An SPH study on viscoplastic surges overriding mobile beds: The many regimes of entrainment

Flow-type landslides entrain mobile bed material, but the processes involved are diverse and require systematic study. We perform direct numerical simulations using the open-source SPH package DualSPHysics with a regularized Herschel-Bulkley rheology. We then compare model output with physical test data, and hence investigate the effects of varying the bed yield stress τy,b and bed depth hb, interpreting the results using a newly-identified set of dimensionless numbers. Results reveal diverse interaction regimes between surges and mobile beds, including “rigid bed”, “lubrication”, “shallow ploughing”, “surfing”, “plunging”, and “deep ploughing”. Shallow, borderline-stable beds “lubricate” the surge: once destabilized, these beds cause strong acceleration of the combined flow front. Deeper borderline-stable beds allow the surge material to “plunge” downward, massively displacing bed material upward and downstream. For stabler beds, “ploughing” and “surfing” are associated with intermediate and high values of τy,b, respectively. In both cases, beds retard the surge, with mobile dams forming for “ploughing” regimes. Across all regimes identified, the influence of τy,b is non-monotonic, with intermediate values decelerating the combined flow fronts the most. Furthermore, the different interaction regimes exhibit unique velocity profiles. We develop phase diagrams based on three dimensionless numbers, demarcating these regimes.