Showing papers by "J.W. Van der Meer published in 2018"

Probabilistic Assessment of Overtopping of Sea Dikes with Foreshores including Infragravity Waves and Morphological Changes: Westkapelle Case Study

Abstract: Shallow foreshores in front of coastal dikes can reduce the probability of dike failure due to wave overtopping. A probabilistic model framework is presented, which is capable of including complex hydrodynamics like infragravity waves, and morphological changes of a sandy foreshore during severe storms in the calculations of the probability of dike failure due to wave overtopping. The method is applied to a test case based on the Westkapelle sea defence in The Netherlands, a hybrid defence consisting of a dike with a sandy foreshore. The model framework consists of the process-based hydrological and morphological model XBeach, probabilistic overtopping equations (EurOtop) and the level III fully probabilistic method ADIS. By using the fully probabilistic level III method ADIS, the number of simulations necessary is greatly reduced, which allows for the use of more advanced and detailed hydro- and morphodynamic models. The framework is able to compute the probability of failure with up to 15 stochastic variables and is able to describe feasible physical processes. Furthermore, the framework is completely modular, which means that any model or equation can be plugged into the framework, whenever updated models with improved representation of the physics or increases in computational power become available. The model framework as described in this paper, includes more physical processes and stochastic variables in the determination of the probability of dike failure due to wave overtopping, compared to the currently used methods in The Netherlands. For the here considered case, the complex hydrodynamics like infragravity waves and wave set-up need to be included in the calculations, because they appeared to have a large influence on the probability of failure. Morphological changes of the foreshore during a severe storm appeared to have less influence on the probability of failure for this case. It is recommended to apply the framework to other cases as well, to determine if the effects of complex hydrodynamics as infragravity waves and morphological changes on the probability of sea dike failure due to wave overtopping as found in this paper hold for other cases as well. Furthermore, it is recommended to investigate broader use of the method, e.g., for safety assessment, reliability analysis and design.

Summary of research work about erodibility of grass revetments on dikes

Abstract: Grass prevents erosion of the subsoil and is an effective control measure for overflowing water as well as overtopping waves. This form of protection has long been used for agriculture drainage channels and on the slopes of dikes. Dutch river dikes usually have clay layers covered with grass on the crest and on both the landward and seaward slopes. Lake and sea dikes with hard revetment in the wave impact zone of the seaward slope also have a grass cover on the crest and the landward slope on a clay type of soil. For grass covers, relatively large forces are required to break up clay aggregates within the soil, while smaller forces may suffice to transport pure sand and small clayey aggregates. Therefore, at the onset of dislocation, a grass cover will experience considerable forces, which may be described by turbulence, especially on steep slopes. From 2007-2014 about 50 experiments at several locations on Dutch and Belgian dikes were conducted with the wave overtopping and also run-up simulator for assessing the erodibility of grass revetments. Based on these prototype tests the cumulative overload method has been developed, a model that predicts the damage of grass mats and failure of dikes provided the load of the wave run-up and/or wave overtopping and the erodibility of the soil are known. This paper discusses engineering methods for predicting the erodibility of grass revetments as well as the erosion of grass at transitions, e.g. at the edges of roads, transitions from hard to grass revetments and objects on dikes, for example trees.

Hydraulic simulators on real dikes and levees

Abstract: The first part of this chapter gives a short description of wave processes on a dike, on what we know, including re-cent new knowledge. These wave processes are wave impacts, wave run-up and wave overtopping. The second part focuses on description of three Simulators, each of them simulating one of the wave processes and which have been and are being used to test the strength of grass covers on a dike under severe storm conditions. Sometimes they are also applied to measure wave impacts by overtopping wave volumes.