Graduation of Lisanne Buis

Wave Transmission over Artificial Reefs, a Physical Model Study

• Professor of graduation: Prof. dr. ir. M. R. A. van Gent

• Supervisors: Dr. ir. D. Wüthrich (TU Delft), Ir. J. P. van den Bos (TU Delft, Boskalis)

   

Currently, 70% of the sandy coasts in the world experience erosion. Simultaneously, the biodiversity in the oceans is decreasing. Coral reefs play an important role in protecting coasts as they provide a sheltered habitat for marine life and absorb wave energy in the foreshore. Therefore, they are essential for the preservation of the biodiversity in the oceans and morphology of the coastlines. In response to this threat, various artificial reefs are being designed. However, as artificial reefs are all very different in design, it is difficult to quantify the functioning of artificial reefs as a coastal breakwater. As it is not realistic to test every artificial reef on its wave transmission, a more fundamental approach to identifying the effect of artificial reefs on the wave transmission is needed. This leads to the following research question that will be answered in this thesis: How do the permeability and the porosity of an artificial coastal reef influence wave transmission and the sheltered habitat of marine life? A physical model is used to answer this question. In the physical model, five trapezoidal breakwaters are tested. One breakwater was impermeable, one was a rubble mound breakwater, and three breakwaters were hollow with a perforated outer surface. Thus, the permeability and (surface) porosity were varied. It was concluded that the volume porosity is of great influence on the wave transmission since the hollow perforated breakwater showed very different wave transmissions than the rubble mound breakwater with an identical surface porosity. However, similar wave transmissions as for a smooth impermeable and rubble mound breakwater were measured when a vertical impermeable screen was positioned inside this hollow perforated structure. From this, it was concluded that the permeability of the vertical screen determines the wave transmission. Moreover, it was concluded that the smooth impermeable breakwater showed mostly lower wave transmissions than the rubble mound breakwater. This shows that the wave dissipation due to more severe wave breaking for impermeable structures is stronger than the dissipation due to roughness and friction inside the structure for permeable structures. As the smooth impermeable, rubble mound, and hollow perforated breakwater with an impermeable screen showed similar wave transmissions, a new empirical relation was derived. This relation takes into account the relative crest freeboard and the relative structure height. This new empirical relation is an improvement on existing empirical relations. For the measured velocities inside the structures, a clear correlation between the velocity and wave height and wave length was observed only in the hollow perforated breakwater with an impermeable screen. Therefore, only this breakwater has been analysed. From the measured velocities it was concluded that the structure reduces the horizontal orbital velocity inside the structure. Remarkable is that the highest velocities measured in the structure, are on the lee side of the impermeable screen and in offshore direction. This is probably caused by the formation of an eddy in the shadow zone of the impermeable screen. The velocities were low enough for marine life to find shelter during the tested storm conditions and therefore, it could function as an artificial reef besides its function as a coastal breakwater. Moreover, the transmitted wave spectrum of the breakwaters has been investigated. From this, it was concluded that hollow perforated structures (also with a perforated screen) transmit mostly longer waves. Whereas the smooth impermeable, rubble mound, and perforated structure with an impermeable screen, transmit mostly shorter waves. For these last structures also a cut in the higher frequencies was observed. From this it was concluded that a cut in higher frequencies is not only caused by the flow ‘through’ the breakwater, but also caused by the ‘permeability’ of the screen.