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  • Modeling shoreline sand waves on the coasts of Namibia and Angola

     Ribas Prats, Francesca; Falques Serra, Alberto; Van den Berg, Niels; Caballería, Miquel
    International journal of sediment research
    Date of publication: 2013-09
    Journal article

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    The southwestern (SW) coast of Africa (Namibia and Angola) features long sandy beaches and a wave climate dominated by energetic swells from the Southsouthwest (SSW), therefore approaching the coast with a very high obliquity. Satellite images reveal that along that coast there are many shoreline sand waves with wavelengths ranging from 2 to 8 km. A more detailed study, including a Fourier analysis of the shoreline position, yields the wavelengths (among this range) with the highest spectral density concentration. Also, it becomes apparent that at least some of the sand waves are dynamically active rather than being controlled by the geological setting. A morphodynamic model is used to test the hypothesis that these sand waves could emerge as free morphodynamic instabilities of the coastline due to the obliquity in wave incidence. It is found that the period of the incident water waves, Tp, is crucial to establish the tendency to stability or instability, instability increasing for decreasing period, whilst there is some discrepancy in the observed periods. Model results for Tp = 7-8 s clearly show the tendency for the coast to develop free sand waves at about 4 km wavelength within a few years, which migrate to the north at rates of 0.2-0.6 km yr-1. For larger Tp or steeper profiles, the coast is stable but sand waves originated by other mechanisms can propagate downdrift with little decay.

  • Shoreline sand waves along the catalan coast

     Falques Serra, Alberto; Caballería, Miquel; Ribas Prats, Francesca; Van den Berg, Niels
    International Conference on Coastal Dynamics
    Presentation's date: 2013-06-28
    Presentation of work at congresses

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    The beach of Calella, north of Barcelona, in the Catalan coast, features a series of shoreline sand waves with wavelengths ranging from 700 to 1400 m that match with similar undulations in the -5 m bathymetric line. Historical satellite images from 2002 till 2010 show that these undulations slightly change in time. The wave climate on that stretch of the Catalan coast has a large proportion of waves from the E-NE and from the SW, i.e., with high angles with respect to shore normal rending the shoreline potentially unstable. Here we show that those sand waves might be due to that instability. Model results, both Linear Stability Analysis and nonlinear time evolution, show that the shoreline is nearly at the threshold for instability and that the emergent wavelengths are roughly consistent with the observed ones.

  • On the predictability of mid-term cross-shore profile evolution

     Fernandez Mora, Maria de Los Angeles; Calvete Manrique, Daniel; Falques Serra, Alberto; Ribas Prats, Francesca; Idier, Deborah
    Journal of coastal research
    Date of publication: 2013
    Journal article

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  • A mechanism for sandbar straightening by oblique wave incidence

     Garnier, Roland Charles; Falques Serra, Alberto; Calvete Manrique, Daniel; Thiebot, Jerome; Ribas Prats, Francesca
    Geophysical research letters
    Date of publication: 2013-06-06
    Journal article

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    Breaker bars in the surf zone of sandy beaches generally evolve between straight bars parallel to the shore and meandering crescentic bars associated with intense (dangerous) currents flowing seaward through rip channels. Understanding the behavior of such systems is fundamental as they control the entire surf zone dynamics, the shape of the coastline, and the exchange of floating material with the shoreface. Although the mechanisms behind the meandering of an originally straight bar have been studied extensively, a clear physical explanation on the crescentic bar straightening was missing. Recent field observations have highlighted that this morphological reset can be due to wave obliquity. By using a two-dimensional horizontal morphological model, we show that the bar straightening by oblique waves occurs because the rip current is both weakened in intensity and shifted downdrift from the channel deepest section. The technique employed is useful for the study of other types of bed forms.

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    Effect of surface rollers on the formation of crescentic bars: large angles of incidence  Open access

     Calvete Manrique, Daniel; Ribas Prats, Francesca; de Swart, H.E.; Falques Serra, Alberto
    International Conference on Coastal Engineering
    Presentation's date: 2012-07-05
    Presentation of work at congresses

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    The formation of crescentic bars is examined using a morphodynamic model based in linear stability analysis. The effect of surface rollers for off-normal wave conditions is examined. The effect of the rollers is to increase the e-folding times with increasing the angle of incidence. For angles large enough the formation of crescentic bars is even inhibit. The main effect of the rollers it be through hydrodynamics. The longitudinal changes in current produced by the rollers cause the maximum of sediment concentration to be shifted towards the coast with the final effect of prevent the formation of crecscentic bars.

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    What determines the wavelength of self-organized shoreline sand waves?  Open access

     Falques Serra, Alberto; Van den Berg, Niels; Ribas Prats, Francesca; Caballeria, M
    International Conference on Coastal Engineering
    Presentation's date: 2012-07-05
    Presentation of work at congresses

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    Shoreline undulations extending into the bathymetric contours with a length scale larger than that of the rhythmic surf zone bars are referred to as shoreline sand waves. Many observed undulations along sandy coasts display a wavelength in the order 1-7 km. Several models that are based on the hypothesis that sand waves emerge from a morphodynamic instability in case of very oblique wave incidence predict this range of wavelengths. Here we investigate the physical reasons for the wavelength selection and the main parametric trends of the wavelength in case of sand waves arising from such instability. It is shown that the existence of a minimum wavelength depends on an interplay between three factors affecting littoral drift: (A) the angle of wave fronts relative to local shoreline, which tends to cause maximum transport at the downdrift flank of the sand wave, (B) the refractive energy spreading which tends to cause maximum transport at the updrift flank and (C) wave focusing (de-focusing) by the capes (bays), which tends to cause maximum transport at the crest or slightly downdrift of it. Processes A and C cause decay of the sand waves while process B causes their growth. For low incidence angles, B is very weak so that a rectilinear shoreline is stable. For large angles and long sand waves, B is dominant and causes the growth of sand waves. For large angles and short sand waves C is dominant and the sand waves decay. Thus, wavelength selection depends on process C, which essentially depends on shoreline curvature. The growth rate of very long sand waves is weak because the alongshore gradients in sediment transport decrease with the wavelength. This is why there is an optimum or dominant wavelength. It is found that sand wave wavelength scales with λ0/β where λ0 is the water wave wavelength in deep water and β is the mean bed slope from shore to the wave base.

    Postprint (author’s final draft)

  • Modeling and analyzing observed transverse sand bars in the surf zone

     Ribas Prats, Francesca; de Swart, H.E.; Calvete Manrique, Daniel; Falques Serra, Alberto
    Journal of geophysical research
    Date of publication: 2012-04-25
    Journal article

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  • Modelling the dynamics of large scale shoreline sand waves  Open access

     Van den Berg, Niels
    Defense's date: 2012-05-11
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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    Shoreline sand waves are shoreline undulations with a length scale of several kilometres and a time scale of years to decades. They occur on many coasts, migrating in the direction of the dominant littoral drift and they introduce a variability into the shoreline position that can be greater than the long term coastal trend. The objective of this thesis is to provide more insight into the formation and dynamics of shoreline sand waves and, in particular, to explore the role of the so called high angle wave instability. Previous studies showed that the shoreline can be unstable under very oblique wave incidence. This high angle wave instability develops due to the feedback of shoreline changes and the associated changes in the bathymetry into the wave field. Wave propagation over this perturbed bathymetry leads to specific gradients in the alongshore transport that can cause the growth and migration of shoreline sand waves. In this thesis a quasi 2D non-linear morphodynamical model is improved and used to explore high angle wave instability and predict the formation and evolution of shoreline sand waves. The model assumes that the large scale and long term shoreline dynamics is controlled by the wave driven alongshore transport so that the details of the surfzone morphodynamics are not resolved. It overcomes some of the limitations of previous modelling studies on high angle wave instability. The wave field is computed with a simple wave module over the evolving bathymetry and an empirical formula is used to compute the alongshore transport. Cross-shore dynamics is described in a parameterized way and the model is capable of describing shoreline perturbations with a finite and dynamic cross-shore extent. The conditions under which shoreline instability can lead to the formation of shoreline sand waves are refined. Generic simulations with constant wave conditions and random initial perturbations show that the shoreline becomes unstable when the wave incidence angle at the depth of closure (i.e., the most offshore extent of the shoreline perturbations) is larger than a critical angle of about 42 degrees and shoreline sand waves develop in unison. The cross-shore dynamics plays an essential role because it determines the offshore extent of the shoreline perturbations. Using default model parameters, wave conditions and cross-shore profile, the sand waves develop with wavelengths between 2 and 5 km, the time scale for their formation is between 5 and 10 years and they migrate downdrift at about 0.5 km/yr. Simulations with a localized large scale perturbation trigger the formation of a downdrift sand wave train. Larger wave obliquity, higher waves and shorter wave periods strengthen the shoreline instability. A more realistic wave climate, with alternating high and low angle wave incidence reduces the potential for shoreline instability. A percentage of about 80% of high angle waves is required for sand wave formation. It is demonstrated that the range of low wave angles that can occur on a coast is larger than the range of high wave angles, and that the stabilizing effect produced by low angle waves (causing diffusion) is bigger than the destabilizing effect produced by high angle waves (causing growth and migration). Even if high angle waves are not dominant, the instability mechanism might still play a role in the persistence and downdrift migration of large scale shoreline perturbations. The model results are in qualitative agreement with observations of shoreline sand waves. The quasi 2D approach provides new insight into the physical mechanisms behind high angle wave instability and the occurrence of a minimal and optimal length scale for sand wave formation. Essential physical processes are wave energy dispersion due to wave refraction, wave energy focusing near the crest of a sand wave and the monotonic decrease of the gradients in alongshore transport for increasing length scales.

    Les ones de sorra a la línia de costa són ondulacions de la línia de costa amb una escala espacial de kilòmetres i una escala temporal d’anys a dècades. Ocorren a moltes costes, migren en la direcció del transport litoral i introdueixen una variabilitat a la línia de costa que pot ser major que la seva tendència a llarg termini. L’objectiu d’aquesta tesi és estudiar amb més profunditat la formació i la dinàmica de les ones de sorra i, més concretament, explorar el rol de l’anomenada inestabilitat d’angle gran. Estudis previs van demostrar que la línia de costa pot ser inestable en cas d’onades obliqües que incideixen amb un angle gran. Aquesta inestabilitat d’angle gran es produeix degut a la retroalimentació entre els canvis a la línia de costa (i els que conseqüentment ocorren a la batimetria) i els canvis al camp d’onades. La propagació de les onades sobre la batimetria pertorbada crea gradients del transport de sediment longitudinal que causen el creixement i la migració de les ones de sorra. En aquesta tesi s’ha millorat un model morfodinàmic quasi 2D i no lineal per usar-lo per explorar la inestabilitat d’angle gran i predir la formació i evolució de les ones de sorra. El model assumeix que la dinàmica a gran escala i llarg termini està dominada pel transport de sediment longitudinal produït per les onades de manera que la morfodinàmica de la zona de rompents no es detalla. S’han superat algunes de les limitacions dels estudis anteriors de modelat de la inestabilitat d’angle gran. El camp d’onades es calcula amb un mòdul senzill de propagació sobre la batimetria canviant i el transport longitudinal s’estima usant una fórmula empírica. La dinàmica transversal es parametritza per descriure pertorbacions de la línia de costa amb una extensió transversal finita i dinàmica. S’han refinat les condicions sota les quals la inestabilitat d’angle gran produeix la formació d’ones de sorra. Les simulacions amb condicions constants d’onades i pertorbacions inicials aleatòries mostren que la línia de costa esdevé inestable quan l’angle d’incidència a la profunditat de tancament és major que un angle de 42 graus i les ones de sorra es desenvolupen a l’uníson. La dinàmica transversal té un rol essencial al determinar l’extensió transversal de les pertorbacions. Usant els valors per defecte dels paràmetres del model, les ones de sorra tenen espaiats d’entre 2 i 5 km i temps de creixement d’entre 5 i 10 anys, i migren en la direcció del transport a uns 0.5 km/any. Les simulacions també mostren que una pertorbació inicial localitzada desencadena la formació d’un tren d’ones de sorra. Com més obliqües i grans són les onades i com menor és el seu període major és la inestabilitat. Un clima d’onatge més realista, alternant onades d’angle d’incidència gran i petit, redueix el potencial de la inestabilitat d’angle gran. Calen almenys un 80% d’onades d’angle gran perquè es formin ones de sorra. El rang d’onades d’angle petit que poden succeir en una costa és major que el d’onades d’angle gran, i l’efecte estabilitzador de les onades d’angle petit (que produeix difusió) és més important que l’efecte desestabilitzador de les onades d’angle gran (que produeix creixement i migració). Fins i tot si les onades d’angle gran no dominen, el mecanisme d’inestabilitat pot tenir un paper important en la persistència i migració de pertorbacions de la línia de costa a gran escala. Els resultats s’assemblen qualitativament a les observacions d’ones de sorra. L’enfocament quasi 2D permet estudiar més detalls del mecanisme físic que hi ha darrere de la inestabilitat d’angle gran i del fet que existeixin longituds d’ona mínima i òptima per la formació d’ones de sorra. Els processos físics essencials són la dispersió de l’energia de l’onatge degut a la refracció, la concentració d’energia de les onades a les crestes de les ones de sorra i el decreixement monòton del transport litoral quan augmenta l’escala espacial.

  • Modeling large scale shoreline sand waves under oblique wave incidence

     Van den Berg, Niels; Falques Serra, Alberto; Ribas Prats, Francesca
    Journal of geophysical research
    Date of publication: 2012
    Journal article

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    Modelling the formation of transverse sand bars: application to Duck beach, USA  Open access

     Ribas Prats, Francesca; de Swart, H.E.; Calvete Manrique, Daniel; Falques Serra, Alberto
    River Coastal and Estuarine Morphodynamics
    Presentation's date: 2011-09-08
    Presentation of work at congresses

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    A morphodynamic model has been applied to explain the formation of transverse sand bars at Duck beach, USA. The model describes the feedback between waves, rollers, depth-averaged currents and bed evolution, so that self-organized processes can develop. The wave and bathymetric conditions measured at Duck are used to perform the simulations. Subsequently, modelled bar characteristics are compared with those observed there. Realistic positive feedback leading to formation of the observed bars only occurs if the resuspension of sediment due to bore turbulence is included in the model. Also, the offshore root mean square wave height must be larger than 0.5 m and the offshore wave incidence angle larger than 15o (offshore boundary is at 8 m depth), conditions that occur at Duck only 25% of the time. The modelled shape (wavelength, cross-shore extent and crest orientation) and growth rate agree with data, but the model overestimates the migration rates.

  • Modelling shoreline sand waves: application to the coast of Namibia

     Falques Serra, Alberto; Van den Berg, Niels; Ribas Prats, Francesca; Caballeria, M
    River Coastal and Estuarine Morphodynamics
    Presentation's date: 2011-09-08
    Presentation of work at congresses

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  • Effect of surface rollers on the formation crescentic bars

     Calvete Manrique, Daniel; de Swart, H.E.; Ribas Prats, Francesca; Falques Serra, Alberto
    River Coastal and Estuarine Morphodynamics
    Presentation's date: 2011-09-08
    Presentation of work at congresses

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    The effect of surface rollers on the formation of crescentic bars/rip channel systems is examined with a morphodynamic model. A linear stability analysis is applied to find the fastest growing bottom patterns that develop as a result of morphodynamics self-organization. Explorations of sensitivity of model results to varying wave conditions have been performed. Special attention has been paid to off-normal wave conditions. There are noticeable differences in growth rates for large/intermediate angles of wave incidence with respect to shore normal conditions. The effect of the rollers is to increase the e-folding times with increasing the angle of incidence. For angles large enough the formation of crescentic bars is even inhibit. The effect of the rollers in wavelengths, migration velocities of the bedforms and bed morphologies of the FGM is minor. Flow velocities on the rips are a factor 2 smaller in the models with rollers than in the model without rollers.

    Postprint (author’s final draft)

  • Long-term evolution of nourished beaches under high angle wave conditions

     Van den Berg, Niels; Falques Serra, Alberto; Ribas Prats, Francesca
    Journal of marine systems
    Date of publication: 2011-10
    Journal article

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    A nonlinear numerical model for large-scale dynamics of shoreline and nearshore bathymetry under wave action is applied to investigate the long-term evolution of a rectilinear coast dominated by high angle wave incidence, which is perturbed by a nourishment or an offshore borrow pit. Previous studies show that a coastline can be unstable due to high angle wave instability, which results from the feedback between shoreline changes and the wave field. In contrast to traditional one-line shoreline models, which always predict a diffusional behaviour, this instability can lead to the growth of shoreline perturbations. Model results suggest that due to high angle wave instability a nourishment or a borrow pit could trigger the formation of a shoreline sand wave train (alternating accretional and erosional zones). Its formation is a selforganised response of the morphodynamic system and can be seen as a spatial-temporal instability. New sand waves are formed downdrift while the old sand waves migrate downdrift and increase in amplitude and wavelength. Instability develops only if the bathymetric changes related to shoreline perturbations extend to a depth where the wave angle is greater than the critical angle of 42°. The potential for coastline instability is therefore limited by the wave incidence angle at the depth of closure and not the angle at deep water as suggested in previous studies. Including a fraction of low angle waves to the wave climate causes saturation of the amplitudes of the sand waves and limits the formation of the sand wave train. Even on a stable coast dominated by low angle waves, the feedback between morphology and the wave field can be crucial for the prediction of nourishment evolution. This feedback leads to relatively slow diffusion of shoreline perturbations and it can lead to downdrift migration. While some existing observations describe downdrift advection, no satisfactory explanation had been provided previously.

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

     Ribas Prats, Francesca; de Swart, H.E.; Calvete Manrique, Daniel; Falques Serra, Alberto
    Journal of marine systems
    Date of publication: 2011-10
    Journal article

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  • Shoreline instability due to very oblique wave incidence: some remarks on the physics

     Falques Serra, Alberto; Calvete Manrique, Daniel; Ribas Prats, Francesca
    Journal of coastal research
    Date of publication: 2011-03
    Journal article

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  • Dynamics of single-barred embayed beaches

     Ojeda Casillas, Elena; Guillén Aranda, Jorge; Ribas Prats, Francesca
    Marine geology
    Date of publication: 2011-02-15
    Journal article

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    Shoreline sand waves and beach nourishments  Open access

     Van den Berg, Niels; Falques Serra, Alberto; Ribas Prats, Francesca
    International Conference on Coastal Engineering
    Presentation's date: 2010-07-04
    Presentation of work at congresses

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    The effects of the feedback between the changing coastal morphology and the wavefield on the generation and propagation of large scale (O(1-10 km)) shoreline sand waves is examined with a quasi-2D morphodynamic model. Traditional shoreline change models do not include this feedback and are only able to describe diffusion of shoreline sand waves and furthermore they are unable to describe migration. It is found with the present model that if there is a dominant littoral drift, the feedback causes downdrift migration of coastline features no matter if they grow or decay. Consistently with previous studies, simulations show that a rectilinear coastline becomes unstable and sand waves tend to grow spontaneously from random perturbations, if the wave incidence angle is larger then about 42º (θc) at the depth of closure (high angle wave instability). The initial wavelengths at which the sand waves develop are 2-3 km and this is similar to previous linear stability analysis. The implications of high angle wave instability for beach nourishments are investigated. The nourished shoreline retreats initially due to cross-shore transport because the nourished profile is steeper than the equilibrium profile. When a dominant littoral drift is present, the nourishment also migrates downdrift. If the wave angle at the depth of closure is below θc the alongshore transport contributes to the diffusion of the nourishment. However, if the angle is above θc (constant high angle wave conditions) the diffusion is reversed and the nourishment can trigger the formation of a shoreline sand wave train. Numerical experiments changing the proportion of "high angle waves" and "low angle waves" in the wave climate show that relatively small proportions of low angle waves slow down the growth of sand waves. These simulations with more realistic wave climates show shoreline sand waves that migrate downdrift maintaining more or less the same amplitude for years.

  • Assessing the suitability of video imaging for studying the Dynamics of nearshore sandbars in tideless beaches

     Ribas Prats, Francesca; Ojeda Casillas, Elena; Price, Timothy D.; Guillén Aranda, Jorge
    IEEE transactions on geoscience and remote sensing
    Date of publication: 2010
    Journal article

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  • MODELIZACIÓN Y MONITARIZACIÓN INTEGRADAS EN MORFODINAMICA DE PLAYAS NATURALES Y REGENERADAS

     Calvete Manrique, Daniel; Ribas Prats, Francesca; Caballeria Suriñach, Miquel; Garnier, Roland Charles; Guillén Aranda, Jorge; Van den Berg, Niels; Fernández Mora, Mª Àngels; Falques Serra, Alberto
    Participation in a competitive project

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    The morphodynamic responses of artificial embayed beaches to storm events  Open access

     Ojeda Casillas, Elena; Guillén Aranda, Jorge; Ribas Prats, Francesca
    Advances in geosciences
    Date of publication: 2010
    Journal article

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    The morphological changes caused by storm events in two Barcelona beaches were recorded using video monitoring techniques during the period 2001–2006. Changes in shoreline position and configuration and submerged bar position and shape were analyzed during the 25 major storm events that occurred during the study period. Beach responses to storms were grouped into three categories: shoreline advance or retreat (including rotation), sandbar migration and/or configuration change (linear or crescentic shape) and formation of megacusps. This work provides examples of the differential adaptation of both beaches to the same storm and of some unexpected morphological responses of both beaches. The response of the beach to storm events is not straightforward because wave conditions are not the only relevant parameter to be considered. In particular, in such embayed beaches it is crucial to take into account their specific morphodynamic configuration prior to the storm.

  • The role of the depth-averaged concentration in coastal morphodynamics

     Calvete Manrique, Daniel; Falques Serra, Alberto; de Swart, H.E.; Dodd, Nicholas; Ribas Prats, Francesca; Garnier, Roland Charles
    International Conference on Coastal Dynamics
    Presentation's date: 2009-09-10
    Presentation of work at congresses

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    In this contribution a discussion is presented on the development of self-organized coastal morphodynamic patterns which are due to the joint action of gradients in the depth-integrated concentration and the flow. This is done in the context of a depth-averaged shallow water model. Two physical mechanisms produce deposition-erosion patterns. Deposition either occurs where the current flows from high to low depth-averaged concentrations (1) or where the flow diverges (2). If flow conditions are quasi steady (i.e., the time scale on which bedforms evolve is much larger than the hydrodynamic time scales) only the former mechanism contributes to the formation of bottom patterns.

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    The role of surface rollers on the formation of surfzone transverse sand bars  Open access

     Ribas Prats, Francesca; de Swart, H.E.; Calvete Manrique, Daniel; Falques Serra, Alberto
    International Conference on Coastal Dynamics
    Presentation's date: 2009-09
    Presentation of work at congresses

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  • Rhythmic surf zone bars and morphodynamic self-organization

     Falques Serra, Alberto; Dodd, N; Garnier, Roland Charles; Ribas Prats, Francesca; Machardy, L C; Larroude, Philippe; Calvete Manrique, Daniel; Sancho, F
    Coastal engineering
    Date of publication: 2008-07
    Journal article

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  • Cambios morfológicos de barras sumergidas en playas artificiales

     Ojeda, E; Guillén Aranda, Jorge; Ribas Prats, Francesca
    Territoris
    Date of publication: 2008-01
    Journal article

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  • Dinámica de las barras sedimentarias en la zona de rompientes y procesos de autoorganización

     Falques Serra, Alberto; Ribas Prats, Francesca; Caballeria, M; Montoto Gayete, Amadeo
    Date of publication: 2008-05
    Book chapter

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  • Topografía rítmica en la playa de Somorrostro (Barcelona): comparación entre medidas topográficas y observaciones de vídeo

     Ortega, A; Guillén Aranda, Jorge; Ribas Prats, Francesca
    VII Congreso Geológico de España
    Presentation of work at congresses

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  • Observation and Modeling of Crescentic Bars in Barcelona Embayed Beaches

     Ribas Prats, Francesca; Garnier, Roland Charles; Ojeda, E; Falques Serra, Alberto; Guillén Aranda, Jorge; Calvete Manrique, Daniel
    Sixth International Symposium on Coastal Engineering and Science of Coastal Sediment Processes. Coastal Sediments 07
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  • Characteristics and dynamics of surfzone transverse finger bars

     Ribas Prats, Francesca; Kroon, A
    Journal of geophysical research
    Date of publication: 2007-09
    Journal article

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  • Bar and shoreline coupling in artificial embayed beaches.

     Ribas Prats, Francesca; Ojeda, E; Guillén Aranda, Jorge
    Date of publication: 2007-05
    Book chapter

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  • Cambios morfológicos de barras sumergidas en playas artificiales

     Ribas Prats, Francesca; Ojeda, E; Guillén Aranda, Jorge
    Date of publication: 2007-01
    Book chapter

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  • Comparing observed surfzone transverse finger bars with model results.

     Ribas Prats, Francesca; Calvete Manrique, Daniel; Falques Serra, Alberto; de Swart, H.E.; Kroon, A.
    Date of publication: 2007-09
    Book chapter

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  • Q2D-morfo: a medium to long term model for beach morphodynamics.

     Falques Serra, Alberto; Ribas Prats, Francesca; Garnier, Roland Charles; Ojeda, E; Guillén, F Ribas y J
    Date of publication: 2007-09
    Book chapter

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  • Observation and modelling of crescentic bars in Barcelona embayed beaches.

     Ribas Prats, Francesca; Falques Serra, Alberto; Calvete Manrique, Daniel
    Date of publication: 2007-05
    Book chapter

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  • Q2D-morfo: a medium to long term model for beach morphodynamics

     Falques Serra, Alberto; Garnier, Roland Charles; Ojeda, E; Ribas Prats, Francesca; Guillén Aranda, Jorge
    5th IAHR Symposium on River, Coastal and Estuarine Morphodynamics (RCEM 2007)
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  • Comparing observed surfzone transverse finger bars with model results

     Ribas Prats, Francesca; Calvete Manrique, Daniel; Falques Serra, Alberto; de Swart, H.E.; Kroon, A
    5th IAHR Symposium on River, Coastal and Estuarine Morphodynamics (RCEM 2007)
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  • Q2D-morfo: una alternativa a los modelos de N lineas. Aplicación a la playa de la Barceloneta.

     Falques Serra, Alberto; Ojeda, E; Ribas Prats, Francesca; Guillén Aranda, Jorge; Garnier, Roland Charles
    IX Jornadas Españolas de Ingeniería de Costas y Puertos
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  • Sensivity of modeled nearshore morphology to wave and sediment transport formulations.

     Ribas Prats, Francesca; de Swart, H.E.; Calvete Manrique, Daniel; Falques Serra, Alberto; Leeuwen, N Dodd y S M van
    Date of publication: 2006-09
    Book chapter

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  • Comparison of coastal processes and monitoring activities between some Bulgarian and Spanish beaches

     Palanques, Alberto; Ribas Prats, Francesca
    Participation in a competitive project

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  • CTM2006-08875 MORFODINÁMICA DE PLAYAS: PREDICCIONES EN LAS GRANDES ESCALAS ESPACIO-TEMPORALES

     Calvete Manrique, Daniel; Falques Serra, Alberto; Ribas Prats, Francesca
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  • Bar and shoreline coupling in artificial embayed beaches

     Ojeda, E; Guillén Aranda, Jorge; Ribas Prats, Francesca
    30th International Conference on Coastal Engineering (ICCE 2006)
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  • Generation of nearshore oblique sand bars: sensitivity to sand transport formulation

     Ribas Prats, Francesca; Falques Serra, Alberto; Vis-Star, N C
    Date of publication: 2005-06
    Book chapter

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  • Sensitivity of Modeled Nearshore Morphology to Wave and Sediment Transport Formulations

     Ribas Prats, Francesca; de Swart, H.E.; Calvete Manrique, Daniel; Falques Serra, Alberto; Dodd, N; Leeuwen, S M Van
    Coastal Dynamics International Conference. Coastal Dynamics
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  • Observaciones de barras de arena transversales en la playa de Noordwijk (Holanda) con el sistema de cámaras Argus

     Ribas Prats, Francesca
    VIII Jornadas Españolas de Ingeniería de Costas y Puertos
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  • On the growth of nearshore sand bars as instability processes of equilibrium beach states  Open access

     Ribas Prats, Francesca
    Defense's date: 2004-01-14
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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    La zona de rompents de les platges està governada er una gran quantitat de processos complexes altament no lineals i a diverses escales de longitud i de temps. Tot i això, algunes vegades s'hi poden trobar estructures morfològiques regulars, com ara punts cuspidals, barres de sorra uniformes en la direcció longitudinal, barres crescèntiques i sistemes rítmics de barres obliqües. La seva regularitat indica que la dinàmica complexa de la zona de rompents a gran escala pot ser explicable en termes de mecanismes físics simples en certes circumstancies. L'origen les propietats dinàmiques d'aquestes curioses estructures morfològiques encara són un problema obert, tot i que la comunitat científica hi està interessada des de fa dècades. En particular, escriure el comportament de les barres de sorra a la zona de rompents és interessant tant des d'un punt de vista científic, com per a contrastar les fórmules de transport de sediment que s'utilitzen actualment en enginyeria de costes.L'objectiu principal d'aquesta tesi és l'estudi teòric d'alguns mecanismes físics que podrien ser responsables de la forma dels perfils d'equilibri de les platges i de l'origen i la dinàmica de les barres de sorra a la zona de rompents (en concret, les barres longitudinals i els sistemes rítmics de barres obliqües). Es pretén així omplir alguns dels buits de coneixement actuals sobre aquestes formes topogràfiques. La principal hipòtesi de treball és que els dos tipus de barres es poden formar per -processos d'auto-organització'. El procediment consisteix en realitzar una anàlisi d'estabilitat per a cada tipus de barra, seguint els passos següents. Primerament es busca un estat d'equilibri rellevant del sistema sense les formes morfològiques. Aleshores s'afegeix una pertorbació a la topografia i s'estudien els efectes que produeix en la hidrodinàmica i en el transport de sediment. Si el transport resultant reforça la pertorbacióo inicial, s'obté una reacció de retro-alimentació positiva' i les barres creixen. Això és el que s'anomena un -procés 'auto-organització' del sistema o un -mecanisme d'inestabilitat' i podria ser una explicació de l'aparició d'aquestes estructures morfològiques complexes, sense estar associades a cap patró regular previ en la hidrodinàmica.El primer model teòric que es presenta en aquesta tesi s'ha construït per a descriure la forma dels perfils d'equilibri i la formació i migració de barres longitudinals, assumint uniformitat en la direcció longitudinal. El model està basat en una equació d'evolució de l'onatge acoblada a una innovadora formula pel transport de sediment transversal. La primera equació descriu la transformació i posterior ruptura d'onades d'alçada aleatòria i incidència perpendicular. La formula pel transport conte tres termes sumats: el transport cap a la costa a causa de les propietats no lineals de les onades, el transport cap al mar produït pels corrents de retorn i el transport gravitacional pendent avall. Aquests tres processos són la base d'un mecanisme físic d'interacció entre la barra i el punt de ruptura', que podria ser l'explicació pel creixement de barres longitudinals de sorra. Al capítol 3 de la tesi es descriuen els resultats d'aquest model quan s'imposen condicions d'equilibri. Els perfils que s'obtenen són similars als de les platges naturals i consisteixen en una terrassa plana i amb poc pendent dins la zona de rompents i una part còncava més inclinada més enllà del punt de ruptura. Quan el sediment és gruixut i les onades incidents són de freqüència baixa (situació típica de platges reflectives), els perfils tenen força pendent i una zona de rompents estreta. Quan la sorra és més fina i les onades són d'alta freqüència (platges més dissipatives), la zona de rompents és molt més ampla i plana i el pendent és menor.Al capítol 4 es presenta una anàlisi complerta de l'estabilitat d'aquests perfils d'equilibri respecte pertorbacions uniformes longitudinalment. Els dos objectius principals són trobar el rang de paràmetres en el qual els estats d'equilibri són estables i posar a prova la hipòtesi de si les barres longitudinals poden aparèixer com a inestabilitats del sistema. Els resultats demostren que tots els perfils d'equilibri obtinguts són estables, tant en el règim lineal com en el no lineal. Per tant, les platges amb terrassa semblen ser forts atractors dinàmics del sistema i caldria estudiar amb més atenció la seva freqüent presencia a les platges naturals.Els resultats del model lineal indiquen que una -inestabilitat de tipus delta de Dirac' podria sorgir a prop del punt de ruptura, però el seu creixement queda sempre inhibit pel transport gravitacional produït pels forts pendents associats. Les evolucions temporals no lineals realitzades posteriorment també tendeixen a estats finals que corresponen als perfils d'equilibri sense barres. Fins i tot començant des d'estats inicials força allunyats de l'equilibri i permetent una possible migració de la línia de costa, aquesta versió idealitzada del mecanisme d'-interacció entre la barra i el punt de ruptura' només ha permès descriure platges amb terrassa. Per tant, verificar quantitativament que el creixement de barres longitudinals pot ser degut a aquest mecanisme encara és un important problema obert de la física del litoral.Finalment, el segon model presentat al capítol 5 intenta reproduir la formació de sistemes rítmics de barres obliqües, partint dels mateixos perfils d'equilibri sense barres perµo ara permetent l'aparició d'inestabilitats no uniformes en la direcció longitudinal. El possible acoblament entre les estructures hidrodinàmiques en el pla horitzontal i la topografia emergent s'examina pel cas d'incidència obliqua de l'onatge. S'utilitza una fórmula clàssica de transport de sediment proporcional a diferents potencies del corrent mitja i amb una certa influència de les onades. En aquest cas, es demostra que pot existir retro-alimentació positiva' i que els diferents mecanismes físics descrits poden explicar el creixement inicial de diversos tipus de sistemes rítmics com els que s'observen en les platges naturals. Els resultats depenen principalment del tipus de transport de sediment dominant i de l'angle d'incidència de les onades. En el cas d'angles d'incidència relativament grans i transport dominat pels corrents mitjans, s'obtenen -barres orientades a favor del corrent'. Això significa que el costat de mar de les barres està desplaçat corrent avall respecte el costat de terra. La orientació és molt obliqua, o sigui que les barres són gairebé paral·leles a la línia de costa. La seva longitud d'ona és de diverses vegades l'amplada de la zona de rompents.En cas contrari, quan els corrents mitjans són febles comparats amb la velocitat orbital de les onades, poden créixer -barres orientades a contra-corrent' o bé -barres crescèntiques orientades a favor del corrent', depenent de l'angle d'incidència i de la influencia de les ones infragravitatòries en el transport. En tots dos casos, l'espaiat és de l'ordre de l'amplada de la zona de rompents. El temps típic de creixement oscil·la entre diverses hores i un parell de dies i les barres migren a favor del corrent amb velocitats fins a desenes de metres per dia.Les condicions que afavoreixen la formació d'aquests sistemes rítmics de barres són onatge regular i estats morfodinàmics intermitjos (situacions ni molt dissipatives ni molt reflexives).

    of the thesisIn spite of the complex behaviour in space and time of the surf zone dynamics, relatively regular morphological patterns dominate quite often the beach topography at length and time scales well above those of incident waves. Well known examples are giant beach cusps, shore-parallel bars, crescentic longshore bars and shore-attached transverse/oblique bar systems. Their regularity indicates that the large scale complex dynamics of the surf zone as a whole can be understood in terms of simple physical mechanisms, at least in some circumstances. Remarkably, after decades of research, no model has been widely accepted to explain the origin and migration of these intriguing large-scale morphological patterns.Understanding the behaviour of nearshore sand bars is not only challenging from a scientific point of view but also very interesting for testing ediment transport formulations with engineering purposes.The main goal of this thesis is to investigate theoretically some physical processes that can be responsible for the shape of equilibrium profiles and the origin and dynamics of nearshore sand bars (in particular, shore-parallel bars and alongshore rhythmic systems of oblique bars are studied). This may fill some of the existing gaps of our current knowledge about these topographic features. The main working hypothesis is that these bars stem from -free instabilities' of the morphodynamical system. A stability analysis is performed for each type of bar system, starting from a steady equilibrium configuration of the beach without the pattern. A small topographic perturbation is then assumed and its effects on the hydrodynamics and on the sediment transport are investigated. If the transport pattern reinforces the topographic perturbation, a -positive feedback' occurs between the topography and the flow. This results in what is called a -free instability' of the system or a -self-organization process'. It can provide an explanation for the emergence of morphological patterns not associated to any previous regular template in the hydrodynamics.The first model is focused on describing the shape of equilibrium beach profiles and the growth and migration of shore-parallel bars, assuming longshore uniformity. A wave transformation equation, describing the shoaling and breaking processes of normally incident random waves, is coupled with an innovatory cross-shore sediment transport formula containing three terms: an onshore contribution due to non-linear wave properties, an offshore contribution due to undertow currents and a term accounting for the downslope gravitational effect. These three latter processes are the basis of the so-called -breakpoint-bar interaction', which has been claimed to be an explanation for the formation of shore-parallel bars. In chapter 3 of the present thesis, this model is used to predict equilibrium beach profiles, which resemble natural non-barred beaches. In case of very dissipative conditions (storm weather and fine sediment), profiles consist of a gently sloping terraced surf zone and a concave-up shoaling zone. For less dissipative conditions, the entire profile gradually becomes more planar with similar surf zone and shoaling zone slopes.Chapter 4 presents a complete stability analysis of such equilibrium profies with respect to arbitrary cross-shore perturbations. The aim is not only finding the range of parameter values leading to stable equilibrium profiles, but also testing whether shore-parallel bars can stem as free instabilities of the system. Results demonstrate that the equilibrium profiles are always stable, both in the linear and in the non-linear regimes. Therefore, the terraced non-barred profiles seem to be a strong attractor of this dynamical system and more attention should be paid to their frequent occurrence on natural beaches. The results of the linear stability analysis indicate that a -Dirac delta instability' tries to emerge at the effective break-point of the equilibrium profiles. However, its growth is always inhibited by the downslope transport that is induced by the inherent infinite slopes. The non-linear temporal evolutions subsequently performed also tend to final states that correspond to the equilibrium non-barred profiles. Even starting from initial states moderately far from equilibrium and allowing for a potential shoreline migration, this idealised version of the breakpoint-bar interaction is only able to reproduce the formation of terraces. Therefore, verifying quantitatively that this interaction can be responsible for the growth of natural shore-parallel bars still remains as an important open question in the nearshore sciences.Finally, the second model presented in chapter 5 aims at reproducing the formation of alongshore rhythmic systems of oblique sand bars, starting from the same equilibrium non-barred profiles but now allowing for alongshore non-uniformities. The potential coupling between the time- and depth-averaged hydrodynamics and the evolving rhythmic topography is examined in case of oblique wave incidence. The used classical sediment transport law is proportional to a power of the depth-averaged current and contains some influence of the waves. In this case, it is shown that -positive feedback' can occur and the different studied underlying physical mechanisms lead to the initial growth of several types of oblique bars.Results mainly depend on the sediment transport conditions and the wave incidence angle.In qualitative agreement with available field observations, for moderately large incidence angles and transport dominated by the mean currents, the emerging bars are -oriented down-current'. This means that their offshore end is shifted down-stream of the equilibrium longshore current with respect to their shore attachment. Their orientation is very oblique, that is to say that the bars are nearly shore-parallel. The wave lengths are of the order of several times the surf zone width. In the opposite situation, when mean currents are weak compared with wave orbital motions, either -up-current oriented bars' or -crescentic/down-current oriented bars' can occur. This depends on the wave incidence angle and on the influence of infragravity waves into the transport. In both cases, the wave lengths are similar to the surf zone width. The e-folding growth times of all these patterns range from a few hours to a few days and they migrate down-flow with velocities that can be up to some few meters per day. The conditions favouring the generation of these rhythmic bar systems are steady waves and intermediate beach states, in between the fully dissipative and the fully reflective situations.

  • Modelling the dynamics of beach profiles, bars and ridges near uninterrupted coasts

     Falques Serra, Alberto; Calvete Manrique, Daniel; Ribas Prats, Francesca; de Swart, H.E.
    Date of publication: 2004-09
    Book chapter

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  • Generation of nearshore oblique sand bars: sensitivity to sand transport formulation

     Ribas Prats, Francesca; Vis-Star, N V; de Swart, H.E.; Falques Serra, Alberto
    Coastal Engineering 2004 (ASCE)
    Presentation of work at congresses

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