Wave current interaction

Abstract

The hydrodynamics of the coastal zone is dominated by two equally important factors: waves and currents. The waves are usually generated by the wind, while the currents may be driven by the tides, the wind, the waves, density variations or river outflows. Their combined effect has many engineering applications. The wave and current fields interact mutually through a number of mechanisms, such as refractin, generation of radiation stresses etc. As a result of these interaction mechanisms, the wave- and current fields in the coastal zone complex and difficult to predict. One of the methods to predict these phenomena is the use of numerical models. In this study, which focuses on the use of bed-stress formulations in hydrodynamical models, two numerical model-systems are used. The first part of the study is performed with use of the model-system Delft3D, developed by WL I Delft Hydraulics. With this system a number of simulations are carried out in order to make a comparison between the implemented bed stress formulations in Delft3D. The simulations show that the use of different bed-stress formulations lead to broadly similar results for the maximum velocities of the longshore current mutual. The mutual differences in the wave-induced current vary up to 20% for the simulated cases. The mutual differences arise in the cases where the waveheight is increased. More understanding in the results of the use of the different bed stress formulations can be obtained by performing a larger number of simulations and by comparing the results of these simulations with field data. Field data is not widely available for longshore currents. To acquire this type of data, large and intensive measuring campaigns are needed. The availability of this data and the comparison of the results of wave driven current computations with this data can lead to better calibration of the model and thus to better results for the simulations. The second part of the study has the objective to develop a link between two models in another model system SIMONA, developed by the National Institute of Coastal and Marine Management. This link is developed analogously to the link between the wave propagation model and the water movement model in Delft3D. It enables the model system to make wave driven current computations. The link in SIMONA is developed using a newer wave model, SWAN, developed by the University of Technology Delft. A number of limitations of HISWA are solved in SWAN. The link between the two models has turned out to be instable. No usable data could be collected. The data from the computations that were carried out successfully is erroneous. The complex model-environment in which the link is developed has lead to this unstable link. It is complicated to use a model not adapted for the SIMONA system with a model in this system. SIMONA is not a flexible system for adapting and adding models. To develop a usable link more research and programming has to be performed. In the limited time available for this study the development of such a stable link has not been realisable. An alternative to this link is the use of the model system Delft3D. This system has the desired functionality. Furthermore SWAN has been implemented in Delft3D. These two issues, the development of a system capable of the computation of wave driven currents and a study on the aspects of the implementation of SWAN in the model system SIMONA were the main reasons to initiate this study. The use of Delft 3D therefore seems a good alternative.