24 research outputs found

    Rip currents on a barred beach

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    This work presents the numerical study of rip current circulation on a barred beach. The numerical simulations have been carried out with the IH-FOAM model which is based on the three dimensional Reynolds Averaged Navier-Stokes equations. The new boundary conditions implemented in IH-FOAM have been used, including three dimensional wave generation as well as active wave absorption at the boundary. Applying the specific wave generation boundary conditions, the model is validated to simulate rip circulation on a barred beach. Moreover, this study addresses the identification of the forcing mechanisms and the three dimensional structure of the mean flow

    Geomorphology, beach classification and seasonal morphodynamic transition of a Mediterranean gravel beach (Sardinia, Gulf of Cagliari)

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    This paper presents an innovative multi-thematic map (1:2500) that integrates morpho-sedimentological data, hydrodynamic processes, seasonal morphodynamic transitions and the distribution of the benthic habitat of a Mediterranean microtidal, wave-dominated gravel beach system. It is part of a larger cartography of coastal areas, and is based on an interdisciplinary sea-land approach that is applicable worldwide and aims to facilitate coastal management practices and future scientific research. The applications to coastal management include: the facilitation of coastal vulnerability assessments; easy-to-access, up-to-date digital geospatial data; and baseline studies for the future assessment and monitoring of environmental changes. The main environmental features that control the marine processes of this gravel beach appear to be linked to geological and morphological contexts such as the presence of the river mouth, the outcropping of a beach-rock along the coastline, the deposition of gravelly sediment in the beachface and the seagrass cover

    Ecogeomorphology and vulnerability in a Mediterranean ria-type coast (La Maddalena Archipelago, NE Sardinia, western Mediterranean)

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    This paper presents a map describing the main geomorphological and sedimentological features, hydrodynamics, benthic habitat distributions and human impact on the coastal and marine areas of the Archipelago of La Maddalena (NE Sardinia, western Mediterranean). This cartography is based on an interdisciplinary sea-land approach, with the aim being to support sustainable and successful beach management in the face of a changing climate and environment, thereby contributing to the achievement of the Agenda 2030 Sustainable Development Goals (13, 14 and 15). In the Main Map (1:14,000 scale), the static and dynamic features of the beach systems and adjacent inner shelf are divided into thematic sections that include the geomorphological elements, hydrodynamics, sedimentological distributions, benthic habitat (mainly Posidonia oceanica meadow) and anthropogenic impacts. The map establishes a fundamental, multidisciplinary benchmark that is able to provide substantial scientific support to policymakers in relation to future vulnerability-assessment activities and the definition of land-management strategies

    Natural vs. Anthropic Influence on the Multidecadal Shoreline Changes of Mediterranean Urban Beaches: Lessons from the Gulf of Cagliari (Sardinia)

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    Urban Mediterranean beaches are often characterized by a fragile and unstable equilibrium that can be easily altered by ongoing climate change and by the increase in human pressure. This may pose serious threats to the survival of beach systems that cannot accommodate these modifications. In this paper, the spatio-temporal shift of the shoreline was investigated along two urban beaches in the Gulf of Cagliari (Poetto and Giorgino; southern Sardinia, western Mediterranean Sea) across a time frame of 62 years (1954–2016). The Digital Shoreline Analysis System (DSAS) ArcGIS™ extension was used to extract different statistical parameters which allowed us to quantify the erosion and accretion rates. These data were further examined in relation to a number of anthropic and natural forcings in order to disentangle the factors controlling shoreline evolution. Eight sectors with interchanging net erosive and accretion trends were identified along the Poetto and Giorgino beaches. In six decades, some sectors of the two study sites appeared to have undergone great shoreline modification as a result of the intense anthropogenic activities impacting these coastal areas. The westernmost portions of both beaches were found to be the most vulnerable to erosion processes; such conditions were likely controlled by the interplaying of local hydrodynamics and by the intense coastal development which affected these sectors. The highest retreat rates (mean end point rate (EPR) = −0.51/year) were recorded in the western limit of Giorgino beach. Along the western limit of Poetto beach, EPR erosion rates (mean EPR = −2.92/year) considerably increased in the years after the artificial beach nourishment carried out in 2002, suggesting that the majority of the nourished material was lost offshore or partly redistributed along the beach. Coastal structures, urban development, river catchment modification, industrial and port activities, beach cleaning and touristic and recreational activities have been identified as the ongoing causes of coastal alteration. If these factors remain constant, under projected climate change scenarios, these beaches are at risk of further increased flooding and erosion. In this context, the application of DSAS appeared as an essential tool, supporting a monitoring system able to provide understanding and, potentially, predictions of the short- to long-term evolution of these beach system

    Boundary layer dynamics in the swash zone under large-scale laboratory conditions

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    This paper presents the results of a laboratory experiment of swash hydrodynamics on a coarse sand barrier beach backed by a lagoon. Boundary layer dynamics have been analyzed using the high-resolution near-bed velocities measured by Acoustic Doppler Velocity Profilers deployed in the swash zone. Swash events have been ensemble-averaged in order to study mean hydrodynamic patterns. A proposed velocity gradient criterion allowed identification of the boundary layer growth during the backwash phase, but it was unable to characterize boundary layer variability during uprush. Cross-shore velocity profiles were well represented by the logarithmic model for a large portion of the ensemble-averaged swash duration. Uprush and backwash logarithmic-estimated friction factors were of the same order of magnitude with a strong variability related to the boundary layer growth during the backwash. The momentum integral method provided smaller bed shear stresses than the logarithmic model, a result possibly related to either the assumptions involved in the momentum integral method or to an underestimation of the boundary layer thickness during uprush. A decrease of friction coefficients for increasing Reynolds numbers at the early backwash was observed. This behavior is consistent with traditional results for steady and uniform flows in a transitional regime

    Transformación de ondas infragravitatorias en playas

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    RESUMEN: El oleaje que se propaga en aguas litorales está caracterizado por procesos altamente no lineales y una amplia gama de escalas espacio-temporales. En aguas someras, los procesos no lineales favorece una transferencia de energía desde la frecuencia de pico del espectro hacia oscilaciones de corto y largo periodo. Munl (1949) y Tucker (1950) reportaron por primera vez la presencia de oscilaciones de superficie libre con periodos de los grupos de ondas cortas y acuñaron el término "surf beat" para estos tipos de movimientos (también el término "ondas infragravitatorias" es frecuentemente usado hoy en día). Desde entonces, las ondas en frecuencias del surf beat han atraído una creciente atención por parte de los investigadores de procesos costeros, habiéndose producido un gran número de trabajos de campo, laboratorio y numéricos publicado sobre este tema. La demostrada importancia de las ondas con frecuencia de surf beat pone de manifiesto la necesidad de desarrollar metodologías que incorporen las ondas largas con el fin de predecir la evolución de la costa y las oscilaciones de run-up. El objetivo principal de este trabajo es mejorar el estado de conocimiento de los procesos hidrodinámicos de baja frecuencia inducidos por el oleaje en playas. Concretamente, se hace énfasis en el estudio de los procesos de asomeramiento, disipación, oscilación y reflexión de ondas infragravitatorias. En este trabajo, el estudio de las ondas de largo periodo ha sido llevado a cabo mediante técnicas numéricas y experimentales. Se ha dedicado una atención especial a las interacciones no lineales y a la generación y liberación de ondas en las zonas de asomeramiento y de rotura. La dispación de energía de baja frecuencia en la zona de rompientes ha sido estudiada identificando los diferentes mecanismos de disipación. Finalmente, este trabajo se ha centrado en las oscilaciones de swash en playas en condiciones altamente disipativas.ABSTRACT: Nearshore processes are highly nonlinear and they encompass a wide range of time and spatial scales. This work deals with low-frecuency phenomena induced by the incident wave field in the surf and swash zone. In order to deal with low-frequency motions in coastal waters, the combined use of laboratory data and numerical simulations is carried out. Capability of the RANS-type IH-2VOF and NLSW-type SWASH models to simulate hydrodynamic processes induced by breaking waves is tested by means of comparison with laboratory observations. Since numerical models provide a means to obtain high spatial and time resolution information that is almost impossible to achieve in the laboratory, they represent a poweful and complementary tool in order to investigate nearshore hydrodynamics. The numerical simulations confirm previous work and improve laboratory observations regarding the study of low-frequency motions on beaches. Special attention is devoted to nonlinear interactions resulting in an enhacement of low frequency waves in the shoaling zone as well as the release of free long waves over a sloping bottom. The low-frequency energy damping inside the surf zone is addressed in detail identifying the dissipation agents and its contribution along the cross-shore profile. Finally, the last part of this work focuses on swash oscillations on the beach face under highly dissipative conditions when saturation extends to surf beat frequencies

    Bed stress estimation in the swash zone of a proto-type laboratory beach

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    A proto-type laboratory experiment examining swash dynamics on a coarse sandy beach was carried out in a large-scale wave flume. This paper addresses boundary layer dynamics using near-bed velocity measurements collected in the swash zone under irregular wave conditions. Bed shear stress estimates were obtained from the velocity measurements using two models: (1) the logarithmic model and (2) the momentum integral model. The logarithmic model provides the most reliable bed shear stress estimates. In general terms, the momentum integral model underestimates bed shear stress during the backwash and performs poorly during the uprush when positive (onshore directed) stresses are hardly observed even though the velocity is onshore. Numerical results suggest that this drawback of the momentum integral method results is likely to be the consequence of the omission of the upper part of the boundary layer thickness in the momentum integration

    An Assessment of Swash Excursion Predictors using Field Observations

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    Accurate predictions of swash excursion are fundamental for improving the understanding of swash zone dynamics and for coastal management and hazard applications. Researchers have provided several formulas for predicting the swash excursion on sandy beaches. However, the problem of the universality of these formulas is still open. In fact, it is not clear whether the existing formulations are applicable to a wide range of beaches and wave conditions. This study verifies 13 existing swash (total, incident and infragravity) formulations using 13 published experiments (636 swash measurements). The experiments were carried out on beaches worldwide (dissipative intermediate and reflective) under a wide range of wave conditions (including extreme events). Results show that formulas behave differently with under and over prediction. However, some trends can be recognized, which are indeed critical for coastal hazards and management applications: generally, for large swash events (swash >1.5 m) large scatter are found for all formulas. During extreme conditions (swash>2.5 m) total and incident swash are strongly underestimated, while infragravity swash varies for each formula. The maximum errors and the root mean square errors can exceed 2.5 m and 1 m, respectively

    Reynolds averaged Navier-Stokes modelling of long waves induced by a transient wave group on a beach

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    This paper presents the numerical modelling of the cross shore propagation of infragravity waves induced by a transient focused short wave group over a sloping bottom. A dataset obtained through new laboratory experiments in the wave flume of the University of Cantabria is used to validate the Reynolds averaged Navier-Stokes type model IH- 2VOF. A new boundary condition based on the wave maker movement used in the experiments is implemented in the model. Shoaling and breaking of short waves as well as the enhancement of long waves and the energy transfer to low-frequency motion are well addressed by the model, proving the high accuracy in the reproduction of surf zone hydrodynamics. Under the steep slope regime, a long wave trough is radiated offshore from the breakpoint. Numerical simulations conducted for different bottom slopes and short wave steepness suggest that this low-frequency breakpoint generated wave is controlled by both the bed slope parameter and the Iribarren number. Moreover, the numerical model is used to investigate the influence that a large flat bottom induces on the propagation pattern of long waves. © 2010 The Royal Society
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