A rip current model based on a hypothesized wave/current interaction

What causes strong, focused rip currents to develop on planar beaches? Obse rvations of waves in the presence of rip currents suggest an interaction be tween waves and currents that causes wave dissipation. A hypothesized mecha nism that would cause such an interaction plays a key role in a rip current model. In this model rip currents can be self-organized, rather than being necessarily forced by bathymetric features or incident wave patterns. The variables in this cellular numerical model interact according to rules that are either direct applications of equations, or abstractions of physical p rinciples. Key processes in the model include: 1) the hypothesized wave-cur rent interaction; 2) onshore water transport by waves; 3) offshore flow cau sed by differences between local radiation-stress gradients and surface-slo pe-generated pressure gradients, 4) alongshore flow driven by alongshore wa ter surface slopes, 5) alongshore mixing of offshore-current momentum, and 6) time and space variations in incident wave heights. Model results indica te that interactions and feedbacks between these processes offer a plausibl e explanation for why rip currents are often narrow and jet-like while also widely spaced, why they can occur on planar beaches as well those with alo ngshore bathymetric variations, and why they are generally dynamic rather t han steady state phenomena.