AbstractsEarth & Environmental Science

The cross-shore sediment transport is of great importance in the coastal water since it may lead to significant change of coastal profile. Many researches have proved that the sediment transport is proportional to velocity moments. In this study, the velocity moments under wave action in the perpendicular direction to the coastline are studied by means of a non-hydrostatic model SWASH and a decomposition technique. The purpose of this project is to perform the velocity moments which contribute to the cross-shore sediment transport with a numerical analysis and the most significant components can be found through the comparisons. These contributed flows to the velocity moments, namely the asymmetric oscillatory flow, wave grouping-induced flow and undertow, are the major objects to be studied. A set of measurements of wave flume experiment are adopted to evaluate the feasibility of the method developed in this study. Firstly, a series of sensitivity analysis of wave decay and mean velocity are presented to investigate the influence of varied settings in SWASH, including the vertical resolution, boundary imposition, bottom friction, discretization schemes for advection terms and water depth in velocity points. The results indicate that predictions of SWASH with proper settings are in good agreement with measurements in terms of wave decay and vertical velocity. Furthermore, a detailed analysis is subsequently conducted with regard to surface elevation, wave decay and undertow. Most of the SWASH predictions are in relatively good agreement with the measurements, but some deviations occurred after wave breaking. This is probably associated to the absence of the production of turbulent energy due to surface roller. A signal decomposition technique is applied to separate the signals with different frequencies. Through the decomposition process, the long and short wave flows are separated by means of filtering out the other part. The SWASH predicted central odd moments of long wave flows is relatively underestimated while the long wave flow variances and the central odd moments of asymmetric flows agree well to the observations. This may correspond to the loss of signal information after decomposition. By summing up the contributions of undertow, wave grouping-induced long wave flow and asymmetric oscillatory flow, the total odd flow moment is easily acquired. Generally, the offshore-directed undertow is the dominant component, the shoreward asymmetric flow is of secondary importance and the contribution of long wave flow is relatively small. As a consequence, the resulted total flow carries the sediment transporting seaward. In overall, SWASH is capable of simulating the wave decay and vertical flow structure correctly. However, some deviations near the breaking point implies that, the implementation of breaking-induced turbulence in SWASH is of great importance.