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    Vulnerability of sandy coasts to climate variability  Open access

     Idier, Deborah; Castelle, Bruno; Poumadère, Marc; Balouin, Yann; Bohn Bertoldo, Raquel; Bouchette, Fréderic; Boulahya, Faiza; Brivois, Olivier; Calvete Manrique, Daniel; Capo, Sylvain; Certain, Raphael; Charles, Elodie; Chateauminois, Eric; Delvallée, Etienne; Falques Serra, Alberto; Fattal, Paul; Garcin, Manuel; Garnier, Roland Charles; Héquette, Arnaud; Larroudé, Philippe; Lecacheux, Sophie; Le Cozannet, Gonéri; Maanan, Mohamed; Mallet, Cyril; Maspataud, Aurélie; Oliveros, Carlos; Paillart, Martin; Parisot, Jean-Paul; Pedreros, Rodrigo; Robin, Nicolas; Robin, Marc; Romieu, Emmanuel; Ruz, Marie-Hélène; Thiebot, Jerome; Vinchon, Charlotte
    Climate research
    Date of publication: 2013-06-13
    Journal article

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    The main objective of the VULSACO (VULnerability of SAndy COasts to climate change and anthropic pressure) project was to investigate present day and potential future vulnerability of sandy coasts at the 2030 horizon, i.e. on a time scale related to climate variability. The method, based on a multidisciplinary approach bringing together geologists, geographers, physicists, social psychologists, engineers and stakeholders, was structured around 4 axes: field data analysis; numerical modelling; analysis of governance and stakeholder perceptions; and development of vulnerability indexes. This approach was designed to investigate vulnerability at a local scale and was applied to 4 contrasting beaches located in France: Sète Lido (Mediterranean Sea), Truc Vert and La Tresson beaches (Atlantic Ocean), and Dewulf (English Channel). The results focus on decadal and multi-annual beach trends at the Truc Vert beach site. There is almost no trend in beach volume at Truc Vert beach, although there is a variation in this parameter on a cycle of 2 to 3 yr, with variations related to wave energy and probably to indexes of climate variability. Numerical modelling identified the sensitivity of beach responses to changes in wave height and direction, especially in terms of subtidal morphology and the potential development of shoreline instability. Together with the observed offshore wave angle at the Biscay Buoy, these model results suggest that a potential change in wave angle due to climate variability could significantly modify the bars¿ morphology. The combination of data analysis and numerical modelling contributed to the development of vulnerability indexes designed for sandy coasts, which take into account climate-dependant variables such as waves. This allowed the differentiation of the sites in terms of vulnerability to erosion: Sète Lido and Truc Vert beach were the most and least vulnerable sites, respectively.

    The main objective of the VULSACO (VULnerability of SAndy COasts to climate change and anthropic pressure) project was to investigate present day and potential future vulnerability of sandy coasts at the 2030 horizon, i.e. on a time scale related to climate variability. The method, based on a multidisciplinary approach bringing together geologists, geographers, physicists, social psychologists, engineers and stakeholders, was structured around 4 axes: field data analysis; numerical modelling; analysis of governance and stakeholder perceptions; and development of vulnerability indexes. This approach was designed to investigate vulnerability at a local scale and was applied to 4 contrasting beaches located in France: Sète Lido (Mediterranean Sea), Truc Vert and La Tresson beaches (Atlantic Ocean), and Dewulf (English Channel). The results focus on decadal and multi-annual beach trends at the Truc Vert beach site. There is almost no trend in beach volume at Truc Vert beach, although there is a variation in this parameter on a cycle of 2 to 3 yr, with variations related to wave energy and probably to indexes of climate variability. Numerical modelling identified the sensitivity of beach responses to changes in wave height and direction, especially in terms of subtidal morphology and the potential development of shoreline instability. Together with the observed offshore wave angle at the Biscay Buoy, these model results suggest that a potential change in wave angle due to climate variability could significantly modify the bars’ morphology. The combination of data analysis and numerical modelling contributed to the development of vulnerability indexes designed for sandy coasts, which take into account climate-dependant variables such as waves. This allowed the differentiation of the sites in terms of vulnerability to erosion: Sète Lido and Truc Vert beach were the most and least vulnerable sites, respectively. These indexes help in identifying the dominant components of beach vulnerability, and provide potential for the study of how anthropogenic factors affect vulnerability. The study of stakeholder perceptions and decision-making with regard to climate-related risk also highlighted potential anthropogenic effects on beach vulnerability, and identified possible site-specific outcomes.

  • 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.

  • Experimental and theoretical analysis of flow and sediment transport in 90-degree fluvial diversions.  Open access

     Herrero Casas, Albert
    Defense's date: 2013-06-07
    Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de Catalunya
    Theses

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    Fluvial bifurcations are systems that have a great influence on human society. Their formation and evolution can affect to activities as agriculture or industry and a good knowledge of the mechanisms that affect the processes involved can be determinant for a proper land management. From a geometrical point of view, diversions or lateral intakes are a particular case of fluvial bifurcations in which one of the downstream branches exits laterally while the other follows the same direction as the incoming channel. Diversions are characterized by a significant trend of sediment to deviate towards the lateral branch. The first part of this document describes the experimental work performed in the Fluvial Morphodynamic Laboratory in BarcelonaTech University. Water and solid discharge distributions, as well as bed and water surface levels are measured along the model, during tests with different total water discharges. Results confirm the sediment distributions reported in the literature and but also highlight the influence of bed bathymetry evolution and three-dimensional flow patterns in the local sediment transport of the diversion area. A threshold value is found for total discharge, above which a vortex is formed in front of the diversion entrance and a scour hole is formed in the first stretch of the main channel downstream of the diversion. An energy balance equation is proposed between upstream and downstream of the diversion taking into account the energy losses associated with the vortex, and a good comparison is found when tested against the experimental results. A nodal equation that relates water discharge to sediment discharge distribution is developed considering separately bed load and suspended sediment transport. Calculated values of sediment discharge ratio compare reasonably well with experimental values. Joining these equations, uniform flow, transport capacity and mass continuity equations for water and sediment, a theoretical model is proposed to evaluate the equilibrium situation of a fluvial diversion in terms of water discharge, sediment discharge, water depth and bed slope in both downstream branches.

    Les bifurcacions fluvials son sistemes amb una gran influència a la societat. La seva formació i evolució pot afectar a activitats com l’agricultura i la indústria, i un bon coneixement dels mecanismes que afecten els processos involucrats pot ser determinant per una correcta gestió del territori. Des d’un punt de vis ta geom ètric, les derivacions o pres es d’aigua laterals són un cas particular de les bifurcacions fluvials , en el qual un dels ramals s urt lateralment i l’altre s egueix la mateixa direcció que el canal entrant. Les derivacions es caracteritzen per una tendència significativa del sediment a ser desviat cap al ramal lateral. La primera part d’aquest document descriu el treball experimental portat a terme al Laboratori de Morfodinàmica Fluvial de la Universitat Politècnica de Catalunya. S’han realitzat experiments amb diferents cabals totals d’aigua, durant els quals s’han pres mesures de la distribució de cabal líquid i sòlid, així com dels nivells del fons i la superfície de l’aigua al llarg de tot el model. Els resultats confirmen el desequilibri de cabals s òlids reportat en estudis anteriors, però també posen de manifest la influència de l’evolució de la batimetria i les característiques tridimensionals del flux en la dinàmica local del transport dels ediments . S’ha trobat un valor límit pel cabal líquid total, per sobre del qual es genera un vòrtex davant de l’entrada del ramal lateral i es form a un fos s at d’eros ió al tram inicial del ramal recte, aigües avall de la derivació. Es proposa una equació de balanç d’energia entre aigües amunt i aigües avall de la derivació tenint en compte les pèrdues d’energia associades al vòrtex, que reprodueix de forma bastant bona els resultats experimentals. Es desenvolupa una equació nodal que relaciona les distribucions de cabal líquid i sòlid, considerant per separat el transport de fons i el transport en suspensió. Els valors del rati de cabal sòlid calculat es corresponen raonablement bé amb els obtinguts experimentalment. Unint aquestes equacions amb les de flux uniforme i transport s òlid a cada ramal, i continuïtat de massa per aigua i sediment, es proposa un model teòric per avaluar la situació d’equilibri d’una derivació fluvial en termes de cabal líquid, cabal sòlid, calat d’aigua i pendent de la llera en cadas cun dels ramals d’aigües avall.

  • PATRONES MORFODINAMICOS COSTEROS: MODELOS, OBSERVACIONES Y APLICACIONES

     Falques Serra, Alberto
    Participation in a competitive project

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    Intertidal finger bars at El Puntal spit, bay of Santander, Spain  Open access

     Garnier, Roland Charles; Medina, Raúl; Pellón, Erica; Falques Serra, Alberto; Turki, Imen
    International Conference on Coastal Engineering
    Presentation's date: 2012-07-05
    Presentation of work at congresses

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    We indentify the presence of an intertidal finger bar system in the swell-protected beaches of El Puntal Spit (Bay of Santander, Spain). The Horus video monitoring system allows us to perform a 2 year survey (June 2008- June 2010). We found that the bar system persists during the survey period with a wavelength of about 25 m and an oblique orientation with respect to the shore. Moreover, the analysis of the position of a particular bar (Bar 10) shows a movement characterized by a net migration to the west and a faster oscillation to the east or the west. Preliminary climate data analysis and morphological model results show that tide seems to be a forcing in the motion of the bars but also wind and waves generated by the wind probably affects the bar dynamics.

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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.

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  • The influence of wave direction on the morphological response of a double sandbar system

     Thiebot, Jerome; Idier, Deborah; Garnier, Roland Charles; Falques Serra, Alberto; Ruessink, Gerben
    Continental shelf research
    Date of publication: 2012-01-01
    Journal article

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    Nearshore sandbars are often characterized by three-dimensional bed patterns. To analyze the influence of wave direction on the morphological response of a double sandbar system, this paper uses the 2DH nonlinear surf zone model MORFO55. Depending on wave direction, different morphologies emerge as free instabilities. These morphological responses differ in terms of geometry (shape of the alongshore rhythmic patterns) and temporal evolution. Nearly shore-normal waves favor the emergence of crescentic patterns along both the inner and outer bars. These instabilities arise from “bedsurf” coupling, which is the positive feedback between the bed perturbations and the wave-breaking patterns resulting in rip-cell circulations. The system is stable for intermediate wave incidence angles because the longshore current shifts the circulation cells alongshore, inhibiting the growth of initial perturbations. For larger angles, the system is again unstable. Two types of oblique bars emerge. The first type forms between the crest of the inner bar and the shoreline and is oriented down-current. The second type is superimposed on the two bars and is oriented up-current. Although previous studies attribute the formation of oblique bars to the deflection of the longshore current (that is a cross-shore process), we show here that this mechanism contributes to the bar formation but the growth rate is mainly governed by a longshore process. Specifically, it is the positive feedback between the bathymetry and the longshore gradient of the sediment concentration. Finally, interactions between the patterns in the two shore-parallel bars are analyzed. In the observed configurations, the influence is always one way as the inner-bar dynamics never influence those of the outer bar. The outer-bar instabilities cause undulations in the inner bar when the outer-bar instabilities grow faster than those of the inner bar.

  • 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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  • Beach inundation and morphological changes during storms using video monitoring techniques  Open access

     Sancho García, Amanda
    Defense's date: 2012-11-16
    Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de Catalunya
    Theses

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    The predictability of beach inundation is mainly related to wave runup and most of the formulations that have been developed focus on the hydrodynamics, disregarding the morphological changes caused during storms. For these reasons, the aim of this thesis is to provide more insight into beach inundation processes at embayed and open beaches and to determine how morphological changes can interfere with these processes. To this end, video measurements of beach inundation and the characteristic morphological changes were carried out at two stretches of coast with different wave climates: the stretch comprising La Barceloneta, Somorrostro and Nova Icaria, which are three artificial, tideless embayed beaches located in Barcelona, Spain (NW Mediterranean); and Noordwijk beach, which is an open, microtidal multibarred beach located in Noordwijk, the Netherlands (North Sea). The effects of a submerged and a detached breakwaters on the morphological and hydrodynamic changes occurring at La Barceloneta during storms is examined in chapter 2. The shoreline response before and after the beach nourishment and the construction of the protective structures was compared using a ten-year video-recorded dataset and hydrodynamics modelled using the SMC model. As a result of the protection works, La Barceloneta was divided into two beaches separated by a salient. A new methodological approach to analyzing beach rotation which eliminates the morphological effect is presented. Results indicate that the beach rotation process has been modified caused by a change in the wave-induced current system from a single dominant alongshore current to one composed of two dominant alongshore currents with opposite directions. In chapter 3, beach inundation affecting the three embayed beaches of Barcelona during the 17 strongest storm events of the period 2001-2008 is analyzed using daily time-exposure images. The shoreline variability due to storms was split into beach planform and morphological features in order to determine its influence on beach inundation measurements. The characterization of the inundation depended on the orientation with respect to the wave direction approach and the morphological features. Beach planform changes are the foremost influence on the inundation of Barcelona beaches. The inundation at the multibarred beach of Noordwijk during the seven strongest storms in the period between 1998 and 2005 is estimated, also using video monitoring techniques in chapter 4. Additionally, the influence of subtidal sandbars on the inundation is analyzed using the XBeach model. To this end, six simulations were carried out using barred profiles measured at Noordwijk but differing in sandbar height and location, and one simulation using a synthetic barless profile. Inundation values ranged from 22 to 105 m, with considerable alongshore variation before the peak of each storm because of the presence of the intertidal bars. The mean inundation values along the beach are well estimated using a simple inundation parameter. The XBeach model shows that the inundation is only affected by the morphology close to the shoreline (i.e. by the intertidal bars or the inner bar if it is wide and closer to shoreline). The outer bar does not seem to influence Beach inundation prediction at Somorrostro beach is evaluated in chapter 5. To this end, inundation measurements using video observations are compared with estimations including the tidal variations and the wave runup formulation of Stockdon et. al (2006) introducing deep water and local wave measurements and computations. The inundation is overestimated if any of the wave heights in the formulation are used. Estimations improved if a local wave height is used, in particular for waves approaching the shore obliquely. Finally, the alongshore variability of the inundation is better captured if the wave runup is assumed proportional to the breaking wave heightBARCELONA07

    Las costas están expuestas a la erosión y la inundación producida por los temporales, los cuales son muy frecuentes y pueden producir grandes daños y pérdidas económicas. La inundación producida por los temporales es debida a la marea astronómica y meteorológica y al remonte del oleaje. La predicción de la inundación de la playa está principalmente relacionada con el remonte y la mayoría de las formulaciones propuestas se centran en parámetros hidrodinámicos sin tener en cuenta los cambios morfológicos causados durante los temporales. Por todo ello, el principal objetivo de esta tesis es proporcionar un mayor conocimiento de los procesos de inundación en playas encajadas y abiertas y determinar cómo los cambios morfológicos pueden interferir con estos procesos. Para ello, se han llevado a cabo medidas de la inundación y de los cambios morfológicos característicos utilizando imágenes de video en dos tramos de costa con climas de oleaje diferente: el tramo que comprende las playas de La Barceloneta, Somorrostro and Nova Icaria, las cuales son tres playas artificiales, encajadas y sin marea localizadas en Barcelona, España (NO Mediterráneo); y la playa de Noordwijk, la cual es una playa abierta, multibarrada y micromareal localizada en Noordwijk, Holanda (Mar del Norte). Los efectos de un dique sumergido y un dique exento en los cambios morfológicos e hidrodinámicos ocurridos en la playa de La Barceloneta durante temporales se examinan en el capítulo 2. La respuesta de la línea de orilla antes y después de la regeneración de la playa y de la construcción de las estructuras de protección se compara utilizando 10 años de video imágenes y la hidrodinámica modelada utilizando el Sistema de Modelado Costero (SMC). Como resultado de estas obras de protección, La Barceloneta quedó dividida en dos playas independientes separadas por un saliente. Una nueva metodología para analizar la rotación de la playa que elimina los cambios morfológicos es propuesta. Los resultados indican que la tendencia erosiva previamente observada en la zona noreste de la playa sigue estando presente en la actual playa noreste y está relacionada con el nuevo dique sumergido. Además, el proceso de rotación de la playa se ha modificado, produciéndose solamente en sentido contrario a las agujas del reloj en la playa noreste y en sentido horario en la playa suroeste. Este nuevo comportamiento es debido al cambio en el patrón de corrientes previamente formado por una corriente longitudinal dominante, a un sistema de corrientes compuesto por dos corrientes longitudinales en direcciones opuestas. En el capítulo 3, la inundación producida en las tres playas encajadas de Barcelona durante los 17 temporales más energéticos del periodo 2001-2008 es analizada utilizando imágenes diarias promediadas. La variabilidad de la línea de orilla debida a los temporales es discriminada en cambios en la forma en planta y morfologías (cúspides de playa, megacúspides, ondulaciones y saliente) para determinar su influencia en las medidas de inundación de la playa. La caracterización de la inundación dependió de la orientación respecto de la dirección de aproximación del oleaje y de las morfologías. Los cambios en la forma en planta de la playa son la máxima influencia en la inundación de las playas de Barcelona. Los cambios en la forma en planta hacia tierra y los cambios en el saliente suponen casi un 50% de la máxima inundación medida, mientras que las megacúspides y la ondulación suponen aproximadamente el 25%. El efecto de las cúspides de playa en la inundación es despreciable. Consecuentemente, la variación en la línea de orilla durante temporales influye significativamente los valores de inundación, especialmente en playas con pendientes fuertes. Pequeñas variaciones en la pendiente de la playa pueden sin embargo afectar sustancialmente la inundación en playas con pendientes suaves. La inundación de la playa multibarrada de Noordwijk durante los 7 temporales más fuertes ocurridos en el periodo comprendido entre 1998 y 2005 es también estimada utilizando medidas de video monitorización en el capítulo 4. Además, la influencia de las barras submareales en la inundación es analizada utilizando el modelo XBeach. Para ello, se realizaron siete simulaciones 1-D sin considerar los cambios morfológicos; seis simulaciones utilizando perfiles barrados medidos en la playa de Noordwijk pero que difieren en altura y localización de las barras, y una simulación utilizando un perfil ideal sin barras. Los valores de inundación oscilaron entre 22 y 105 m, con variaciones considerables a lo largo de la playa antes del pico del temporal debido a la presencia de barras intermareales. La inundación promedio a lo largo de la playa es estimada considerablemente bien utilizando un parámetro de inundación sencillo que incluye la pendiente intermareal y supramareal, la altura de ola y la longitud de onda en aguas profundas y la marea meteorológica. El modelo XBeach muestra que la inundación está solamente afectada por la morfología próxima a la línea de orilla, esto es, por las barras intermareales o por la barra interna si es ancha y próxima a la orilla. La barra externa no parece tener influencia en el comportamiento de la inundación. La predicción de la inundación en la playa de Somorrostro es evaluada en el capítulo 5. Para ello, medidas de la inundación utilizando observaciones de imágenes de video se comparan con estimaciones de la inundación incluyendo las variaciones de la marea y el fórmula del remonte de Stockdon et al. (2006), introduciendo medidas locales y en aguas profundas del oleaje así como oleaje modelado. Debido a que las observaciones corresponden a un remonte medio y las estimaciones usan el remonte excedido un 2% (R2%), la inundación es sobreestimada para cualquier altura de ola utilizada en la fórmula. Sin embargo, las estimaciones mejoran si una altura de ola medida a 10 m es utilizada en la fórmula en particular para oleajes con una aproximación oblicua a la línea de orilla. Finalmente, las diferencias entre las observaciones y las estimaciones varían a lo largo de la playa, siendo mayores en la zona de curvatura de la playa. La variabilidad de la inundación a lo largo de la playa es mejor caracterizada si el remonte se asume igual que la altura de ola en rotura.

  • 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.

  • Potential instabilities of Catalan coastline induced by high-angle waves

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

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    The relatively long sandy beaches of El Maresme, and the bimodal wave climate of the Catalan coast, with dominant E-ENE and SSW waves that leads to high angle of incidence, propitiate good conditions for high angle of wave instability. Subtle undulations of the shoreline and the more intense undulations of the bathymetric contour line of 5 meters depth, with wavelengths of about 1300 m, can be found at that stretch of the coast. A morphodynamic model has been used to test if such undulations could be generated by high angle wave instability. Model results, for wave period of 4 s, show that at El Maresme coast high angle wave instability may develop with time rate of about 1 year and with dominant wavelength that ranges from 600 to 1400 m. For the wave climate of the Catalan coast, wave heights of 0.5 – 1 m and a mean peak period of 5.6 s, and at the steep beaches of El Mareme, it has been found that the wavelength of the instability is in good agreement with the observed undulations, and depends in a sensitive way of the mean slope of the bathymetric profile, and on the length of the bathymetric perturbation.

    Postprint (author’s final draft)

  • 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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  • Shoreline instability under low-angle wave incidence

     Idier, Deborah; Falques Serra, Alberto; Ruessink, Gerben; Garnier, Roland Charles
    Journal of geophysical research
    Date of publication: 2011-12
    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.

  • Modeling sandbar morphodynamics by linear stability analysis: application to the Lido de Sete beach

     Thiebot, Jerome; Idier, Deborah; Falques Serra, Alberto; Calvete Manrique, Daniel; Certain, Raphael; Garnier, Roland Charles
    Houille blanche
    Date of publication: 2010
    Journal article

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    Mechanisms controlling crescentic bar amplitude  Open access

     Garnier, Roland Charles; Dodd, Nicholas; Falques Serra, Alberto; Calvete Manrique, Daniel
    Journal of geophysical research
    Date of publication: 2010-04-15
    Journal article

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    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.

  • Evolución de remolinos mesoescalares a partir de derivadores y un modelo de advección-difusión de velocidad angular

     AULADELL MESTRE, MARICEL
    Defense's date: 2010-12-14
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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

     Garnier, Roland Charles; Ortega Sánchez, Miguel; Losada ., Miguel Angel; Falques Serra, Alberto; Dodd, Nicholas
    Scientia marina
    Date of publication: 2010-09
    Journal article

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    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.

  • 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.

  • A mechanism inhibiting rip channel formation for oblique waves

     Garnier, Roland Charles; Dodd, Nicholas; Falques Serra, Alberto; Calvete Manrique, Daniel
    International Conference on Coastal Dynamics
    Presentation's date: 2009-09-10
    Presentation of work at congresses

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    Previous numerical modelling studies based on 2DH morphodynamical model show that oblique waves tend to inhibit the formation of rip channel systems, but the mechanisms were not investigated. Field observations do not always agree with this model result, thus, understanding the mechanisms seems essential. To this end, the global analysis technique, originally developed to describe the long term behavior of bars (saturation of the bar growth), is also applied here to the initial stage of the bar evolution (formation of the bars). As a result, rip channels grow slower for larger wave angle because of the weakening of the instability mechanism -that only depends on the cross-shore current- rather than the increase of the damping due to the diffusive bedslope transport.

  • Coastline sand waves on a low-energy beach at El Puntal spit, Spain: linear stability analysis

     Medellín, Gabriela; Falques Serra, Alberto; Medina, Raul; González, Mauricio
    Journal of geophysical research
    Date of publication: 2009-03
    Journal article

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  • Linear stability ananlysis in coastal morphodynamics: essential or useless?

     Falques Serra, Alberto
    Participation in a competitive project

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

     Garnier, Roland Charles; Calvete Manrique, Daniel; Falques Serra, Alberto; Dodd, N
    Journal of geophysical research
    Date of publication: 2008-07
    Journal article

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    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.

  • Coastline sand waves on a low-energy beach at "El Puntal" spit, Spain

     Medellin, G; Medina, R; Falques Serra, Alberto; Gonzalez, M
    Marine geology
    Date of publication: 2008-05
    Journal article

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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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  • Modelling the formation and the nonlinear evolution of crescentic bars of the Aquitanian coast

     Garnier, Roland Charles; Bonneton, P; Falques Serra, Alberto; Calvete Manrique, Daniel
    Houille blanche
    Date of publication: 2008-07
    Journal article

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    Crescentic bars emerge as free instabilities of the coupling between topography and water motion. Their long term behaviour will be studied in the real case of the French Aquitaine beaches by using the 2DH numerical model MORFO55. For the first time, the equilibrium state of a crescentic bar system is obtained, in the case of steady incident wave conditions. Due to non linear interactions, the final wave length of the system does not inevitably increase with the incident wave height. Finally, the study of variable incident wave conditions suggests that the equilibrium state may depend on the initial conditions.

  • waves on a Low-Energy Beach at el 'Puntal' Spit, Spain: Linear Stability Analysis

     Medellín-Mayoral, G; Falques Serra, Alberto; Medina, R; González, M
    Journal of geophysical research
    Date of publication: 2008-01
    Journal article

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  • Multiscale techniques in turbulence: application to geophysical fluids and operational oceanography  Open access

     NIEVES CALATRAVA, VERONICA URSULA
    Defense's date: 2008-11-25
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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  • The role of cross-shore profile dynamics on shoreline instability due to high-angle waves

     Falques Serra, Alberto; van den Berg, Niels; Calvete Manrique, Daniel
    International Conference on Coastal Engineering
    Presentation's date: 2008-09-05
    Presentation of work at congresses

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    If the wave climate is dominated by a wave approach in deep water that is very oblique with respect to the shoreline, the littoral drift may render the rectilinear trend of the shoreline unstable. This instability causes large scale shoreline sand waves (L ~ 1-10 km, T~ 1-10 yr) that grow and propagate along the coast, implying erosional hot spots. This was known to be primarily due to gradients in alongshore transport rate. However, it is here shown that the cross-shore transport is also essential to the instability, the faster the relaxation to the equilibrium profile, the stronger the instability. Furthermore, it is found that the cross-shore location of the initial bathymetric perturbation is very important. The maximum efficiency in triggering the instability corresponds to perturbations spread across both the surf and shoaling zones. In contrast, for a perturbation confined only in the surf zone, the development may be so slow that the instability is hardly relevant to coastal engineering. Preliminary implications for beach nourishments are discussed.

    Postprint (author’s final draft)

  • Modelling the characteristics and dynamics of surfzone transverses sand bars observed at Noordwijk beach

     Falques Serra, Alberto
    Dynamics Days Europe 2008
    Presentation's date: 2008-08-25
    Presentation of work at congresses

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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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  • Modelling the formation and the long term behaviour of rip channel systems from the deformation of a longshore bar

     Garnier, Roland Charles; Calvete Manrique, Daniel; Falques Serra, Alberto; Dodd, N
    Journal of geophysical research
    Date of publication: 2007-12
    Journal article

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    (Un)predictability in rip channel systems  Open access

     Calvete Manrique, Daniel; Coco, Giovanni; Falques Serra, Alberto; Dodd, N
    Geophysical research letters
    Date of publication: 2007-03
    Journal article

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    Predicting the development of rip channel systems on beaches is relevant for beach safety, nearshore mixing and dispersion, and has been puzzling researchers for decades. Field observations and computer simulations have focused on predicting the spacing and growth time of rip channels as a function of wave characteristics. A satisfactory predictor of rip channel spacing and growth time has not yet been proposed. Here, we show that the lack of predictability of rip channels is an inherent property of the system related to the high sensitivity to the bathymetry prior to pattern development. Sensitivity to the initial cross-shore profile appears to be as important as sensitivity to wave height. Although we might be able to predict rip channel characteristics, the information required by such predictors will need to account for the underlying bathymetry. Implications arising from this study are that other geomorphic patterns could display similar sensitivity.

  • Linear evolution of a shoreface nourishment

     Leeuwen, Van S; Dodd, N; Calvete Manrique, Daniel; Falques Serra, Alberto
    Coastal engineering
    Date of publication: 2007-05
    Journal article

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    The morphological evolution of a shoreface nourishment is investigated by interpreting the nourishment as a linear perturbation of the natural system. The nourishment is projected onto the subset of linear eigenmodes with negative growth rates of the morphodynamical system. The evolution of these linear modes then determines the temporal behaviour of the shoreface nourishment. The method is presented, and results are shown for shoreface nourishments of different length scales on a straight coast and subject to normal incidence. Shoreface nourishments are represented by their expansions according to the projection method on a 1:50 plane beach profile. All nourishments are shown primarily to be diffusive features, with long scale nourishments diffusing more slowly than shorter length scale nourishments. Long scale nourishments also exhibit a shoreward movement during their decay. This all indicates that long length scale nourishments may be more beneficial in coastal engineering projects. This study is a first step towards nonlinear projection to study shoreface nourishment behaviour.

  • NONLINEAR MODELLING OF SURF ZONE MORPHODYNAMICAL INSTABILITIES  Open access

     Garnier, Roland Charles
    Defense's date: 2007-03-21
    Department of Applied Physics, Universitat Politècnica de Catalunya
    Theses

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    Esta tesis se dedica en un estudio de estabilidad no lineal de la morfodinàmica de la zona de rompientes de playas de arena. El modelo numérico MORFO55 resuelve las ecuaciones de aguas someras no lineales para la hidrodinámica y actualiza la topografía a partir del transporte de sedimento. En primer lugar, se aplica en el caso de playas complejas longitudinalmente no uniformes con objeto de probar sus distintas formulaciones. En secundo lugar, se usa para estudiar la generación de estructuras rítmicas en playas longitudinalmente uniformes. Entre estos patrones se distinguen las barras transversales y oblicuas, las barras crescenticas y los sistemas de barras/surcos (ridges/runnels). La hipótesis de que emergen a partir de inestabilidades internas del acoplamiento entre la topografía y la hidrodinámica se investiga. Los estudios previos de modelización numérica se limitaban a las etapas iniciales de la generación de las barras. En particular, mostraban que las barras transversales y oblicuas pueden formarse en playas planas mientras que las barras crecenticas aparecen en playas con barra. La formación de los sistemas de barras/surcos se explica con modelos conceptuales de la observación mediante satélite según los cuales emergerán a partir de la deformación de la barra intermareal. Esta tesis estudia el régimen no lineal de la evolución de todos estos sistemas. Particularmente se obtiene un estado 'nal de equilibrio. Los resultados generales coinciden cualitativamente con las barras observadas en la naturaleza. Se da una interpretación física de la formación, de la evolución y de la saturación del crecimiento de las barras.

    This thesis performs a nonlinear stability study of the surf zone morphodynamics of sandy beaches. To this end the MORFO55 model based on a wave and depth averaged nonlinear shallow water equations solver with wave driver, sediment transport and bed updating is presented. It is first applied to complex longitudinally non-uniform beaches in order to test different model formulations. Second, it is applied to study the generation of surf zone rhythmic features on alongshore uniform beaches. Shore-attached transverse or oblique bars, crescentic bars and ridge and runnel systems are well known examples of such features. The hypothesis that they emerge by self organisation of the coupling between topography, waves and currents is here tested. In absence of shore-parallel bars, the initial formation of transverse and oblique bars had been shown by previous modelling studies of linear stability analysis but is now extended to the finite amplitude regime. In most of barred beaches, crescentic bars and ridge and runnel systems appear. Conceptual models based on field observations suggest that ridges and runnels could emerge by the deformation of the alongshore intertidal bar intercepted by crescentic bars. Up to now, only the formation of crescentic bars had numerically succeeded with linear and non linear models. This study shows that a dynamical equilibrium state of each of these rhythmic bar systems may be described with a numerical model. General results are in qualitative agreement with the bar systems observed in nature. A physical explanation for their formation, their evolution and the saturation of their growth is given.

  • 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
    Presentation of work at congresses

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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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