Modelling the formation and the long‐term behavior of rip channel systems from the deformation of a longshore bar

Abstract

[1] A nonlinear numerical model based on a wave-and depth-averaged shallow water equation solver with wave driver, sediment transport, and bed updating is used to investigate the long-term evolution of rip channel systems appearing from the deformation of a longshore bar. Linear and nonlinear regimes in the morphological evolution have been studied. In the linear regime, a crescentic bar system emerges as a free instability. In the nonlinear regime, merging/splitting in bars and saturation of the growth are obtained. In spite of excluding undertow and wave-asymmetry sediment transport, the initial crescentic bar system reorganizes to form a large-scale and shore-attached transverse or oblique bar system, which is found to be a dynamical equilibrium state of the beach system. Thus the basic morphological transitions ''Longshore Bar and Trough'' ! ''Rhythmic Bar and Beach'' ! ''Transverse Bar and Rip'' described by earlier conceptual models are here reproduced. The study of the physical mechanisms allows us to understand the role of the different transport modes: The advective part induces the formation of crescentic bars and megacusps, and the bedslope transport damps the instability. Both terms contribute to the attachment of the megacusps to the crescentic bars. Depending on the wave forcing, the bar wavelength ranges between 180 and 250 m (165 and 320 m) in the linear (nonlinear) regime.Citation: Garnier, R., D. Calvete, A. Falqués, and N. Dodd (2008), Modelling the formation and the long-term behavior of rip channel systems from the deformation of a longshore bar,

Origine des courants d´arrachement bien établis

“…Le modèle 2DH non-linéaire MORFO55 (CABALLERIA et al, 2002et GARNIER et al, 2006 a été appliqué pour illustrer le mécanisme décrit précédemment: la figure 4a,b montre le développement d´une structure rythmique de barres en croissant au niveau de la barre parallèle, avec une longueur d´onde de 200m (résultats commentés par GARNIER et al, 2008). Ce mécanisme peut aussi s´appliquer pour les plages planes (dépourvues de barres parallèles) comme le montre la figure 4c,d.…”

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

“…The formation of realistic crescentic bars has been reproduced by many linear (Deigaard et al, 1999;Falqués et al, 2000;Calvete et al, 2005) and nonlinear (Caballeria et al, 2002;Damgaard et al, 2002;Reniers et al, 2004;Garnier et al, 2008) morphodynamic models. A self-organization physical mechanism called 'bed-surf coupling', first described by Falqués et al (2000), explains the formation of crescentic bars in the majority of these models.…”

Beach cusps and inner surf zone processes: growth or destruction? A case study of Trafalgar Beach (Cádiz, Spain)

“…However, the bed level at the wall is allowed to move vertically and we can consider that a horizontal oscillation of the shoreline is induced by the deposition/erosion of sediment next to the wall generating a horn/embayment at the shoreline, respectively. This assumption has been considered to explain the formation of megacusps (Calvete et al, 2005;Garnier et al, 2008), which are well known surf zone features.…”

“…They are due to surf zone processes and commonly correspond to the shore attachment of transverse/oblique bars. These bars and the associated rip current circulation are very often related to an off-shore crescentic bar system (Inman and Guza, 1982;Wright and Short, 1984;Short, 1999;Ortega-Sánchez et al, 2003;Calvete et al, 2005;Castelle et al, 2007;Garnier et al, 2008).…”