Modelling tephra dispersal and ash aggregation: application to the 26 April 1979 eruption of La Soufrière St. Vincent

On 26 April 1979, La Soufrière St. Vincent volcano (West Indies) erupted producing a tephra fallout that blanked the main island and neighbouring island of Bequia, located southwards. Using deposit measurements and available observations reported in Brazier et al. (1982), we reproduce the main features and processes of the eruption and estimate by best-fit the eruption source parameters such as the Mass Eruption Rate (MER), the Total Erupted Mass (TEM), and validate the Total Grain-Size Distribution (TGSD). Tephra transport and deposition is simulated using the 3D time-dependent Eulerian model FALL3D. The TGSD reconstructed by Brazier et al. (1982) showed a bi-modal pattern having a coarse mode around 0.5 mm and a fine mode around 0.06 mm. Because ash aggregation was significant during the eruption, we perform a comparative study by neglecting and accounting for aggre- gation using three different aggregation models. The models for ash aggregation in the plume assume wet conditions consistently with the eruptive phreatomagmatic features, considering both the effects of air moisture and magmatic water. The sensitivity to the driving meteoro- logical model (WRF-ARW) is also investigated considering model spatial resolutions of 5 and 1 km showing that, for this kind of eruptions, high-resolution meteorology is pivotal. The optimal best-fit results indicate a column height of ~13 km above the vent, a MER of ~7×106 kg/s which, for an eruption duration of 370 seconds, gives a TEM of ~2.7×10^9 kg. The opti- mal aggregate mean diameter obtained is 1.5Φ with a density of 520 kg/m3 contributing to ~28 % of the deposit loading.