Equilibrium bays - A numerical study after the behaviour ofequilibrium bays

Abstract

At the present day, the far field impact of coastal structures can be simulated beforehand with the aid of numerical models. The consequences of new coastal defence schemes can be visualised before they are applied in reality. Therefore, the objective of this study is on the one hand, to investigate the capabilities of the numerical model Delft2D-MOR to predict the coastline evolution behind a series of emerged breakwaters and on the other hand, to increase our understanding of the hydraulic and morphological behaviour of bays in order to make deliberate decisions about the application of headland control in future. Since the work described this thesis is a continuation of the work done by Blankers (1999), morphological simulations are executed and the effect of the wave height on the suspended sediment transport in the surf zone is examined. Although a reversal in longshore transport direction is observed (indicating a certain bay shape with a zero transport rate), bay shapes which are stable in the long run have still not been found. Local longshore (and cross-shore) transport gradients alter the bathymetries, which in their turn influence the overall longshore sediment transport. The result is a change in geometry. Since with the present models it is not yet possible to calculate the equilibrium bathymetry via a long simulation, new geometries and bathymetries are suggested. Rewriting an empirical formula leads to two geometrical requirements which describe the shape of a bay in a simplified manner. However, this logarithmic spiral is based on aerial photos and hence do not include important aspects such as wave height, bottom material and bottom slopes. Analysing bottom maps of existing stable bays showed that the slope behind the headland (or breakwater) is flatter than the slope in the centre. Combining the geometrical requirements (which define the plan view of the bay) with slope information results in bathymetries which are rather different from the bathymetries used so far in the simulations. This explains partly why equilibrium has yet not been found. An advantage of creating bathymetries via the elliptical approach used so far, is that ellipses can be mathematically expressed and hence, many bays can be used in the simulations in relatively short time. However, in order to find equilibrium further research should be done with different bathymetries. Some simulations already executed show promising results.