چكيده به لاتين
Abstract: Numerous studies of the hydrodynamics and sediment transport in the swash zone in recent years have found the importance of swash processes in terms of science advancement and practical applications. Evidently, the hydrodynamics of swash zone are complex and not fully understood. Key hydrodynamic processes occur at both high frequency infra-gravity motions, 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.
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, wave and coastal aquifer interactions are unavailable yet. Therefore, numerical models for wave and sediment transport models are widely used by coastal engineers. Existing numerical models cannot describe wave breaking satisfactory and as yet no comprehensive numerical investigations of the combined surf and swash zones have been presented., In particular, no model has been validated carefully for the both turbulent and mean velocity fields. In this study, we investigate wave breaking and describe the ensuring hydrodynamic processes in the surf and swash zones.
First, a two-dimensional numerical model was presented for the simulation of wave breaking, runup and turbulence in the surf and swash zones. The main components of the model are the Reynolds-Averaged Navier-Stokes (RANS) equations describing the average motion of a turbulent flow, a k-e turbulence closure model describing the transformation and dissipation processes of turbulence and a Volume-Of-Fluid (VOF) technique for tracking the free surface motion. Nearshore wave evolution on a sloping bed, the velocity field and other wave characteristics were investigated. First, the results of the model were compared with experimental results for different surf zone hydrodynamic conditions. Spilling and plunging breakers were simulated and numerical model investigated for different wave parameters. The turbulence field was also considered and spatial and time-dependent variations of turbulence parameters were discussed. In the next stage of the study, numerical results were compared with two sets of experimental data in the swash zone. Generally, there is good agreement except for turbulence predictions near the breaking point where the model does not represent well the physical processes. On the other hand, turbulence predictions were found to be excellent for the swash zone. The model provides a precise and efficient tool for the simulation of the flow field and wave transformations in the nearshore, especially in the swash zone. The numerical model can simulate the surface elevation of the vertical shoreline excursion on sloping beaches, while swash-swash interactions within the swash zone are accounted for.
Then, hydrodynamics and sediment transport in the nearshore zone were modeled numerically taking into account turbulent unsteady flow. The flow field was computed using the RANS equations with a k-e turbulence closure model, while the free surface was tracked using the VOF technique. This hydrodynamical model was supplemented with a cross-shore sediment transport formula to calculate profile changes and sediment transport in the surf and swash zones. Based on the numerical solutions, flow characteristics and the effects of breaking waves on sediment transport were studied. The main characteristic of breaking waves, i.e., the instantaneous sediment transport rate, was investigated numerically, as was the spatial distribution of time-averaged sediment transport rates for different grain sizes. The analysis included an evaluation of different values of the water friction factor and an empirical constant characterizing the uprush and backwash. It was found that the uprush induces a larger instantaneous sediment transport rate than the backwash, indicating that the uprush is more important for sediment transport than the backwash. The results indicate that the sediment transport at the wave-breaking point is at least an order of magnitude greater than at the start of surf zone and greatest erosion takes place in the vicinity of the wave-breaking point. In addition, the bars are continuously generated and march offshore. Generally, the results of the present model are in reasonable agreement with other numerical and physical models of nearshore hydrodynamics. The model was found to predict well cross-shore sediment transport and thus it provides a tool for predicting beach morphology change.
Finally, the numerical investigation was carried out to advance mechanistic understanding sediment transport under sheet flow conditions. An Euler-Euler coupled two-phase flow model was developed to simulate fluid-sediment oscillatory sheet flow. Since the concentration of sediment particles is high in such flows, the kinematics of the fluid and sediment phases are strongly coupled. This model includes interaction forces, intergranular stresses and turbulent stress closure. Each phase was modeled via the RANS equations, with inter-phase momentum conservation accounting for the interaction between phases. The generation and transformation of turbulence was modeled using the two-equation k-e turbulence model. Concentration and sediment flux profiles were compared with experimental data for sheet flow conditions considering both symmetric and asymmetric oscillatory flows. Sediment and fluid velocity variations, concentration profiles, sediment flux and turbulence parameters of wave-generated sheet flow were studied numerically with a focus on sediment transport characteristics. In all applications, the model predictions compared well with the experimental data. Flow velocities, sediment concentrations are maximum at the start of the uprush and end of backwash. Unlike previous investigations in which the flow is driven by a horizontal pressure gradient, the present model solves the Navier-Stokes equations under propagating waves. Therefore, this model increases insight into realistic presentation of sediment transport mechanism in oscillatory sheet flow under water waves. The model’s ability to predict sediment transport under oscillatory sheet flow conditions was demonstrated, allowing the model to be used as a practical tool for understanding the evolution of beach morphology.
Keywords: swash zone, two-phase flow modeling, sediment transport, turbulence, hydrodynamics, uprush, backwash
