Modelling the impact of an offshore breakwater on the shore

Modelling the impact of an offshore breakwater on the shore

Bos, K.J.

Booij, N. (mentor)
Van de Graaff, J. (mentor)
Roelvink, J.A. (mentor)
D' Angremond, K. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

1996-05-01

This report is the result of a master thesis of the author, student at Delft University of Technology, faculty of Civil Engineering. The study was carried out at DELFT HYDRAULICS. In this report the effects of an offshore breakwater on the shore are investigated with the morphodynamic models DELFT2D-MOR and DELFT3D. These, respectively 2-dimensional horizontal and quasi 3-dimensional models, were developed by DELFT HYDRAULICS. The offshore breakwater lay-out simulated in this study serves as a test case for these two models. The breakwater lay-out simulated in this study is subjected to the action of both normal and oblique incident waves. In the former case no current is present since the computations are executed in the absence of tide. In the case of oblique waves a wave-driven longshore current is present. Within the 2-dimensional model the cross-current transport mechanisms are neglected which result in unrealistic bottom profile evolutions. However, bearing this in mind the results are still very useful for getting insight into the littoral processes. A comparison of the model results with laboratory and field data shows that the model is able to simulate the dominant morphodynamic features induced by the offshore breakwater. In the case of normal incident waves, sediment is trapped into the lee of the breakwater from both lateral sides forming a tombolo in the equilibrium state. In the case of oblique incident waves also a tombolo is created while heavy erosion occurs at the down-stream side. However, after tombolo forming accretion occurs at the up-stream shore which finally results in bypassing and filling of the down-stream scour hole. Before simulating the offshore breakwater lay-out with the quasi 3-dimensional model a number of errors had to be detected and removed from the numerical model. Furthermore, two improvements had to be made. First, it was noticed that the dissipation had to be incorporated into the 'continuity corrections' in order to reduce the number of calls of the wave and flow module. This reduces the required computational time considerably. Second, a better estimation of the bed-level celerity was made in order to guarantee numerical stability. Next the breakwater lay-out was applied to the Q3D model. In these computations the sediment transport by the secondary current and the resulting bottom profile evolution is clearly visible. Furthermore, the resulting bathymetry has less steep slopes due to the crosscurrent transport mechanisms. However, the resulting bathymetry contains very irregular bottom contour lines. So locally high transport rates occur which result in a decrease of the computational time step and, consequently, an increase of the required computational time. It is shown that accounting for the flow velocities in the wave computation (wave-current interaction) improves these results considerably. The resulting bathymetry contains less irregular contour lines.

offshore breakwater
coastal protection
coastal morphology
Delft3D

http://resolver.tudelft.nl/uuid:ca395867-dbbb-49a7-bd8c-f4888b578262

Student theses

master thesis

(c) 1996 Bos, K.J.