Overtopping resistant dikes
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Introduction
Within the THESEUS-project several innovative structures will be developed or further developed in order to defend specific coastal zones from erosion of breaching. This report is somewhat a strange one in this context. This report describes tests in order to develop guidelines for determining the strength of some typical coastal structures. The knowledge further may be used to determine how to strengthen these structures by replacing or adding materials on the cover of these structures. The structures meant in this context are mainly dikes and levees. 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 the safety standard was determined on basis of testing with overflow instead of overtopping. Overtopping is a pulsating load on the landward slopes of dikes whereas overflowing is a continuous load. From the safety assessments it was found out that more failure mechanisms were not adequate described and the mechanisms not enough understood. This lead to a national research program on loads on and strength of flood defences. One of the projects within this program is Overtopping and Strength of Grass Covers. As part of this Dutch research project Overtopping and Strength of Grass Covers, within the SBWprogram (Strength of and Loads on Water Defences), from the Dutch Ministry of Transport, Public works and Water Management, destructive tests were performed at several dikes in the Netherlands. This part of the program has the objective to create a Technical Report with guidelines to assess the erosion strength of grass covers on inner slopes of dikes under wave overtopping. Also the influences of obstacles on the slopes and transitions from the slope to horizontal parts, is subject of the research. All tests were and will be performed with the Wave Overtopping Simulator. A Wave Overtopping Simulator is a device to perform destructive tests on inner slopes of real dikes in order to establish the erosion resistance against overtopping waves from severe storms. This document describes first the principles of this device and measurements of flow depth and flow velocity on the inner slope with recently developed instruments. The main part of the report is focused on results of four years of destructive testing, on observations obtained and on preliminary conclusions. This leads then to a discussion on implementation of results in practice, where distinction can be made between modeling of failure mechanisms and implementation in safety assessment or design procedures. The last part of this report is on possibilities to expand the results of these tests with the help of numerical modeling. Numerical modeling 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 or turbulence induced by local effects at the armour stones and breakwater cover layers play an important role in order to determine wave induced dynamics. The lack of knowledge in the interpretation of many of the factors present in the wave-structure interaction has motivated their study through two-dimensional physical tests, especially small-scale model tests. 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. Although great progress has been made in the last decade in wave generation, wave absorption and measurement techniques, the repeatability of the experiments is not yet fully solved in laboratory tests, especially when dealing with overtopping studies. A large number of experiments have to be carried out in order to define confidence intervals. This increases the number of experiments, and also their costs. 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. It can be concluded that physical tests do not fully represent the wave structure interaction process and formulations have a narrow range of applicability. 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. New forms of study are applied to the analysis of wave-structure interaction, in order to gain better knowledge of the associated processes. An analytical approach is not possible because of the complexity of the problem. A great effort has been made over the last decades in the numerical modelling of wave interaction with coastal structures to overcome these limitations. Several approaches have been followed to study coastal structure induced hydrodynamics. Among other existing approaches, 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. In section 3.2 a detailed description of the different approaches will be presented.
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