Breaching and liquefaction in subaqueous retrogressive flow slides

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.