Coupling mechanisms in double sandbar systems. Part 2: impact on alongshore variability of inner‐bar rip channels

Castelle, Bruno; Ruessink, Gerben; Bonneton, Philippe; Marieu, Vincent; Bruneau, Nicolas; Price, Timothy

doi:10.1002/esp.1949

Cited by 34 publications

(26 citation statements)

References 42 publications

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“…The general fi eld observation that inner-bar rip channels are more irregularly spaced than outer-bar crescents suggests that morphological coupling may be more important to understanding and predicting the evolution of inner-bar rip channels than previously envisaged. This issue is explored in the companion paper (Castelle et al, 2010).…”

Section: Discussionmentioning

confidence: 98%

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

Castelle

Ruessink

Bonneton

et al. 2010

Earth Surf Processes Landf

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Crescentic sandbars and rip channels along wave-dominated sandy beaches are relevant to understand localized beach and dune erosion during storms. In recent years, a paradigm shift from hydrodynamic template models to self-organization mechanisms occurred to explain the formation of these rhythmic features. In double sandbar systems, both the inner-and outerbar rip channels and crescentic planshapes are now believed to be free instabilities of the nearshore system arising through selforganization mechanisms alone. However, the occasional occurrence of one or two inner-bar rip channels within one outer-bar crescent suggests a forced, morphologically coupled origin. Here we use a nonlinear morphodynamic model to show that alongshore variability in outer-bar depth, and the relative importance of wave breaking versus wave focussing by refraction across the outer bar, is crucial to the inner-bar rip channel development. The coupling patterns simulated by our model are similar to those observed in the fi eld. Morphological coupling requires a template in the morphology (outer-bar geometry) which, through the positive feedback between fl ow, sediment transport and the evolving morphology (that is, self-organization) enforces the development of coupling patterns. We therefore introduce a novel mechanism that blurs the distinction between self-organization and template mechanisms. This mechanism may also be extended to explain the dynamics of other nearshore patterns, such as beach cusps. The impact of this novel mechanism on the alongshore variability of inner-bar rip channels is investigated in the companion paper.

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Section: Discussionmentioning

confidence: 98%

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

Castelle

Ruessink

Bonneton

et al. 2010

Earth Surf Processes Landf

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113

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“…This enables both the detection of the dominant spatial modes of variability and how these modes vary in space. As in some previous wavelet applications to sandbar data [e.g., Ruessink et al , 2007; Castelle et al , 2010b], we used the Morlet wavelet ψ 0 because of its good localization characteristics in both the space and frequency domains. In all results below, the term wavelength refers to the Fourier wavelength λ equivalent to the wavelet scale s. We determined the statistical significance of W n z ( s ) against synthetic red noise series, as detailed by Torrence and Compo [1998].…”

Section: Methodsmentioning

confidence: 99%

On the impact of an offshore bathymetric anomaly on surf zone rip channels

et al. 2012

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[1] We use a nonlinear morphodynamic model to demonstrate that the presence of a single persistent offshore bathymetric anomaly strongly affects the formation, nonlinear evolution and saturation of surf zone rip channels. In the case of an offshore bump or trough and waves with oblique incidence, a rip channel shoreward of the anomaly is enforced by the more seaward alongshore variability in depth. The degree of rip channel enforcement is controlled by the strength of the rotational nature of surf zone rip current circulations, which is, in turn, driven by differential broken wave energy dissipation induced by wave refraction across the offshore bathymetric anomaly. The alongshore location of this forced rip channel is more stable with increasing offshore anomaly amplitude, decreasing offshore wave obliquity and decreasing bathymetric anomaly distance to the shore. Simulations show that rip channel behavior downdrift and updrift of the offshore perturbation are different. In our numerical experiments, downdrift rip channels have systematically larger alongshore scales, smaller alongshore migration rates and more erosive megacusps than those updrift. Rip channels therefore self-organize into patterns of different alongshore scales and migration rates as a result of an alongshore perturbation in the wave forcing enforced by wave refraction across an offshore bathymetric anomaly. These simulations are qualitatively corroborated by video observations of sandbar behavior during a down-state sequence at a site with a persistent offshore trough.Citation: Castelle, B., V. Marieu, G. Coco, P. Bonneton, N. Bruneau, and B. G. Ruessink (2012), On the impact of an offshore bathymetric anomaly on surf zone rip channels,

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“…The morphological evolution of the inner bar, therefore, is driven by a mixture of self‐organization and outer‐inner bar coupling mechanisms [ Castelle et al , , ]. Although morphological coupling between two sandbars suggests a reciprocal influence, observations [ Ruessink et al ., ] and modeling results [ Castelle et al , , ; Thiébot et al , ] indicated that the inner‐bar morphology does not affect the outer‐bar morphodynamics.…”

Section: Introductionmentioning

confidence: 99%

Coupled sandbar patterns and obliquely incident waves

et al. 2013

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[1] In double sandbar systems, the alongshore variability in the inner bar often resembles that of the outer bar, suggesting that the outer bar acts as a morphological template for the inner bar. Earlier observations have indicated that this resemblance, also termed "coupling," may take several forms. Here we apply a nonlinear 2DH morphodynamic model with time-invariant forcing to show that the angle of wave incidence (Â ) is crucial for the alongshore-variable morphodynamic evolution of the inner bar, for a given crescentic outer bar. In contrast to previous modeling efforts of double-barred systems, which mostly used highly idealized boundary conditions, we force our model with realistic hydrodynamics and bathymetrical data derived from video observations at the double-barred Gold Coast, Australia. The results show that for small angles of wave incidence (Â < 10 ı ) over a crescentic outer-bar, cell-circulation patterns govern the flow at the inner bar, giving rise to rip channels that incise the inner bar at the locations of the landward perturbations in the outer bar (horns). On the other hand, for obliquely incident waves (Â = 10 ı -20 ı ) over a crescentic outer bar, the circulatory nature of the flow disappears and gives way to a meandering alongshore current. The offshore-directed sections of this meandering alongshore current erode the inner-bar downdrift of the outer-bar horns, leading to landward perturbations of the inner bar that are coupled to the outer-bar horns; an observed coupling type that had not been reproduced as yet. Oblique wave incidence thus proves to be crucial to the development of this type of sandbar coupling, as previously hypothesized from sandbar-coupling observations at the Gold Coast. Additional simulations including tidal water level variations and bar depth variations demonstrate the robustness of our findings.

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Coupling mechanisms in double sandbar systems. Part 2: impact on alongshore variability of inner‐bar rip channels

Cited by 34 publications

References 42 publications

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

On the impact of an offshore bathymetric anomaly on surf zone rip channels

Coupled sandbar patterns and obliquely incident waves

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