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This chapter gives an introduction to deliverable D5.3 - Interaction between currents, waves, structure and subsoil – with respect to the MERMAID project. The deliverable focuses on the conditions in European waters such as the four sites that is addressed in the MERMAID project. The most important physical processes will be described and ways to technical address the challenges will be proposed.

This study investigates particle motions near the bed in an oscillating tunnel with a smooth bed. Trajectories of a heavy particle were recorded in two dimensions (horizontal and vertical) and in time. The wave boundary layer Reynolds number is Re = 520000. Kinematical quantities such as the probability distribution of particle position in the vertical, and the horizontal and vertical particle velocities, among others, are determined. The particle is observed to reach heights of 2.5-3d, similar to that characterizing a typical bedload particle in sediment transport.

It has been suggested that vertical perforated tubes placed below the beach surface will increase the drainage of the beach, and hence increase the deposition of sand on the beach. The system is called the PEM-system, Pressure Equalization System, and the Danish company SIC (www.shore.dk) is doing the marketing. Although it for a coastal engineer seems obvious that such a device can’t drain the beach (nearly no driving forces ), SIC has succeeded in installing the system in more than 75 locations around the world (according to the company). In Denmark a full scale experiment at the exposed west coast has been performed through 2005-08, and a similar Dutch test is going on right now at Egmond, Holland. In this paper, we model the flow in the beach taking into account the presence of (high-permeable) tubes and demonstrate that the drainage effect is negligible. Further, the morphodynamic behavior of the coast in relation to the Danish field test is described, and it is concluded that all morphological changes can be explained by natural causes.

In this work, the flow and bed load transport over ripple profiles under the influence of oscillatory will be investigated. The investigation is made through a parametric study, where the bed shape and the ripple steepness are varied. For the sediment transport, the Shields parameter relative to the critical one is varied in the range 1.31-3.45. A detailed flow description in terms of phase resolved quantities and period averaged quantities are given. The former are phase lag between the free stream flow and the separation and a description of the lee side vortices behind vortex ripples. The latter are the recirculation zone and its penetration into the main flow. This will be compared to the period averaged bed shear stress. An analysis of the bed load transport, both as a transport capacityin the phase resolved space and an analysis of possible stable ripples is conducted. This leads to some stable ripples and the general conclusion is that sharp crested ripples is thought unlikely to become stable under oscillatory flow due to the large increase in bed shear stresses because of the rapid contraction of the flow around the crest. Further, a dynamically moving ripple crest is investigated. The key focus is to investigate how a perturbation on top of the ripple influences the flow. It will be seen that both the shape and the velocity of the crest are affecting the period averaged flow globally.

In accordance with the agreement of 18 August 2004 between Skagen Innovation Center (SIC) and the Danish Governmental Coastal Authority (KDI) a field test with the purpose of demonstrating the efficiency of the SIC vertical drain method as a mean for coastal protecting was initiated in a meeting 24 August 2004. The test period is three years after which a final report has to be presented. The report shall contain an evaluation of the drain system with respect to qualitative and quantitative fficiency and environmental impact, as well as a related comparison with conventional coastal protection methods. In accordance with the agreement of 18 August 2004 between Skagen Innovation Center (SIC) and the Danish Governmental Coastal Authority (KDI) a field test with the purpose of demonstrating the efficiency of the SIC vertical drain method as a mean for coastal protecting was initiated in a meeting 24 August 2004. The test period is three years after which a final report has to be presented. The report shall contain an evaluation of the drain system with respect to qualitative and quantitative fficiency and environmental impact, as well as a related comparison with conventional coastal protection methods.

In accordance with the agreement of 18 August 2004 between Skagen Innovation Center (SIC) and the Danish Governmental Coastal Authority (KDI) a field test with the purpose of demonstrating the efficiency of the SIC vertical drain method as a mean for coastal protecting was initiated in a meeting 24 August 2004.The test period is three years after which a final report has to be presented. The report shall contain an evaluation of the drain system with respect to qualitative and quantitative efficiency and environmental impact, as well as a related comparison with conventional coastal protection methods.