Abstract

The BIG'95 debris flow affected 2,200 km2 of the Ebro continental slope and rise in the western Mediterranean Sea. The resulting deposit of sandy and silty mud has a volume of 26 km3 and is up to 150 m thick. Dating yields a minimum failure age of ca. 11,500 cal yr BP. Swath bathymetry data, very-high-resolution seismic reflection profiles, side-scan sonographs, and sediment cores indicate that the debris flow consisted of two main components: relatively coarse, more mobile material, which was mostly remolded during flow; and finer, more cohesive material, which moved as independent blocks and partially kept its internal coherence. These blocks were partially buoyant and were pushed and dragged up to 15 km by the mobile material. During transport the blocks broke into smaller fragments. The more mobile material kept on flowing, reaching almost 2,000 m water depth in the Valencia Channel after traveling ~ 110 km.

In this paper, a conceptual and numerical model of the BIG'95 debris flow is investigated. The more mobile component has been modeled as a Bingham fluid with assigned rheological properties. The more cohesive material has been idealized as consisting of rigid blocks. Using a simplified end-member scenario, the blocks are modeled as being at rest and then moved by interaction with the mobile material. As the mobile material comes into contact with the blocks, the latter are subjected to interaction forces: the impact force, static earth-pressure force, shear force, and water drag, in addition to gravity and the Coulomb friction with the seafloor. Theoretical values for the internal friction angle of the blocks and the yield strength of the mobile material were adjusted to reproduce the observed run-outs of 15–20 km for the blocks and 70 km for the mobile material. The numerical results demonstrate that the proposed conceptual model is physically possible with input values of the order of 1° for the Coulomb frictional angle and 800 Pa for the yield strength. Such values, even if certainly low, are in line with recent estimates for other subaqueous debris flows, and might be explained by processes aiding mobilization such as hydroplaning and entrapment of more mobile material underneath the blocks. Alternatively, it might imply that the blocks were already in motion before the impact of the mobile material took place.

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