Rip currents are the strong narrow seaward current produced by waves breaking on the coast. Of the many types of rip currents, the present study investigates, experimentally and numerically, the rip current system produced by the wave reflection on a finite-length shore-normal structure (corresponding to groin, jetty and headland in a real-word environment) with strong wave reflection strength. The incident wave condition of an obliquely propagating monochromatic surface gravity wave is considered for the bottom topographies of planar and barred beaches without or with a rip channel. The corresponding laboratory experiment was conducted and used to validate the numerical model. A set of higher order Boussinesq equations is used to reproduce the experimental observation and produce the results for the related prototype cases, with the latter considering more factors which may influence the formation and evolution of rip current flow patterns. The resulting rip current system contains two parts: the node rips formed due to the presence of the longshore standing wave in the wave reflection field and the deflection rip formed due to the interaction between the incoming longshore current and the reflection wave field mentioned above. A new formation mechanism of the deflection rip has been proposed, i.e., the deflection rip is formed by the meeting and then deflection of the longshore current and lateral flow. The effects of different wave reflection strengths of the structure, wave incident angles and bathymetric conditions on the pattern and flow volume of the deflection rip are also studied.

Multiple submerged breakwaters (MSBWs) are commonly used coastal protection structures due to their specific advantages over the emerged ones. Rip currents, as the inevitable natural hazard in the gaps of these constructions, are investigated numerically in the present study. A fully nonlinear mild-slope equation (NMSE) model possessing both fully nonlinear and fully dispersive properties is validated and adopted in the simulations. With four monochromatic wave conditions of different wave heights, periods and incidences representing low-energy, typical, storm and oblique waves tested, the flow patterns and the low-frequency oscillations of the rip currents are studied. For the convenience of risk assessment, the rip risk level is divided into three degrees according to the maximum rip flow speed. The effects of the configurations of the MSBWs on the rip current system as well as the rip risk level are examined, considering different breakwater widths, heights, forms, gap widths and gap numbers. Simulation results suggest that the cross-shore configurations of MSBWs influence the rip risk level by inducing different wave energy dissipations but the longshore configurations of MSBWs by changing flow field patterns.

A set of time-dependent fully nonlinear mild slope equations is established in terms of velocity vector at still water level and free surface elevation, which is derived from the Euler equations without approximation in nonlinearity and only with the potential flow assumption adopted for bottom slope terms. Apart from the fully nonlinear property, the model is also different from previous nonlinear mild slope equations by adopting the velocity expression accurate to the first order in bottom slope. This makes the new expression for higher order terms in bottom slope be established in the model equations and the fully nonlinear terms containing the bottom slope be included. This also makes the established equations be able to be reduced to the fully nonlinear Boussinesq equations for shallow water. The above advantages of the present model are validated against the related laboratory experimental results.

In this study, a set of accurate dispersive nonlinear wave equations is established, using the wave velocity and free surface elevation as variables. These equations improve upon previous equations in which the velocity potential is used as a variable by considering the rotational wave motion and by adding a second-order bottom slope term that applies to general situations, allowing the equations to consider the influence of rapidly changing, horizontal, two-dimensional bottom topographies. The problem of the inaccuracy of the integral calculations used in previous equations in nearshore areas is solved by approximating the integral calculations into differential calculations, and a set of coupled wave equations is established by keeping the free surface elevation and the horizontal velocity constant, thus allowing the calculation of nearshore wave-generated currents. The benefits of the current model are confirmed through comparisons with corresponding laboratory experimental findings and are illustrated through a comparison with the numerical outcomes of other pertinent models.