Nearshore efffects of submerged breakwaters

Abstract

This master thesis is a contribution to the Dynamics of Beaches project, which is a part of the EU funded Human Capital and Mobility program (HCM). In this study, the 3D-effects of detached segmented submerged breakwaters, in the nearshore zone, primarily on the hydrodynamic processes and only secondarily on the morphodynamics, are investigated. First the results of previous laboratory experiments, performed in a wave basin by Delft University of Technology, are discussed. After that, the outcomes of different series of simulations with the 2DH version of the numerical model DELFT3D (Delft2D-MOR), developed by Delft Hydraulics, are analysed. The primary aim of these simulations is to investigate whether the model is able to reproduce the basin experiments. The secondary aim is to analyse the effects of different submerged breakwaters configurations on the hydrodynamic processes. From the laboratory experiments, it resulted that the submerged breakwaters induce wave breaking and consequently wave energy dissipation. The effect of the submerged breakwaters, in dissipating wave energy, weakens as the wave height becomes small compared to the submergence of the crest of the structures. A water volume is transported over the submerged breakwaters shorewards. Then a longshore current in the direction of the gaps develops, due to a longshore gradient in the average water level sloping down from the submerged breakwaters to the gaps. A strong off-shore current through the gaps can be observed. In comparison with the tests without submerged breakwaters, a larger amount of sediment from the shoreface is transported seawards through the gaps. From the experiments, it seems that the overall effects of segmented submerged breakwaters are more harmful than beneficial. Although the basin condition is different from a prototype condition, the use of segmented submerged breakwaters as a protection tool for sandy coast should be carefully studied. Comparing Delft2D-MOR output with the basin experiments, some positive conclusions can be drawn. In the area close to the submerged breakwaters, the numerical model correctly reproduces the current field and the pattern of the average water level. In agreement with the measurements, sediment is transported seawards through the gaps. Nevertheless, some discrepancies with the measurements are found. Erosion at the lee side of the submerged breakwaters is overestimated. This is due to the high dissipation concentrated in this area and to the low resolution of the computational grid. Furthermore, the retreat of the shoreline is not modelled, yielding an unrealistic trough at the shoreline. Improvements to Delft2D-MOR are requested mainly in the area landwards of the submerged breakwaters. The flow field has been analysed while enlarging the exposure ratio (ratio of gap width to the sum of submerged breakwater length and gap width) artificially from zero (one single breakwater, no gap) to unity (no submerged breakwater). In the region around the submerged breakwaters, the longshore and cross-shore currents first increase, until an exposure ratio of approximately two third is reached, then a constant decrease can be observed. Since sediment transport and current field are closely correlated, the presence of gaps seems to be more harmful than the situation without submerged breakwaters. By decreasing the off-shore distance of the structures, the efficiency in dissipating energy seems to be reduced.