Overtopping resistant dikes
In 1953 the Netherlands experienced a major flooding. Studies determined that a lot of the breaches of the dikes were caused by overtopping (and even overflowing) of the dikes. The failure mostly started on the landward part of the structures. All dikes back then were constructed with a relative low crest and a steep landward slope. Since then all major water defences were raised and the landward slopes were made more gentle. The heightening of the dikes was done in such a manner that statistically only once in 10.000 years (in the western part of the Netherlands) 0.1 l/s per m of overtopping would occur. In the 1990ties by law it was decided that all major water defences should be assessed for safety. In these safety assessments it was found that al lot of dikes again should be raised in order to comply with the safety standards. This raised the question if the method to determine the rate of wave overtopping was correct or not. Also the question was put if the safety standard of 0.1 l/s per m once in 10.000 years was adequate. It turned out that that flooding the mechanisms not enough understood and therefore the safety standard was wrongfully determined on basis of testing with overflow instead of overtopping. Additionally, failure mechanisms were not adequate described. This lead to a Dutch national research program on loads on and strength of flood defences. To assess the erosion strength of grass covers on inner slopes and transition zones between slope to horizontal flats, destructive tests using a Wave Overtopping Simulator were performed at several dikes in the Netherlands.
A Wave Overtopping Simulator performs destructive tests on inner slopes of real dikes in order to establish the erosion resistance against overtopping waves from severe storms. The most relevant hydraulic processes to be considered at wave-structure interaction encompass wave reflection, wave dissipation, wave transmission resulting from wave overtopping and wave penetration through the porous structures, wave diffraction, run-up and wave breaking. Focusing on overtopping, additional processes such as trapped air on broken waves and turbulence, induced by local effects at the armour stones and breakwater cover layers, play an important role in order to determine wave induced dynamics. Formulations derived from these experimentations, are, in most of the cases, semiempirical in nature with their form based on physical considerations but empirical constants determined by fitting to experimental data. The role of scaling factors for dissipation mechanisms due to wave breaking, turbulence and generation of eddies in the fluid region as well as turbulence and friction within the porous material, is also not well established in the physical test. Besides the problem of the scaling technique, other features related with the duration of the experiment programs, wave flume dimensions or economical cost have to be considered. Due to poor repeatability, a large number of experiments have to be carried out in order to define confidence intervals. Moreover, experimental investigation on large-scale models is expensive and measurements within breaking waves can be very complex, due to the aerated and transient nature of the water surface. As a consequence, formulations extracted from the experimental tests present several restrictions. They can only be applied to a structure with a geometry similar or almost identical to the one tested and under identical wave characteristics. An analytical approach is not possible because of the complexity of the problem. A great effort has therefore been made over the last decades in the numerical modelling of wave interaction with coastal structures to overcome these limitations. Nonlinear Shallow Water (NSW), Boussinesq-type and Navier-Stokes equations models have traditionally been used. SPH models have also appeared in the last years as an alternative. However, they are in an early stage to be used as predictive tool.
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