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Edited by:
   [~trp]

   [Geeralt A. van den Ham|~vdh]

   [Dick R. Mastbergen|~mastber]

   [Frank Engering|~eg]


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Here 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|GEO:Literature^Handreiking Toetsen Voorland Zettingsvloeiing tbv verlengde derde toetsronde.pdf]. For more detailed information about the stability of sandy and silty under water slopes you can also download our [brochure|GEO:Literature^PB_Stability of sandy and silty under water slopes.pdf].
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!D-Flowslide-pic1.png|border=1!
Figure 1: Examples of damage to a dike due to a flow slide

h4. Introduction

Flow slides form a major threat for flood defences along (estuary) coastlines and riverbanks in the Netherlands. Such flow slides may result in severe damage to dikes and structures, eventually leading to flooding of the hinterland (see Figure 1). Measures to prevent, mitigate, or even repair flow slides are costly. Due to the complexity of flow slides, methods that enable an accurate quantitative risk assessment are under-developed, especially compared to methods currently available for other failure mechanisms (e.g. piping below the dike or macro-instability of the dike body).

h4. Flow Slide: description of processes

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.

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  !Introduction_Process.png|border=1!
Figure 2: Processes possibly involved in a flow slide

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*{_}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.


h4. !D-Flow Slide.jpg!


h4. D-FLOW SLIDE

*D-FLOW SLIDE* is a user-friendly software tool developed for performing safety assessment on the failure mechanism flow slide. The safety assessment includes the *[Global|GEO:D-Flow Slide - Global]* assessment rules according the [VTV-2006 |GEO:Literature^vtv2006.pdf]and a probabilistic *[Detailed|GEO:D-Flow Slide - Detailed]* assessment method based on the First Order Reliability Method. Furthermore advanced models for static liquefaction (*[SLIQ2D|GEO:D-Flow Slide - Advanced]*) and breach flow ([*HMBreach/HMTurb*|GEO:D-Flow Slide - Advanced]) have been implemented in *D-FLOW SLIDE*.
*D-FLOW SLIDE* has been implemented in the software DAM and will in future be implemented in RingToets.
Implementation in DAM makes it possible to do calculations with *D-FLOW SLIDE* in combination with stochastic subsurface models and various probabilistic methods (probabilistic library).
Flow slide is a failure mode of the foreland of the levee and may not necessarily directly result in failure of the levee and therewith flooding of the hinterland. However, failure of the foreland may induce a so-called "direct" failure mechanism of the levee, such as backward erosion and macro-instability. Since DAM includes calculation models of several direct failure modes, it is possible to calculate the combined probability on a flow slide and the subsequent failure of the levee by a "direct" failure mode.

h4. Licence / Support

In the near future you can obtain a licence during short period, by sending an email to Sales: <[sales@deltaressystems.nl|mailto:sales@deltaressystems.nl]>.
-If problems are encountered or if you have questions about this program, please send an e-mail to the Deltares Systems Support team: <--[support@deltaressystems.nl|mailto:support@deltaressystems.nl]-->.-

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