Modeling tephra sedimentation from a Ruapehu weak plume eruption

We present a two‐dimensional model for sedimentation of well‐mixed weak plumes, accounting for lateral spreading of the cloud, downwind advection, increase of volumetric flux in the rising stage, and particle transport during fallout. The 17 June 1996 subplinian eruption of Ruapehu produced a bent‐over plume that rose to a height of 8.5 km in a wind field with an average velocity of 24 m s −1 and generated a narrow deposit on land extending up to 200 km from vent. The sedimentation from the Ruapehu plume was dominated by coarse ash, with all the blocks and most of the lapilli falling while the plume was still rising. Particles with diameter <125 μm show three accumulation maxima, one coincident with a secondary maximum in the total tephra deposit about 150 km from the vent. Numerical modeling shows that the plume started spreading horizontally beyond about 30 km from vent. Investigations also show that Fickian diffusion can be a good approximation to the crosswind spreading of both “vigorous” and “low‐energy” weak plumes, but a wide range of diffusion coefficients is to be expected, with the largest values necessary to describe the most vigorous weak plumes such as Ruapehu. One‐dimensional and two‐dimensional simulations show that tephra sedimentation must have been subvertical. This feature, also supported by direct observations, can be the result of the turbulence structure of the local wind field and convective instabilities, which could also have played an important role in the generation of the pronounced sinuosity shown by the Ruapehu deposit.