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Abstract In the present study, the three-dimensional, turbulent, free-surface flow, developing by the oblique propagation of breaking waves over a constant slope bed is numerically simulated. Then, two-dimensional simulations of suspended sediment transport, induced by normal to the shore wave breaking over the same beach, are performed. The main objective is to investigate in depth the flow structure in the surf zone, as well as the behavior of sediment suspension. Predicted profiles of the undertow and the longshore current at several positions in the surf zone, are presented. For the sediment transport simulations, cases of sediments of various grain sizes are investigated. During wave breaking and also in the surf zone, strong uplift of bed sediment is observed. Introduction Significant coastal processes, such as wave breaking, wave induced currents, sediment transport and bed morphology evolution come as a result of the interaction between coastal waves and sea bottom. Wave breaking, which takes place when the wave height and steepness become very large as the water depth becomes shallower, is the generating mechanism of currents that develop in the surf zone where wave energy dissipation takes place after breaking. The main wave-induced currents are the longshore current, which is developed when the direction of the breaking wave is oblique to the shoreline, and the cross-shore current, which is also known as the undertow current. These currents owe their existence to the mean shear stress field, developing in the surf zone, in order to balance the pressure gradient and the momentum fluxes due to wave set-up and wave energy dissipation. The undertow current, which is seawards directed close to the bottom and shorewards directed close to the free surface, was firstly observed experimentally by Bagnold (1940). Several years later, a qualitative description of the phenomenon was reported in Dyhr- Nielsen and Sørensen (1970). The capturing of the longshore profile demands much more effort, however it has also been studied, mainly by means of physical modeling (e.g. Visser, 1991; Hamilton and Ebersole, 2001; Wang et al. 2002). Coupled with wave breaking and the wave induced-currents is the sediment transport, which is divided into two basic modes, i.e. bed load and suspended load transport. The bed load is in more or less continuous contact with the bed during the transport, determined by the effective bed shear stress, while the suspended load is moving without continuous contact with the bed as a result of the agitation of fluid turbulence (Fredsoe and Deigaard, 1992). The foundations of sediment transport research can be found in the studies of du Boys (1879), Einstein (1950) and Vanoni (1975). In addition, sediment transport in marine environments has been investigated more recently in depth (e.g. Van Rijn, 1984a;b;c; Fredsoe, 1993).
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- North America > United States (0.95)
Morphological Evolution of a Bed Profile Induced By a Storm Event at the Belgian Coast Predicted By XBeach Model
Kolokythas, Gerasimos A. (Flanders Hydraulics Research, Flemish Government) | Raquel, Silva (Flanders Hydraulics Research, Flemish Government) | Blanco, Maria Rosalia Delgado (Antea Group)
Abstract In the present study, the impact of the storm event on the bed morphology of a typical Belgian coastal profile, is numerically simulated. The simulations are performed using the open-source model XBeach, which is capable of representing hydrodynamic and morphodynamic processes in coastal areas. The objective is to evaluate the performance of XBeach through a sensitivity analysis, in which various model parameter (numerical and physical) settings, are considered. It was found that the model's predictions are, in general, reasonable, while extensive grid resolution analysis led to suggestions for the optimal use of non-equidistant grids in the cross-shore direction. Introduction The protection of coastal areas from eroding processes is an issue of high importance that has motivated many scientists and researchers to investigate in depth related processes, such as wave propagation, sediment transport and bed morphology evolution. Over the past few decades, tools for addressing the combined action of the aforementioned phenomena have been developed (e.g. Vellinga, 1986; Steetzel,1993; Larson et al., 2004). XBeach (Roelvink et al., 2009) is an open-source, process-based model, which was developed to represent hydrodynamic and morphodynamic processes and impacts on coastal areas. As for the hydrodynamics, XBeach includes the processes of short wave transformation (shoaling, refraction and breaking) and long wave transformation, while bed load and suspended sediment transport and avalanching are the main processes supported by the morphodynamic module of XBeach. Two modes of XBeach are available, i.e. the hydrostatic and the nonhydrostatic mode. The hydrostatic mode is based on the solution of the time-dependent (instationary) version of wave-action balance equation for the calculation of short-wave forcing, which is introduced in the non-linear shallow water equations for the (low-frequency) flow calculation (Roelvink et al., 2009). In the non-hydrostatic mode, the calculation of short and long wave transformation is achieved by the non-linear shallow water equations, which include a dynamic pressure correction term following the procedure introduced by Zijlema et al. (2011). One of the main advantages of XBeach is the capability to account for the generation of long waves in the surf zone and therefore reproduce their impact on the bed morphology evolution close to the shoreline. As mentioned in Van Dongeren et al. (2003), these lowfrequency motions were confirmed to contain a significant portion of the total energy in the wave field, especially in shallower water, and has been hypothesized that they have important effect on the nearshore morphology.