Numerical simulation of wave motion and its interaction with groundwater flow in coastal aquifers

Groundwater regime has an important role in coastal sediment transport near the coast and beach profile changes. Hitherto, accurate and complete nature of the interaction between wave motions, underground water and beach sediment transport is not yet known. Most existing numerical models for studying the beach groundwater are related to the tidal forces. In this study, interaction of fresh groundwater, salt transport and wave motion was investigated via the process-based model. The hydrodynamic part is computed using the Navier-Stockes equations in conjunction with a k-ε closure turbulence model and Volume of Fluid (VOF) technique. The bed deformation is calculated using empirical sediment transport formula according the influence of watertable changes. Furthermore, the wave-induced watertable fluctuations and effects of infiltration/exfiltration on the nearshore sediment transport are discussed. The groundwater flow and salt transport are considered based on density-variable model. The results reveal that in the surf zone, the sediment transport rate increases towards the shore until the wave breaks whereas in the swash zone it decreases and the greatest erosion takes place in the vicinity of the wave breaking point.

Wave-induced watertable fluctuations, sediment transport and beach profile change: Modeling and comparison with large-scale laboratory experiments

Roham Bakhtyar, David Andrew Barry, Alessandro Brovelli

Coastal groundwater systems can have a considerable impact on sediment transport and foreshore evolution in the surf and swash zones. Process-based modeling of wave motion on a permeable beach taking into account wave-aquifer interactions was conducted to investigate the effects of the unconfined coastal aquifer on beach profile evolution, and wave shoaling on the water table. The simulation first dealt with wave breaking and wave runup/rundown in the surf and swash zones. Nearshore hydrodynamics and wave propagation in the cross-shore direction were simulated by solving numerically the two-dimensional Navier-Stokes equations with a k-ε turbulence closure model and the Volume-Of-Fluid technique. The hydrodynamic model was coupled to a groundwater flow model based on SEAWAT-2000, the latter describing groundwater flow in the unconfined coastal aquifer. The combined model enables the simulation of wave-induced water table fluctuations and the effects of infiltration/exfiltration on nearshore sediment transport. Numerical results of the coupled ocean/aquifer simulations were found to compare well with experimental measurements. Wave breaking and infiltration/exfiltration increase the hydraulic gradient across the beachface and enhance groundwater circulation inside the porous medium. The large hydraulic head gradient in the surf zone leads to infiltration across the beachface before the breaking point, with exfiltration taking place below the breaking point. In the swash zone, infiltration occurs at the upper part of the beach and exfiltration at the lower part. The simulations confirm that beaches with a low water table tend to be accreted while those with a high water table tend to be eroded.

Elsevier2011

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

Modeling sediment transport in the swash zone: A review

Roham Bakhtyar, David Andrew Barry

A critical review of conceptual and mathematical models developed in recent decades on sediment transport in the swash zone is presented. Numerous studies of the hydrodynamics and sediment transport in the swash zone in recent years have pointed out the importance of swash processes in terms of science advancement and practical applications. Evidently, the hydrodynamics of the swash zone are complex and not fully understood. Key hydrodynamic processes include both high frequency bores and low frequency infragravity motions, and are affected by wave breaking and turbulence, shear stresses and bottom friction. The prediction of sediment transport that results from these complex and interacting processes is a challenging task. Besides, sediment transport in this oscillatory environment is affected by high-order processes such as the beach groundwater flow. Most relationships between sediment transport and flow characteristics are empirical, based on laboratory experiments and/or field measurements. Analytical solutions incorporating key factors such as sediment characteristics and concentration, waves and coastal aquifer interactions are unavailable. Therefore, numerical models for wave and sediment transport are widely used by coastal engineers. This review covers mechanisms of sediment transport, important forcing factors, governing equations of wave-induced flow, groundwater interactions, empirical and numerical relations of cross-shore and longshore sediment transport in the swash zone. Major advantages and shortcomings of various numerical models and approaches are highlighted and reviewed. These will provide coastal modelers an impetus for further detailed investigations of fluid and sediment transport in the swash zone.

2009