Impacts of 3D wave and tidal forcing on nearshore hydrodynamics

Numerical investigations covering all the physical processes occurring in the surf and swash zones, including the cross-shore and longshore flows, have not been fully developed, although considerable work in 2D has appeared in recent years. However, given the interplay of mechanisms inherent in nearshore processes – sediment and contaminant transport driven by tides and waves– it is not possible to readily extrapolate 2D results to 3D. Comprehensive 3D numerical models are thus required. The main objective of this study was to de¬velop further mechanistic understanding of nearshore hydrodynamics under different oceanic forcing conditions. The main tool to achieve this purpose was a com¬prehensive 3D numerical model – incorporating the existing model Delft3D – of ocean hydrodynamics that relaxes many existing model simplifications. The nearshore hydrodynamics were modeled using the 3D Reynolds-averaged Navier-Stokes equations, combined with the k-ε turbulence closure model to simulate oceanic motion near the shoreline. A series of numerical experiments under a range of incident wave and tide conditions were run to demonstrate the model’s capacity to generate insights into nearshore processes. The results were interpreted according to existing understanding of surf and swash zone processes. The results confirmed that both cross-shore flow and longshore current generated by obliquely incident breaking waves are important in nearshore hydrodynamics and as a result in sediment transport sediment in the surf-swash zones. In addition, results revealed that the longshore velocity varies across the nearshore zone and the angle between the crest line of the approaching wave and the shoreline defines the direction and strength of the longshore current.