Impacts of wave and tidal forcing on 3D nearshore processes on natural beaches. Part I: Flow and turbulence fields

The major objective of this study was to develop further understanding of 3D nearshore hydrodynamics under a variety of wave and tidal forcing conditions. The main tool used was a comprehensive 3D numerical model – combining the flow module of Delft3D with the WAVE solver of XBeach – of nearshore hydro- and morphodynamics that can simulate flow, sediment transport, and morphological evolution. Surf-swash zone hydrodynamics were modeled using the 3D Navier-Stokes equations, combined with various turbulence models (k-ε, k-L, ATM and H-LES). Sediment transport and resulting foreshore profile changes were approximated using different sediment transport relations that consider both bed- and suspended-load transport of non-cohesive sediments. The numerical set-up was tested against field data, with good agreement found. Different numerical experiments under a range of bed characteristics and incident wave and tidal conditions were run to test the model’s capability to reproduce 3D flow, wave propagation, sediment transport and morphodynamics in the nearshore at the field scale. The results were interpreted according to existing understanding of surf and swash zone processes. Our numerical experiments confirm that the angle between the crest line of the approaching wave and the shoreline defines the direction and strength of the longshore current, while the longshore current velocity varies across the nearshore zone. The model simulates the undertow, hydraulic cell and rip-current patterns generated by radiation stresses and longshore variability in wave heights. Numerical results show that a non-uniform seabed is crucial for generation of rip currents in the nearshore (when bed slope is uniform, rips are not generated). Increasing the wave height increases the peaks of eddy viscosity and TKE (turbulent kinetic energy), while increasing the tidal amplitude reduces these peaks. Wave and tide interaction has most striking effects on the foreshore profile with the formation of the intertidal bar. High values of eddy viscosity, TKE and wave set-up are spread offshore for coarser grain sizes. Beach profile steepness modifies the nearshore circulation pattern, significantly enhancing the vertical component of the flow. The local recirculation within the longshore current in the inshore region causes a transient offshore shift and strengthening of the longshore current. Overall, the analysis shows that, with reasonable hypotheses, it is possible to simulate the nearshore hydrodynamics subjected to oceanic forcing, consistent with existing understanding of this area. Part II of this work presents 3D nearshore morphodynamics induced by the tides and waves.

Impacts of 3D wave and tidal forcing on nearshore hydrodynamics

Roham Bakhtyar, David Andrew Barry

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.

2011

Numerical Simulation of Two-Phase Flow for Nearshore Hydrodynamics under Wave-Current Interactions

Roham Bakhtyar, David Andrew Barry, Amir Mehdi Razmi

Surf and swash zone hydrodynamics are complex, being composed of a highly energetic water-air mixture under periodically unsteady motion. Wave-current interactions have a major effect on the results of hydrodynamic simulations, and consequently on predictions of mixing and sediment transport processes. An Euler-Euler air-water two-phase flow model was used to simulate hydrodynamic processes under waves and currents in the nearshore zone. The flow field was computed with the Reynolds-Averaged Navier-Stokes equations in conjunction with the Volume-Of-Fluid method and RNG turbulence closure scheme. Different flow fields were simulated and compared (wave, current, wave+current). The results indicated that the characteristics of oscillatory flow were affected by the combined current and wave motions, especially near the bed, which is important for sediment transport. This study demonstrates that currents modify the hydrodynamics of wave-driven motion in the surf and swash zones. The kinetic energy magnitude was much improved in the wave breaking process and the breaking point occurred further offshore than for purely wave-driven motion. The effects of currents on the flow field were found to be different depending on their magnitudes. Vertical variations of the velocity distribution (undertow), turbulence characteristics, void fractions, and non-hydrostatic pressure were used to assess the hydrodynamics of the nearshore zone and wave-breaking processes under different combinations of wave and current. These numerical studies simulate the different types of wave breaking and accompanying nearshore zone hydrodynamics that affect the physical processes in surf and swash zones.

Coastal Education and Research Foundation2011

Impacts of wave and tidal forcing on 3D nearshore processes on natural beaches. Part II. Sediment transport

Roham Bakhtyar, David Andrew Barry

This is the second of two papers on the 3D numerical modeling of nearshore hydro- and morphodynamics. In Part I, the focus was on surf and swash zone hydrodynamics in the cross-shore and longshore directions. Here, we consider nearshore processes with an emphasis on the effects of oceanic forcing and beach characteristics on sediment transport in the cross- and longshore directions, as well as on foreshore bathymetry changes. The Delft3D and XBeach models were used with four turbulence closures (viz., k-ε, k-L, ATM and H-LES) to solve the 3D Navier-Stokes equations for incompressible flow as well as the beach morphology. The sediment transport module simulates both bed load and suspended load transport of non-cohesive sediments. Twenty sets of numerical experiments combining nine control parameters under a range of bed characteristics and incident wave and tidal conditions were simulated. For each case, the general morphological response in shore-normal and shore-parallel directions was presented. Numerical results showed that the k-ε and H-LES closure models yield similar results that are in better agreement with existing morphodynamic observations than the results of the other turbulence models. The simulations showed that wave forcing drives a sediment circulation pattern that results in bar and berm formation. However, together with wave forcing, tides modulate the predicted nearshore sediment dynamics. The combination of tides and wave action has a notable effect on longshore suspended sediment transport fluxes, relative to wave action alone. The model’s ability to predict sediment transport under propagation of obliquely incident wave conditions underscores its potential for understanding the evolution of beach morphology at field scale. For example, the results of the model confirmed that the wave characteristics have a considerable effect on the cumulative erosion/deposition, cross-shore distribution of longshore sediment transport and transport rate across and along the beach face. In addition, for the same type of oceanic forcing, the beach morphology exhibits different erosive characteristics depending on grain size (e.g., foreshore profile evolution is erosive or accretive on fine or coarse sand beaches, respectively). Decreasing wave height increases the proportion of onshore to offshore fluxes, almost reaching a neutral net balance.

Techno-Press2016

Impacts of wave and tidal forcing on 3D nearshore processes on natural beaches. Part II. Sediment transport

Roham Bakhtyar, David Andrew Barry

Techno-Press2016