Deposits from ash-cloud surges associated with dome collapse can, under certain conditions, be remobilised to form surge-derived pyroclastic flows (SDPFs). Using numerical modelling, we reproduce the emplacement of these flows and investigate the conditions that favour their genesis. We use the new version of the numerical model VolcFlow, which simulates the two components of a pyroclastic flow: the basal avalanche and the overriding ash-cloud surge. The basal avalanche (primary block-and-ash flows and SDPFs) are simulated using three previously published rheological laws: plastic, frictional and frictional velocity-weakening rheologies. Applied to the 25 June 1997 dome collapse at Soufrière Hills Volcano, the models reproduce to different degrees the deposit footprints formed by the block-and-ash flows, the ash-cloud surges and the SDPFs. In the plastic model, SDPFs occur if the ash-cloud surge deposit exceeds a threshold thickness that allows it to remobilise and flow. In the frictional models, SDPFs occur only if ash-cloud surge deposition takes place on a slope exceeding the friction angle of the ash. Results also highlight that SDPFs appeared so clearly in 1997 at Montserrat due to a combination of topographic factors: (i) a bend in the Mosquito Ghaut drainage that allowed the ash-cloud surges to detach, (ii) a depositional area on the watershed between the eastern and western drainage channels and (iii) a network of tributaries that drained all the remobilised mass into Dyer’s River to form a single, large SDPF. Our model could be a promising tool for the future forecasting of hazards posed by surge-derived pyroclastic flows.
