A Numerical Model For Sand Beach Evolution Under Nonlinear Waves

The evolution of nearshore topography is a major concern in coastal protection and coastal engineering. Wave is one of the most important dynamic factors in causing such evolution. The strong nonlinearity of waves in shallow water significantly affects the sediment transport and, consequently, also the topography evolution. Development of a numerical model for nearshore wave motion including the nonlinear effects so that the topography evolution can be more accurately predicted has been a hot issue in the study of coastal sediment transport.This study is based on the phase-averaged wave model. The periodic wave processes are reconstructed based on their relations with the wave parameters such as wave height water depth, wave period, etc. The reconstructed process of wave velocity can be used in unsteady sediment transport calculation, and then a topography evolution model can be developed. The model developed in this study keeps the efficiency of the phase-averaged model, and also describes the effect of the temporal variation of the velocity on sediment transport, which is the same with the phase-resolved wave model.There are three modules in the model: nearshore wave module, wave processes reconstruct module, sediment transport and seabed evolution module. The nearshore wave height is calculated based on wave energy balance equation, and a modification to including the effect of wave nonlinearity is introduced. Using the traditional wave energy balance equation, a significant underestimation on wave height is found when the wave nonlinearity is strong, especially in case of regular waves. This is because the relationship between wave energy and wave height is based linear theory in the traditional wave energy balance equation. Therefore, the modification is proposed in the wave model for regular waves, which is proved can greatly improve the accuracy of regular wave. The reconstruction of periodic wave processes is based on previous study on the analytical approximation of wave form. The periodic wave process is then linked with the wave parameters given in the wave model, such as wave height, water depth and wave period. Using this procedure, the asymmetry and skewness of nearshore waves can be well described. And compared with phase-resolved wave model, this method has obvious advantage on computational efficiency. In the module of sediment transport and seabed evolution, a sediment transport formula which can describe the effect of the wave nonlinearity is adopted and the effect of the bed slope is concerned.Firstly, the model is verified and applied in simulation of the topography evolution under regular wave s. In the wave module, three nonlinearity modification methods are juxtaposed and the result suggests that all of the three methods can improve the computational accuracy of regular wave height simulation. Discussion on wave break models is also made and the result suggests that the breaking model adopted is widely applicable. The wave processes reconstruction module is proved to be reliable by comparing with the result of the NS-VOF method. The whole model is then applied in reproducing of the topography evolution experiment in a large scale wave flume. The model is used to predict the evolution of various slope beaches under the regular wave with various wave height and period. The results show a good agreement with experiment data.The model is further applied in the irregular wave condition. The wave breaking formula using in the wave model is changed to fit the random behavior of irregular waves. The model is applied to simulate irregular wave propagation on the uniform and various slope, and topography evolution under irregular waves is also simulated. The simulation results present good agreement with experiment data. Compared with XBeach model, which is widely used in simulating nearshore morphology evolution, the model performs better in simulation of the bar formation under short waves.