Sediment size effects on rip channel dynamics

Field observations indicate that both wave conditions and sediment characteristics exert the controlling effects on the beach states and determine the development of rip channel system. Although the role of wave conditions on the rip channel dynamics has been extensively studied, little attention has been paid to the effects of sediment characteristics. In this paper, we use a nonlinear morphodynamic model, XBeach, to investigate the impact of grain size on the formation and nonlinear evolution of rip channels under the condition of uniform sediments. We find that the grain size can affect a number of aspects of rip channel dynamics. With increasing grain size, both longshore migration speed and growth rate decrease significantly, while the response and saturation times increase. On the other hand, the influence of grain size on the geometrical properties of the rip channel seems to be much less pronounced, as the wavelength of the channels hardly changes and the three-dimensionality features only vary slightly. The "global analysis" method is applied to investigate the mechanism underlying the variations of growth rate and migration speed for varying sediment size. The calculations reveal that these variations are mainly caused by the amount of sea bed sediments being stirred up which is clearly grain-size-dependent. Quantitative relationships between predicted rip channel characteristics, sediment grain size and the hydrodynamic conditions are tentatively established, which is amendable to improvement by extending the analysis to other or more general situations.