Boussinesq modeling of the influence of wave energy converters on nearshore circulation

Abstract

In Chapter 4 it was found that particle transport is much more sensitive to waves originating in the infra-gravity (IG) band. These are waves which typically have much smaller amplitudes than the typical waves seen on the ocean, and their long period and wavelength makes them hard to spot while looking at out at sea. Nevertheless it was found that the movement of particles have a significant component in IG frequency offshore. As the waves break in the surf zone the IG band grows in importance before the movement associated with them are dominant in the surf zone. This explains multiple effects that has been found to be dominated by IG frequencies such as the location of the river plume, net movement of inertial particles, and sand flux. As it was found that the phase-resolving nearshore wave model Boussinesq Ocean and surf zone model (BOSZ) can generate these kind of waves, it also makes sense that the model can be used to somewhat accurately predict the movement of actual particle paths. In Chapter 5 this was put to the test by comparing the simulated paths in BOSZ with actual orange paths outside Sylt. Oranges was tracked through pictures taken with a stereo camera and combined with GPS data to recreate their position in a coordinate system. Combining this data about their position with data regarding the bathymetry, tide levels, and wave compositions it was possible to numerically recreate the wave conditions of the day. In the resulting simulated movements we find that the numerical paths matches the actual paths to a satisfactory degree. Further it was found that the drifters moved slowly in one direction while having much faster oscillating movements while travelling there. This of course follows from the discussion that the slowly varying IG waves dominate the movements. It was also found that these IG waves is critical to accurately simulate particles in the surf zone, as the phases of the waves determined the direction of movement of the particles. Only by averaging paths by different phases could we get the right paths, a results which is necessary to include in all further work in tracking near shore particles. Along the coast multiple large scale fluid patterns can be found. There exists regions of flow which primarly move towards the beach or out of the beach, the latter of which is known as rip currents, as well as alongshore flow and lastly large scale circular patterns called vortexes. In Chapter 6 and 7 it was found that vortexes in the surf zone are dependent upon the tide level, the mean direction the waves were coming from, and if they all came in at the same angle or not. The radii, strength and the number of these circular movements for different values of these parameters were compared, and it was found through statistical testing that some of the parameters only influenced the radii of the vortexes, while others influenced the strength or number of vortexes. Oscillating wave surge converter (OWSC) devices are a relatively new type of wave energy converter of recent interest as these can capture a large percentage of the energy of the waves (capture factor). In Chapter 7 a method of analytically calculating the capture factor for a wave was retold. A way of numerically calculate this factor was developed, and how to include this in the BOSZ model was explained. The inclusion of a farm of OWSCs in the bathymetry had the effect of reducing all the radii and circulatory strength in the surf zone, while keeping most of the relationships from the case with an open beach. A better understanding of the vortex structures can shed light into the intensity of mixing. This goes beyond this project but we might be able to recalculate the vertical exchange of water by knowing the size and intensity of eddies. The fact that we found in Chapter 6 that vortexes at the beach match the fundamental Rankine vortex yields a better feeling of what to expect from computations. It might be possible to use the computed results in combination with analytical solutions to create better estimates for mixing ratios in the future. This can among other help to understand the influence OWSC farms have on the local ecology and morphology.