Influence of beach grain size and bed slope on nearshore hydro- and morpho-dynamics

Two major parameters that determine the beach type are sediment grain size and beach slope. Intermediate beaches normally have steep slopes and are associated with coarse-grained sands and narrow surf zones, while dissipative beaches generally have mild slopes and are related to fine sands and wider surf zones. In numerical experiments, the Delft3D and Xbeach models were combined to resolve the 3D Navier-Stokes equations for incompressible flow and beach morphology. The sediment transport module supports both bed-load and suspended load transport of non-cohesive sediments. Numerical simulations were run for different hydrodynamic conditions, but with a focus on different beach slopes and grain sizes, and considering hydrodynamic processes, sediment transport in cross- and alongshore directions, as well as foreshore bathymetry changes. Larger grain sizes tend to generate more complex nearshore hydrodynamic patterns. The transformation of incoming waves as they reach shallow water occurs closer the shoreline for steeper profiles. Consistently, the peaks in eddy viscosity, turbulence dissipation rate (TDR), turbulent kinetic energy (TKE) and wave set-up are shifted onshore for steeper slopes. High values of eddy viscosity, TKE and wave set-up are spread offshore for coarser grain sizes. The TDR is an order of magnitude smaller for the coarsest grains compared with other cases. The numerical results showed that TKE, sediment concentrations and sediment transport rate are greater on steep beaches than on mildly sloped beaches. The beach morphology exhibits different erosive characteristics depending on grain size (e.g., foreshore profile evolution is erosive and accretionary on fine and coarse sand beaches, respectively). The results confirmed that wave energy, beach grain size and bed slope are the main factors influencing sediment transport and beach morphodynamics.

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

Numerical experiments on breaking waves on contrasting beaches using a two-phase flow approach

Roham Bakhtyar, David Andrew Barry

A mechanistic understanding of beach environments needs to account for interactions of oceanic forcing and beach materials, in particular the role of waves on the evolution of the beach profile. A fully coupled two-phase flow model was used to simulate nearshore fluid-sediment turbulent flow in the cross-shore direction. It includes the Reynolds- Averaged Navier-Stokes equations and turbulent stress closures for each phase, and accounts for inter-granular stresses. The model has previously been validated using laboratory-scale data, so the results are likely more reliable for that scale. It was used to simulate wave breaking and the ensuing hydrodynamics and sediment transport processes in the surf/swash zones. Numerical experiments were conducted to investigate the effects of varying beach and wave characteristics (e.g., beach slope, sediment grain size, wave periods and heights) on the foreshore profile changes. Spilling and plunging breakers occur on dissipative and intermediate beaches, respectively. The impact of these wave/beach types on nearshore zone hydrodynamics and beach morphology was determined. The numerical results showed that turbulent kinetic energy, sediment concentrations and transport rate are greater on intermediate than on dissipative beaches. The results confirmed that wave energy, beach grain size and bed slope are main factors for sediment transport and beach morphodynamics. The location of the maximum sediment transport is near the breaking point for both beach types. Coarse- and fine-sand beaches differ significantly in their erosive characteristics (e.g., foreshore profile evolutions are erosive and accretionary on the fine and coarse sand beaches, respectively). In addition, a new parameter (based on main driving factors) is proposed that can characterize the sediment transport in the surf and swash zones. The results are consistent with existing physical observations, suggesting that the two-phase flow model is suitable for the simulation of hyperconcentrated mixed water-sediment flows in the nearshore. The model thus has potential as a useful tool for investigating interactions between nearshore hydrodynamics and beach morphology.

Elsevier2012

Numerical simulation of wave breaking and wave runup in the nearshore zone

Roham Bakhtyar

When the wave approaches to coast, the wave steepness, which is the ratio of wave height to water depth increases due to depth re-duction. This increase continues to a certain extent, until finally at this limit, the wave breaks. Wave breaking results in the releasing of wave energy, production of coastal currents and increase of the average water elevation. The surf zone and swash zone is the most active region from the aspect of hydrodynamic processes. In this region sediment transport and seabed changes are due to the breaking waves and its induced nearshore currents. Numerical models based on the Navier-Stokes equations can model the flow field very accurately. The generation, transformation and dissipation of turbulence can be delicately modeled by closuring a proper turbulence model to the main model. Besides the free surface shape can be estimated using an appropriate numerical algorithm. As a final remark it can be concluded that numerical modeling can accurately model wave breaking. The main objective of this research is to define the wave breaking in shallow water and describing the relevant hydrodynamic processes such as wave run up, generation and dissipation of turbulence. In this paper, a two-dimensional numerical model has been developed to study wave breaking and wave run up on a sloping beach. The model is based on solving the Reynolds Averaged Navier-Stokes (RANS) equations, and a k-e turbulence closure model. The free surface is tracked by the VOF technique; the log-law profile for the mean velocity is applied at the bottom. Propagation, deformation, breaking and overturning of an Airy wave have been numerically studied with the main emphasis on velocity and turbulence characteristics.

2007

Numerical experiments on breaking waves on contrasting beaches using a two-phase flow approach

Roham Bakhtyar, David Andrew Barry

Elsevier2012

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

Roham Bakhtyar, David Andrew Barry

Techno-Press2016