Design of a breach retardant dike

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Abstract

Sand dikes have a sand core and a clay layer to protect the core against erosion. It is not unlikely that this layer fails due to several failure mechanisms, exposing the sand core to water. This can be catastrophically when water is overtopping the dike. In case the protective cover fails, water can flow over the core and erode the sand, eventually causing the dike to breach. This breaching process is described and modelled for a better understanding. The BRES-model (BReach Erosion in Sand dikes) was specifically created for sand dikes and is used to simulate this process. It determines, among others, the final breach width, flow rate through the breach and duration of the breaching process. These breach parameters are important for determining rise rates and flow velocities of the water in a polder. By reducing rise rates and flow velocities, which can be achieved by retarding the breaching process, the number of casualties can be reduced. The safety of the inhabitants depends on the mortality during a flood event, i.e. the fraction of casualties of inhabitants in a polder. The safety increases when the mortality reduces, i.e. the amount of casualties decreases. To reach a desired ten times higher safety level, the mortality has to decrease by a factor 10. The breaching process can be retarded in several ways, such as altering the dike shape, increasing the cohesion or strength of the sand core, adding components to the dike or influencing the erosion parameters of sand. Literature research results show that adding a few percent bentonite clay to sand, cementing sand with a biological process or mixing sand with fibres are the most promising options. For these options erosion formulas were determined to implement into the BRES-model. The most promising options were modelled in the BRES-model using a norm dike. The breaching process of this dike was simulated in case it was constructed of sand, mixed with bentonite or fibres, or biologically cemented. The norm dike is the dike which was constructed for the ZWIN’94 experiment. The model simulates the breaching process of these options and the results are compared to each other. From this comparison it is concluded that adding bentonite or polypropylene fibres to sand, or biologically cementing sand lead to similar reductions of the breach parameters. To test the results of the model outcome, laboratory experiments were executed. Sand, sand-bentonite and sand-fibre mixtures were subjected to direct shear, permeability and erosion tests. These tests show that sand-fibre mixtures do not significantly influence the measured parameters of the sand. The sand-bentonite mixtures show a significant reduction of the permeability and erosion velocity compared to sand. The erosion velocities of these mixtures can be modelled in the BRES-model with the Van Rijn-Van Rhee formula. Using the results from the laboratory experiments the effects of bentonite on the breaching process of the norm dike were determined. Even adding a few percent of bentonite reduced the final breach width, maximum breach flow and inundation velocity (rise rate) significantly. For the norm dike it was calculated that 5.4% bentonite is sufficient to reduce the inundation velocity below the threshold value of 0.5 m/h. This results in a reduction of the mortality of approximately a factor ten. Preliminary research indicates it is possible to mix bentonite with sand in-situ, using a Mixed-In-Place technique. A cost indication shows that improving a dike with bentonite using this technique might cost approximately the same as a traditional dike reinforcement. Depending on the circumstances this solution may lead to a decrease of mortality and thus an increase in safety, even by a factor 10.