Roland Garnier | Universidad de Cantabria (original) (raw)
Papers by Roland Garnier
Journal of Geophysical Research, 2009
1] The instability leading to the formation of rip currents in the nearshore for normal waves on ... more 1] The instability leading to the formation of rip currents in the nearshore for normal waves on a nonbarred, nonerodible beach is examined with a comprehensive linear stability numerical model. In contrast to previous studies, the hypothesis of regular waves has been relaxed. The results obtained here point to the existence of a purely hydrodynamical positive feedback mechanism that can drive rip cells, which is consistent with previous studies. This mechanism is physically interpreted and is due to refraction and shoaling. However, this mechanism does not exist when the surf zone is not saturated because negative feedback provided by increased (decreased) breaking for positive (negative) wave energy perturbations overwhelms the shoaling/refraction mechanism. Moreover, turbulent Reynolds stress and bottom friction also cause damping of the rip current growth. All the nonregular wave dissipations examined give rise to these hydrodynamical instabilities when feedback onto dissipation is neglected. When this feedback is included, the dominant effect that destroys these hydrodynamical instabilities is the feedback of the wave energy onto the dissipation. It turns out that this effect is strong and does not allow hydrodynamical instabilities on a planar beach to grow for random seas.
Journal of Geophysical Research, 2009
1] The instability leading to the formation of rip currents in the nearshore for normal waves on ... more 1] The instability leading to the formation of rip currents in the nearshore for normal waves on a nonbarred, nonerodible beach is examined with a comprehensive linear stability numerical model. In contrast to previous studies, the hypothesis of regular waves has been relaxed. The results obtained here point to the existence of a purely hydrodynamical positive feedback mechanism that can drive rip cells, which is consistent with previous studies. This mechanism is physically interpreted and is due to refraction and shoaling. However, this mechanism does not exist when the surf zone is not saturated because negative feedback provided by increased (decreased) breaking for positive (negative) wave energy perturbations overwhelms the shoaling/refraction mechanism. Moreover, turbulent Reynolds stress and bottom friction also cause damping of the rip current growth. All the nonregular wave dissipations examined give rise to these hydrodynamical instabilities when feedback onto dissipation is neglected. When this feedback is included, the dominant effect that destroys these hydrodynamical instabilities is the feedback of the wave energy onto the dissipation. It turns out that this effect is strong and does not allow hydrodynamical instabilities on a planar beach to grow for random seas.
Journal of Geophysical Research, 2010
1] The formation of crescentic bars from self-organization of an initially straight shore-paralle... more 1] The formation of crescentic bars from self-organization of an initially straight shore-parallel bar for shore-normal incident waves is simulated with a two-dimensional horizontal morphodynamical model. The aim is to investigate the mechanisms behind the saturation process defined as the transition between the linear regime (maximum and constant growth of the crescentic pattern) and the saturated state (negligible growth). The global properties of the morphodynamical patterns over the whole computational domain are studied ("global analysis"). In particular, consideration of the balance of the potential energy of the emerging bar gives its growth rate from the difference between a production term (related to the positive feedback leading to the instability) and a damping term (from the gravity-driven downslope transport). The production is approximately proportional to the average over the domain of the cross-shore flow velocity times the bed level perturbation. The damping is essential for the onset of the saturation, but it remains constant while the production decreases. Thus, it is notable that the saturation occurs because of a weakening of the instability mechanism rather than an increase of the damping. A reason for the saturation of the crescentic bar growth is the change in bar shape from its initial stage rather than the growth in amplitude itself. This change is mainly characterized by the narrowing of the rip channels, the onshore migration of the crests, and the change in the mean beach profile due to alongshore variability. These properties agree with observations of mature rip channel systems in nature.
Scientia Marina, 2010
Large beach cusps (LBC, wavelength of ~ 30 m) are intertidal features that can alternately exist ... more Large beach cusps (LBC, wavelength of ~ 30 m) are intertidal features that can alternately exist in the swash and in the inner surf zone due to tidal sea level changes. They have a larger cross-shore extent (up to 50 m) than traditional cusps. This extent has been explained by a shift of the swash zone during falling tide. The cusps immerse at rising tide and previous studies indicate that surf zone processes are exclusively destructive. Here, the behaviour of large beach cusps in the inner surf zone is investigated by using a 2DH morphological numerical model applied to Trafalgar Beach (Cádiz, Spain). The model results indicate that the inner surf zone processes do not always destroy the cusps but can in fact reinforce them by considering neither the swash processes nor the tidal changes. More generally, in conditions favouring the presence of the LBC the surf zone of a beach can be unstable, leading to the formation of transverse/oblique sand bars that can have characteristics similar to the LBC. Thus, in principle, the LBC could emerge not only due to swash zone morphodynamics but also due to surf zone morphodynamics or a combination of both.
Houille Blanche-revue Internationale De L Eau, 2010
Le Nord-Est de la plage du Lido de Sète est caractérisé par la présence de deux barres sédimentai... more Le Nord-Est de la plage du Lido de Sète est caractérisé par la présence de deux barres sédimentaires d'avant-côte. L'étude de la morphodynamique de la plage, par observations in situ, a mis en évidence une relative stabilité de la barre externe et une variabilité plus importante de la barre interne. Dans la configuration la plus fréquente, les deux barres sont rectilignes et parallèles à la côte. Lors de tempêtes, la barre interne peut se segmenter et se réorienter pour faire face aux vagues incidentes. L'objectif de la présente étude est d'identifier les mécanismes à l'origine de l'évolution de la barre interne. Une analyse de stabilité linéaire a été effectuée avec un modèle numérique de stabilité nommé MORFO60. Les simulations montrent que des instabilités morphodynamiques peuvent se développer le long de la barre interne à la suite de tempêtes qui génèrent des houles de Sud ou d'Est. Ces évolutions morphodynamiques calculées par le modèle sont conformes aux évolutions observées sur le site.
Journal of Geophysical Research, 2008
1] A nonlinear numerical model based on a wave-and depth-averaged shallow water equation solver w... more 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.
Journal of Geophysical Research, 2011
A. Falqués, UPC, C/ Jordi Girona 1-3, Modul B4/B5, despatx 103 -E-08034, Abstract.
Journal of Geophysical Research, 2010
1] The formation of crescentic bars from self-organization of an initially straight shore-paralle... more 1] The formation of crescentic bars from self-organization of an initially straight shore-parallel bar for shore-normal incident waves is simulated with a two-dimensional horizontal morphodynamical model. The aim is to investigate the mechanisms behind the saturation process defined as the transition between the linear regime (maximum and constant growth of the crescentic pattern) and the saturated state (negligible growth). The global properties of the morphodynamical patterns over the whole computational domain are studied ("global analysis"). In particular, consideration of the balance of the potential energy of the emerging bar gives its growth rate from the difference between a production term (related to the positive feedback leading to the instability) and a damping term (from the gravity-driven downslope transport). The production is approximately proportional to the average over the domain of the cross-shore flow velocity times the bed level perturbation. The damping is essential for the onset of the saturation, but it remains constant while the production decreases. Thus, it is notable that the saturation occurs because of a weakening of the instability mechanism rather than an increase of the damping. A reason for the saturation of the crescentic bar growth is the change in bar shape from its initial stage rather than the growth in amplitude itself. This change is mainly characterized by the narrowing of the rip channels, the onshore migration of the crests, and the change in the mean beach profile due to alongshore variability. These properties agree with observations of mature rip channel systems in nature.
Houille Blanche-revue Internationale De L Eau, 2008
Modelling the formation and the nonlinear evolution of crescentic bars of the Aquitanian coast.
Journal of Geophysical Research, 2008
1] A nonlinear numerical model based on a wave-and depth-averaged shallow water equation solver w... more 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.
Journal of Geophysical Research, 2009
1] The instability leading to the formation of rip currents in the nearshore for normal waves on ... more 1] The instability leading to the formation of rip currents in the nearshore for normal waves on a nonbarred, nonerodible beach is examined with a comprehensive linear stability numerical model. In contrast to previous studies, the hypothesis of regular waves has been relaxed. The results obtained here point to the existence of a purely hydrodynamical positive feedback mechanism that can drive rip cells, which is consistent with previous studies. This mechanism is physically interpreted and is due to refraction and shoaling. However, this mechanism does not exist when the surf zone is not saturated because negative feedback provided by increased (decreased) breaking for positive (negative) wave energy perturbations overwhelms the shoaling/refraction mechanism. Moreover, turbulent Reynolds stress and bottom friction also cause damping of the rip current growth. All the nonregular wave dissipations examined give rise to these hydrodynamical instabilities when feedback onto dissipation is neglected. When this feedback is included, the dominant effect that destroys these hydrodynamical instabilities is the feedback of the wave energy onto the dissipation. It turns out that this effect is strong and does not allow hydrodynamical instabilities on a planar beach to grow for random seas.
Journal of Geophysical Research, 2009
1] The instability leading to the formation of rip currents in the nearshore for normal waves on ... more 1] The instability leading to the formation of rip currents in the nearshore for normal waves on a nonbarred, nonerodible beach is examined with a comprehensive linear stability numerical model. In contrast to previous studies, the hypothesis of regular waves has been relaxed. The results obtained here point to the existence of a purely hydrodynamical positive feedback mechanism that can drive rip cells, which is consistent with previous studies. This mechanism is physically interpreted and is due to refraction and shoaling. However, this mechanism does not exist when the surf zone is not saturated because negative feedback provided by increased (decreased) breaking for positive (negative) wave energy perturbations overwhelms the shoaling/refraction mechanism. Moreover, turbulent Reynolds stress and bottom friction also cause damping of the rip current growth. All the nonregular wave dissipations examined give rise to these hydrodynamical instabilities when feedback onto dissipation is neglected. When this feedback is included, the dominant effect that destroys these hydrodynamical instabilities is the feedback of the wave energy onto the dissipation. It turns out that this effect is strong and does not allow hydrodynamical instabilities on a planar beach to grow for random seas.
Journal of Geophysical Research, 2010
1] The formation of crescentic bars from self-organization of an initially straight shore-paralle... more 1] The formation of crescentic bars from self-organization of an initially straight shore-parallel bar for shore-normal incident waves is simulated with a two-dimensional horizontal morphodynamical model. The aim is to investigate the mechanisms behind the saturation process defined as the transition between the linear regime (maximum and constant growth of the crescentic pattern) and the saturated state (negligible growth). The global properties of the morphodynamical patterns over the whole computational domain are studied ("global analysis"). In particular, consideration of the balance of the potential energy of the emerging bar gives its growth rate from the difference between a production term (related to the positive feedback leading to the instability) and a damping term (from the gravity-driven downslope transport). The production is approximately proportional to the average over the domain of the cross-shore flow velocity times the bed level perturbation. The damping is essential for the onset of the saturation, but it remains constant while the production decreases. Thus, it is notable that the saturation occurs because of a weakening of the instability mechanism rather than an increase of the damping. A reason for the saturation of the crescentic bar growth is the change in bar shape from its initial stage rather than the growth in amplitude itself. This change is mainly characterized by the narrowing of the rip channels, the onshore migration of the crests, and the change in the mean beach profile due to alongshore variability. These properties agree with observations of mature rip channel systems in nature.
Scientia Marina, 2010
Large beach cusps (LBC, wavelength of ~ 30 m) are intertidal features that can alternately exist ... more Large beach cusps (LBC, wavelength of ~ 30 m) are intertidal features that can alternately exist in the swash and in the inner surf zone due to tidal sea level changes. They have a larger cross-shore extent (up to 50 m) than traditional cusps. This extent has been explained by a shift of the swash zone during falling tide. The cusps immerse at rising tide and previous studies indicate that surf zone processes are exclusively destructive. Here, the behaviour of large beach cusps in the inner surf zone is investigated by using a 2DH morphological numerical model applied to Trafalgar Beach (Cádiz, Spain). The model results indicate that the inner surf zone processes do not always destroy the cusps but can in fact reinforce them by considering neither the swash processes nor the tidal changes. More generally, in conditions favouring the presence of the LBC the surf zone of a beach can be unstable, leading to the formation of transverse/oblique sand bars that can have characteristics similar to the LBC. Thus, in principle, the LBC could emerge not only due to swash zone morphodynamics but also due to surf zone morphodynamics or a combination of both.
Houille Blanche-revue Internationale De L Eau, 2010
Le Nord-Est de la plage du Lido de Sète est caractérisé par la présence de deux barres sédimentai... more Le Nord-Est de la plage du Lido de Sète est caractérisé par la présence de deux barres sédimentaires d'avant-côte. L'étude de la morphodynamique de la plage, par observations in situ, a mis en évidence une relative stabilité de la barre externe et une variabilité plus importante de la barre interne. Dans la configuration la plus fréquente, les deux barres sont rectilignes et parallèles à la côte. Lors de tempêtes, la barre interne peut se segmenter et se réorienter pour faire face aux vagues incidentes. L'objectif de la présente étude est d'identifier les mécanismes à l'origine de l'évolution de la barre interne. Une analyse de stabilité linéaire a été effectuée avec un modèle numérique de stabilité nommé MORFO60. Les simulations montrent que des instabilités morphodynamiques peuvent se développer le long de la barre interne à la suite de tempêtes qui génèrent des houles de Sud ou d'Est. Ces évolutions morphodynamiques calculées par le modèle sont conformes aux évolutions observées sur le site.
Journal of Geophysical Research, 2008
1] A nonlinear numerical model based on a wave-and depth-averaged shallow water equation solver w... more 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.
Journal of Geophysical Research, 2011
A. Falqués, UPC, C/ Jordi Girona 1-3, Modul B4/B5, despatx 103 -E-08034, Abstract.
Journal of Geophysical Research, 2010
1] The formation of crescentic bars from self-organization of an initially straight shore-paralle... more 1] The formation of crescentic bars from self-organization of an initially straight shore-parallel bar for shore-normal incident waves is simulated with a two-dimensional horizontal morphodynamical model. The aim is to investigate the mechanisms behind the saturation process defined as the transition between the linear regime (maximum and constant growth of the crescentic pattern) and the saturated state (negligible growth). The global properties of the morphodynamical patterns over the whole computational domain are studied ("global analysis"). In particular, consideration of the balance of the potential energy of the emerging bar gives its growth rate from the difference between a production term (related to the positive feedback leading to the instability) and a damping term (from the gravity-driven downslope transport). The production is approximately proportional to the average over the domain of the cross-shore flow velocity times the bed level perturbation. The damping is essential for the onset of the saturation, but it remains constant while the production decreases. Thus, it is notable that the saturation occurs because of a weakening of the instability mechanism rather than an increase of the damping. A reason for the saturation of the crescentic bar growth is the change in bar shape from its initial stage rather than the growth in amplitude itself. This change is mainly characterized by the narrowing of the rip channels, the onshore migration of the crests, and the change in the mean beach profile due to alongshore variability. These properties agree with observations of mature rip channel systems in nature.
Houille Blanche-revue Internationale De L Eau, 2008
Modelling the formation and the nonlinear evolution of crescentic bars of the Aquitanian coast.
Journal of Geophysical Research, 2008
1] A nonlinear numerical model based on a wave-and depth-averaged shallow water equation solver w... more 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.