Multiple Flow Slide Experiment in the Westerschelde Estuary, The Netherlands

Flow slides form a major threat to flood defences along coastlines and riverbanks in the Netherlands. Due to the uncertainties with respect to the process in combination with the severity of the consequences and costs for prevention measures, there is a need to improve existing models for prediction or occurrence of, and damage by, flow slides. One of the key questions to be answered is whether slope failure by a flow slide is caused by up-slope migrating breaches or by static liquefaction. Although fundamentally different mechanisms, both result in a flowing sand-water mixture or turbidity current that eventually redeposits on a gentle slope. Over the last decades numerical models have been developed for both mechanisms, based on flume experiments. Upscaling these experiments is complex, as scaling rules are different for the various processes involved. To evaluate the failure mechanism on a natural scale, validate numerical models and test new technology to monitor the occurrence of flow slides, a large, controlled field test was performed. The test site was situated in the Westerschelde estuary, in the south-western part of the Netherlands (Fig. 24.1). Several flow slides of 105-106 m3 have occurred in this area in the past. In advance of the experiment, cone penetration tests and boreholes were performed on the test location. Pore water pressure sensors were installed in the sand. Triaxial tests and grain size distribution measurements were performed on collected soil samples. The flow slides were initiated by means of steepening of the slope by dredging. Eventually several autonomously retrogressing flow slides were observed, running several hours over a maximum distance of about 100 m and resulting in a total displaced volume of several 103 m(3) of sand. During the test the evolution of the slope topography was monitored continuously by three multibeam survey vessels. This resulted in a full multibeam survey of the area almost every quarter of an hour and, assisted with other advanced instruments, enabling us to witness the early development of a flow slide.