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Although retrogressive flow slides in subaqueous sandy slopes can be very large and cause substantial damage, the failure mechanisms of such slides are not very clear yet. This study analyses two well-monitored flow slides in a shoal margin in the Western Scheldt estuary in the Netherlands: a natural flow slide that eroded 300 m into the edge of the shoal and an artificially induced flow slide that was triggered by dredging and eroded only 30 m of the shoal margin. Both slides were simulated with a newly developed numerical model that describes the physics of slow retrogressive breaching and the much faster retrogression of statically liquefied fine to medium (silty) sands. The simulations show that the differences in trigger and size can be explained by assuming that in the larger slide both retrogressive breaching and static liquefaction took place, while in the smaller one only breaching occurred. The main contribution of retrogressive liquefaction to the larger slide was the generation of a temporary high-erosive density flow that proved sufficient to create such a high, near-vertical slope that the breaching process could continue over a long period and distance. It is therefore likely that both breaching and static liquefaction play a role in large natural flow slides. Key words: flow slide, retrogressive breaching, static liquefaction, numerical model, subaqueous slope instability, 1. Introduction This paper deals with subaqueous flow slides in fine to medium (silty) sands that may include thin, discontinuous mud layers. Such flow slides may encompass several million cubic [...]

Field surveys and experiments in a tilting flume on hyperconcentrated sand-water mixture (SWM) flows were conducted. At slopes slightly smaller than the equilibrium slope, a rapid development of sand bars propagating upstream was observed. These bars consist of an almost horizontal terrace with subcritical flow, a steep lee-side slope with supercritical flow, and a hydraulic jump between subsequent bars. The erosion processes on the bar are limited by the high sand concentrations and flow velocities on the steep slopes. As a result, the erosion rate is more moderate than predicted by the pickup functions used in classical sediment-transport theory. A simple one-dimensional model is presented to study the behavior of triangular schematized bars. From an analysis of the equations it is concluded that an increase in erosion rate requires an increase in bed slope to maintain equilibrium flow conditions. Finally, simulations with the model are compared with observations in the field and in the tilting flume. The agreement appears to be reasonable.