| A depth-integrated, shortwave-averaged quasi 3D nearshore circulation model, called SHORECIRC, is developed, which combines semi-analytical solutions for the 3D current profiles with numerical solutions of the depth-integrated equations. These equations are based on the Reynolds equations with as few additional assumptions as possible. The model's purpose is to gain an enhanced prediction and analysis capability for nearshore circulation on any bathymetry and under any hydrodynamical condition.; Special emphasis is placed on the development of accurate numerical boundary conditions. At the seaward boundaries an absorbing-generating boundary condition is developed, which reflection errors are shown to be very small in a number of formal tests. At the shoreline an inundation-storage boundary condition is implemented, which shows good agreement with a number of analytical and numerical solutions.; It is emphasized that SHORECIRC can be used with any bathymetry and hydrodynamical condition. In this thesis, however, it is applied to a number of specific cases: The generation of infragravity waves forced by normally-incident wave groups ("surf beat") is analyzed. This process can be characterized by three external parameters: the relative slope steepness, the breakpoint location and the breaking mechanism. A variation of these parameters results in a variation in the ratio of outgoing to incoming wave energy. This ratio can be interpreted as an integral measure of the net transfer of energy to the infragravity waves as a result of short-wave shoaling and breaking.; The model is further applied to the start-up of a longshore current and to infragravity waves forced by obliquely-incident wave groups. It is shown that the quasi 3D terms have a significant effect on the results and represent terms in the momentum equations that can be at least of the same order as the terms corresponding to the 2DH equations.; The magnitude of the quasi 3D coefficients is analyzed. It is found that the time-varying forcing has a large effect on the size of these terms. The analysis of the quasi 3D contributions to the momentum equations shows that a number of these terms can be neglected in the cases shown here. However, the assessment of the relative magnitude of these terms should be performed for each application. |
