In this space you can find information about the program D-FLOW SLIDE for analysing flow sliding in submerged slopes (in Dutch: zettingsvloeiing). The programme has been developed in 2012 and is based on the Handreiking Voorland Zettingsvloeiingen. For more detailed information about the stability of sandy and silty under water slopes you can also download our brochure.
Figure 1: Examples of damage to a dike due to a flow slide
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Flow slide is a complex failure mechanism that includes both soil mechanical and hydraulic features, of which the elementary ones are depicted in the flow chart in Figure 2. Two important physical (sub-)mechanisms are static soil liquefaction and breaching. For most of the documented cases of flow slides it is not clear to what extent static soil liquefaction and/or breaching played a role. Both mechanisms result in a flowing sand-water mixture, that eventually resediments under a very gentle slope.
Figure 2: Processes possibly involved in a flow slide
Static liquefaction entails the sudden loss of strength of loosely packed saturated sand or silt, which may result in a sudden collapse of the sand body. Contrary to “ordinary” slope failure, in which a clear rupture surface can be distinguished over which the instable soil mass slides while staying more or less intact, in a liquefaction flow slide the instable mass of sand (or silt) flows laminar like a viscous fluid.
Unlike liquefaction, breaching only takes place at the soil surface: a local steep part of the slope retrogresses upslope and generates a turbulent sand-water mixture flow over the sand surface of the under water slope. If the mixture carries enough sand and if the local slope is steep enough, the thickness of the mixture will grow by erosion of the sand surface. Although strongly dependent on the properties of the sand or silt, a breaching flow slide in general takes much more time (several hours) than a liquefaction flow slide (several minutes).
Regardless of the mechanisms involved, a flow slide needs a trigger. Soil liquefaction may be initiated by a rapid drop of outer water level, a small earthquake, or a change in geometry by erosion or local instability, resulting in an unfavourable change of stress conditions within the loosely packed sand or silt. Breaching requires an initial breach, which may be formed by scour, by a local slip failure or by a local liquefaction flow slide. The triggers for both liquefaction and breaching are presented in the top of the flow chart in Figure 2.
D-FLOW SLIDE
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