Nearshore Wave-induced Currents And 2～(nd) Order Long Waves

Waves, one of the most important dynamic factors in coastal engineering, have significant effects on the safety and cost of coastal structure construction. The variation of wave-induced nearshore current fields and long period oscillation of water surface result in sediment transport, resonance of harbors, ship failures, diffusion of pollutants and difficult sea rescues, drawing scientific researchers' concern.In this paper , a quasi-3D model of nearshore wave-induced currents and long waves is established based on the combination of an averaged integration of surface wave-induced currents equation along water depth, a 2D equation for the solution of vertical structures of the currents and a nonlinear parabolic approximate wave reflection-diffraction equation of mild slope. First, based on Navier-Stokes equation, and with the assumption of water particle velocity ui = average velocity of currents Ui + oscillationvelocity of currents Ui + turbulent velocity of currents Ui and free surface height ξ =average height ξ + linear wave surface height ξ, + turbulent height ξ, an averaged wave energy equation and a generalized wave action conservation equation and a surface wave-induced current equation considering factors such as bottom friction, tangential stress of free surface and energy dissipation (increase) are presented. As regards the determination of the vertical structure of wave-induced currents, a modified Vriend-Faria method is put forward with the assumption of weak nearshore currents by Faria & Stanton (1995) and that of wave-induced currents consisting of fundamental currents and secondary currents by Vriend & Stive (1987). Such a method reduces the quantity of calculation in Vriend et al.'s method and raises precision of calculation in Faria et al.'s one . Then, methods are given to determine the most important factors to wave-induced currents such as radiation stress, bottom tangential stress, free surface stress and Reynolds stress as well as some equations under typical conditions. Based on the nonlinear parabolic approximate wave reflection-diffraction equation of mild slope, a model of wave propagation on a fan-shaped coordinate is put forward considering winds, bottom friction and wave breaking, etc. The existing calculation shows that the model can be employed to determine wave fields in large water areas. Finally, some cases of computation are given, showing that though no obvious nearshore wave-induced currents occur in some water areas before construction of projects, the projects may have great effects on wave induced currents. It is important to study sediment transport and local scour near coastal structures, and much attention should be paid.