameters involved in the generation of waves. For this purpose, we investigate experimentally the effect of pyroclastic flow density on tsunami generation by considering variably fluidized granular flows denser or less dense than water, referred to as heavy and light granular flows, respectively, by varying the particle density. Qualitative observations show that differences in bulk flow density mainly affect the propagation of granular flows underwater. In contrast, the bulk flow density has little effect on the amplitude of the leading and largest wave. In fact, the wave amplitude is initially similar to the local water depth along the inclined plane, and then reaches a maximum value that depends mainly on the other flow parameters (i.e., velocity, thickness, volume of flows). Far from the shoreline, we provide evidence of the bulk flow density effect on the wave amplitude, while other characteristics of the leading wave remain broadly unaffected in the range of parameters considered. Finally, a main difference on the tsunami generation between light and heavy granular flows is related to the energy distribution between the leading largest wave and the wave train, which is attributed to different modes of interaction of the two flow types with the water. For tsunami hazard assessment, our study suggests that the contribution of the bulk flow density on tsunami generation has a second-order effect compared to other flow parameters.