Modeling waves, currents and sandbars on natural beaches: The effect of surface rollers

Ribas, Francesca; Swart, H.E. de; Calvete, D.; Falqués, Albert

doi:10.1016/j.jmarsys.2011.02.016

Cited by 23 publications

(29 citation statements)

References 29 publications

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“…The values of γ b and k a were adjusted so that the profiles of H o and V o could be modeled with the minimum RMS error. The default values of β rol and M were chosen for being standard widely used values, and they were subsequently shown to give nearly the smallest RMS errors of H o and V o [ Ribas et al , 2011]. An example of measured and modeled H o and V o on 18 October 1998 at Egmond beach (when the wave conditions were similar to those of the default case of the present study) is shown in Figure 3.…”

Section: Modelmentioning

confidence: 99%

“…Performing a more quantitative comparison between model results and field observations is the second essential step to test the bed‐flow mechanism and obtain a better understanding of the underlying physical processes. Ribas et al [2011] recently studied the effect of surface rollers on the profiles of the current and

scriptC

da in the surf zone. They presented a new self‐organization model that included the roller dynamics in the hydrodynamic equations and its effect in the sediment transport.…”

Section: Introductionmentioning

confidence: 99%

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Modeling and analyzing observed transverse sand bars in the surf zone

et al. 2012

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[1] A morphodynamic model has been applied to explain the characteristics of transverse sandbars observed in the inner surf zone of open beaches. The model describes the feedback between waves, rollers, depth-averaged currents and bed evolution, so that self-organized processes can develop. The modeled bar characteristics, i.e. wavelength (30-70 m), crest orientation (up-current) and the e-folding growth time (about 12 hr) are in good agreement with those of observed transverse bars at Noordwijk beach, the Netherlands, but modeled migration speeds (tens of meters per day), turn out to be a factor 2 larger than those observed. The wavelength increases with the distance between the shoreline and the peak of the longshore current and the migration speed is correlated with the maximum longshore current. The model also explains why transverse bar formation at Noordwijk occurs for obliquely incident waves of intermediate heights. Realistic positive feedback leading to formation of up-current oriented bars like those observed is only obtained if a term related to the turbulence sediment resuspension created by the rollers is included in the transport formula. In that case, the depth-averaged sediment concentration decreases seaward across the inner surf zone, enhancing the convergence of sediment transport in the offshore directed flow perturbations that occur over the up-current bars. This offshore current deflection is mainly caused by frictional torques, but the roller radiation stresses also play an important role.

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Section: Modelmentioning

confidence: 99%

scriptC

da in the surf zone. They presented a new self‐organization model that included the roller dynamics in the hydrodynamic equations and its effect in the sediment transport.…”

Section: Introductionmentioning

confidence: 99%

Modeling and analyzing observed transverse sand bars in the surf zone

et al. 2012

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“…The default offshore wave conditions (H off = 0.8 m, T p = 9 s, and θ off = 30 o ) are the mean values measured at the Gold coast at the start of the finger bar events (Table 1). The default values of the physical parameters of the model parameterizations are the same that were used in the previous applications of Duck and Noordwijk (Ribas et al 2011a(Ribas et al , 2012 and were a result of a model calibration with data from a Dutch beach close to Noordwijk (Ribas et al 2011b).…”

Section: Methodsmentioning

confidence: 99%

“…These studies showed that accounting for the longshore currents generated by obliquely incident waves and wave rollers (which not only affect the currents but also generate additional sediment resuspension) is essential to successfully explain the formation of finger bars at Noordwijk (Ribas et al 2011b(Ribas et al , 2012 and at Duck (Ribas et al 2011a). A depth-averaged sediment concentration that decreases in the seaward direction together with an offshore deflection of the longshore current over the up-current oriented bars are necessary conditions for the formation of finger bars.…”

Section: Introductionmentioning

confidence: 99%

“…A depth-averaged sediment concentration that decreases in the seaward direction together with an offshore deflection of the longshore current over the up-current oriented bars are necessary conditions for the formation of finger bars. Ribas et al (2011b) showed that including the roller-induced sediment resuspension creates an increase of the depth-averaged sediment concentration near the shoreline in such a way that this quantity decreases in the seaward direction across the inner surf zone.…”

Section: Introductionmentioning

confidence: 99%

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Observations and modeling of surf zone transverse finger bars at the Gold Coast, Australia

et al. 2014

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The occurrence and characteristics of transverse finger bars at Surfers Paradise (Gold Coast, Australia) have been quantified with 4 years of time-exposure video images. These bars are attached to the inner terrace and have an oblique orientation with respect to the coastline. They are observed during 24 % of the study period, in patches up to 15 bars, with an average lifetime of 5 days and a mean wavelength of 32 m. The bars are observed during obliquely incident waves of intermediate heights. Bar crests typically point toward the incoming wave direction, i.e., they are upcurrent oriented. The most frequent beach state when bars are present (43 % of the time) is a rhythmic low-tide terrace and an undulating outer bar. A morphodynamic model, which describes the feedback between waves, currents, and bed evolution, has been applied to study the mechanisms for finger bar formation. Realistic positive feedback leading to the formation of the observed bars only occurs if the sediment resuspension due to roller-induced turbulence is included. This causes the depth-averaged sediment concentration to decrease in the seaward direction, enhancing the convergence of sediment transport in the offshore-directed flow perturbations that occur over the up-current bars. The longshore current strength also plays an important role; the offshore root-mean-square wave height and angle must be larger than some critical values (0.5 m and 20 • , respectively, at 18-m depth). Model-data comparison indicates that the modeled bar shape characteristics (up-current orientation) and the wave conditions leading to the bar formation agree with data, while the modeled wavelengths and migration rates are larger than the observed ones. The discrepancies might be because in the model we neglect the influence of the large-scale beach configuration.

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Understanding coastal morphodynamic patterns from depth‐averaged sediment concentration

Ribas

Falqués

Swart

et al. 2015

Reviews of Geophysics

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This review highlights the important role of the depth‐averaged sediment concentration (DASC) to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern: beach cusps, surf zone transverse and crescentic bars, and shoreface‐connected sand ridges. We present a formulation and methodology, based on the knowledge of the DASC (which equals the sediment load divided by the water depth), that has been successfully used to understand the characteristics of these features. These sand bodies, relevant for coastal engineering and other disciplines, are located in different parts of the coastal zone and are characterized by different spatial and temporal scales, but the same technique can be used to understand them. Since the sand bodies occur in the presence of depth‐averaged currents, the sediment transport approximately equals a sediment load times the current. Moreover, it is assumed that waves essentially mobilize the sediment, and the current increases this mobilization and advects the sediment. In such conditions, knowing the spatial distribution of the DASC and the depth‐averaged currents induced by the forcing (waves, wind, and pressure gradients) over the patterns allows inferring the convergence/divergence of sediment transport. Deposition (erosion) occurs where the current flows from areas of high to low (low to high) values of DASC. The formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features, but the physical mechanisms for their migration, their finite‐amplitude behavior and their decay can also be explored.

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Modeling waves, currents and sandbars on natural beaches: The effect of surface rollers

Cited by 23 publications

References 29 publications

Modeling and analyzing observed transverse sand bars in the surf zone

Modeling and analyzing observed transverse sand bars in the surf zone

Observations and modeling of surf zone transverse finger bars at the Gold Coast, Australia

Understanding coastal morphodynamic patterns from depth‐averaged sediment concentration

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