Wave Interaction with Vertical Slotted Walls as a Permeable Breakwater

The development of coastal areas depends on shore protection against waves and currents. Solid breakwaters are commonly used along shorelines, but they are often unsuitable due to environmental impacts. Permeable breakwaters like rows of piles have been suggested as a more environmentally friendly alternative, but the performance of piles alone has been proven as too weak. Breakwaters with impermeable skirts in combination with piles are assumed to perform better. However, wave-structure-interaction and flow behavior of this type are more complicated, but have to be analyzed before designing.The objective of the present dissertation thesis is to describe the flow behavior and the hydraulic performance of this kind of permeable breakwaters. A numerical model has been developed based on an Eigen function expansion method for wave interaction with a single and a double vertical slotted wall. Experimental tests have been conducted on a model scale of 1 to 25 to validate the numerical model and to assess the performance characteristics of the reflection (CR), transmission (CT) and energy losses (CE). Additional, experimental tests have been conducted to measure and analyze the velocity distribution in front and behind of the vertical slotted wall and to understand the pattern that dissipates wave energy.To fulfill the above-mentioned objectives, this thesis is divided into the following Chapters: Chapter 1 gives an introduction into the problem. Chapter 2 is dealing with the state of the art and an extensive literature review. A numerical model based on Eigen fuction expansion is described in Chapter 3. The numerical model is suitable to determine the wave interaction with single or double vertical slotted wall breakwaters. Furthermore, Stokes second-order wave theory has been compared to the linear wave theory assumption. In Chapter 4, a series of experimental tests are shown, which have been conducted in the wave flume of the University of Wuppertal. The set-up and the measurement devices are explained. Additional, attention has been given to the measurement of the velocities via PIV. The results have been discussed and analyzed with emphasis on the interaction of waves with the vertical slotted walls. In Chapter 5, the results of the numerical model are compared with previous studies and the experimental work of this study. Chapter 6 closes with a summary, concluding remarks, recommendations and suggestions for future studies. The major results from this study are the following:- The numerical model has been validated by comparisons with previous studies and experimental results of this study. The agreement is generally satisfying. - The degree of target protection can be achieved through a combination of permeability area and its location. - The coefficient of friction f and the coefficient of porosity E have significant influence on CR, CT and CE of the permeable breakwaters, while the influence of added mass coefficient cm is low and can be neglected for this configuration. - For the case of double walls, the second wall should be constructed at a distance of an uneven multiple of a quarter of the wavelength (0.25 L, 0.75 L and 1.25 L). This position can increase the dissipation of the energy up to 40 % than a single wall. - PIV measurements can be used in the laboratory for measuring the co-existing and transmitted waves and to visualize the wave interaction with a permeable breakwater. The achievable accuracy of PIV measurement within this set-up is a function of the relative time increment S t /T.Finally, it is recommended to use vertical slotted walls as breakwaters for the protection against waves, whenever it is possible. The progressively decreasing depth of the permeability part of the wall can be used to minimize the transmission of wave energy. For double rows of vertical slotted walls, the spacing between rows should be an uneven multiple of a quarter of the wavelength.