Flow models of natural debris flows originating from overconsolidated clay materials

In this paper, slides and debris flows in overconsolidated clay materials are simulated numerically. As a case study, the models are applied to the Storegga slide in the Norwegian Sea and in particular to the sub-region called Ormen Lange, where the information available is the most precise for a subaqueous debris flow. Three different models for the rheology of clay are used: a viscoplastic (Bingham) fluid model, a viscoplastic fluid with interspersed solid blocks, and a viscoplastic model with yield strength increasing with depth. The small-scale debris flows in the Ormen Lange area can be reasonably well understood in terms of a pure Bingham model without granular effects and blocks. The presence of intact blocks in the region, however, indicates that at least the top layer of the sliding sediments was not destroyed by the flow. It suggests that the flow occurred mainly at high shear rate in a lubricating layer of mud deriving partly from the disintegration of the block's own material, and possibly from the entrainment of hemipelagic sediments along the flow path while the top part was left unsheared. The failure of the model with blocks probably stems from the use of the Coulomb friction law to represent the interaction between the block and the seabed. The Bingham model works better because during the flow of such fluids an unsheared plug region is formed naturally, even in unconsolidated materials. Combining the simulations with these three models, a possible scenario for the Ormen Lange debris flows is deduced according to which the lubricating layer supporting the blocks has a yield strength of about 10–15 kPa.