Characteristics and dynamics of crescentic bar events at Castelldefels beach

Crescentic sand bars have been studied intensively in the last decades, resulting in good knowledge of some of their characteristics, but the processes behind their formation and destruction are not yet clear. This study aims to increase our understanding of the dynamics of crescentic bars at an open, dissipative Mediterranean beach (Castelldefels, 20 km southwest of Barcelona). Their dynamics have been analysed using a 4.25-year dataset of video images. The crescentic bar events have been identified using visual analysis, including the formation and destruction moments. The results show that crescentic bars hardly occurred when the sandbar was located close to the beach, whilst they developed often when the sandbar was further offshore. Wave conditions during crescentic bar formation were low- to intermediate-energy waves with both oblique and shore-normal angles of incidence. Sandbar straightening was preferably observed for oblique waves (of both intermediate and high energy). The alongshore wavelength and cross- shore amplitude of the crescentic bars have been also quantified, giving some 245 m and 10 m on average, respectively.Peer ReviewedPostprint (published version

Morphological changes, beach inundation and overwash caused by an extreme storm on a low-lying embayed beach bounded by a dune system (NW Mediterranean)

The geomorphological evolution of a low-lying, micro-tidal sandy beach in the western Mediterranean, Pals beach, was characterized using airborne Light Detection and Ranging (LiDAR) data. Data were collected in prior to and six months after the impact of an extreme storm with a return period of approx. 50 years, with the aim of characterizing the beach's response to the storm. The use of repeated high-resolution topographic data to quantify beach geomorphic changes has allowed assessment of the accuracy of different proxies for estimating beach volume changes. Results revealed that changes in the shoreline position cannot accurately reproduce beach volume changes on low-lying beaches where overwash processes are significant. Observations also suggested that volume estimations from beach profiles do not accurately represent subaerial volume changes at large profile distances on beaches with significant alongshore geomorphological variability. Accordingly, the segmentation of the beach into regularly spaced bins is proposed to assess alongshore variations in the beach volume with the accuracy of the topographic data. The morphological evolution of Pals beach during the study period showed a net shoreline retreat (- 4 m) and a significant sediment gain on the subaerial beach (+ 7.5 m3/m). The net gain of sediment is mostly due to the impact of the extreme storm, driving significant overwash processes that transport sediment landwards, increasing volume on the backshore and dunes. The increase of volume on the foreshore and the presence of cuspate morphologies along the shoreline also evidence post-storm beach recovery. Observed morphological changes exhibit a high variability along the beach related to variations in beach morphology. Changes in the morphology and migration of megacusps result in a high variability in the shoreline position and foreshore volume changes. On the other hand, larger morphological changes on the backshore and larger inundation distances occur when the beach and the dunes are lower, favouring the dominance of overwash. The observed storm-induced morphological changes differ from predicted beach storm impacts because of spatial and temporal variations in the beach morphology, suggesting that detailed morphological parameters and indicators used for predicting beach vulnerability to storms should be regularly updated in order to represent the pre-storm beach conditions. Finally, observed morphological changes in Pals Bay evidenced a different behaviour between natural and urban areas, with better post-storm beach recovery on natural areas where the beach is not artificially narrowed.Peer ReviewedPostprint (author's final draft

Cambios morfológicos en barras sumergidas de playas artificiales

Se analizan los cambios morfológicos que experimentan las barras sumergidas de dos playas artificiales de la ciudad de Barcelona durante un período de tres años. Los desplazamientos transversales de las barras alcanzaron más de 50 m, con tasas máximas de migración hacia mar de 5 m/día y hacia tierra de 3 m/día que ocurren asociadas a determinados temporales. La configuración en planta de la barra puede estar estrechamente relacionada con la línea de costa, especialmente cuando se desarrollan dos cúspides en una de las playas que favorecen la generación de una configuración «crescéntica» (oscilante! de la barra durante más de un año. Aunque se trata de playas artificiales, el comportamiento de las barras es equivalente al descrito en playas naturales

The responses of artificial embayed beaches to storm events

Peer ReviewedPostprint (published version

Camera calibration for coastal monitoring using available snapshot images

Joint intrinsic and extrinsic calibration from a single snapshot is a common requirement in coastal monitoring practice. This work analyzes the influence of different aspects, such as the distribution of Ground Control Points (GCPs) or the image obliquity, on the quality of the calibration for two different mathematical models (one being a simplification of the other). The performance of the two models is assessed using extensive laboratory data (i.e., snapshots of a grid). While both models are able to properly adjust the GCPs, the simpler model gives a better overall performance when the GCPs are not well distributed over the image. Furthermore, the simpler model allows for better recovery of the camera position and orientation.Postprint (author's final draft

Sediment transport and dispersal in the nearshore of “flash-flood” rivers

River sediment dispersal on the near-shore of “flash-flood” rivers is investigated using a coupled wave-current-sediment transport model. Besòs and Llobregat rivers (short and mountainous rivers in NW Mediterranean Sea, near to Barcelona City) are used as examples to study the sediment transport under “flash-flood” regime. The modeling system COWAST which includes the coupling between the water circulation model ROMS and the wave model SWAN, is applied to assess the sediment dispersal mechanisms and deposition in the coastal area off the two river mouths. Preferential depositional areas such as mud-belts were identified from the simulations. The sediment dispersal pattern obtained by the model agrees with observational measurments. Complementary numerical simulations revealed sorting of sediment grain size in the cross-shelf direction.Peer ReviewedPostprint (published version

Observed alongshore sandbar and shoreline variability at an open, fetch-limited beach

andbars often display an alongshore variable morphology (crescentic bars), which is often mimicked at the shoreline (megacusps). The processes resulting in these patterns are not yet clear and particularly the coupling between sandbar and shoreline deserves more attention. This contribution presents 8 years of video observations on the dynamics of crescentic bars and megacusps, including coupling, at the open, fetch-limited beach of Castelldefels (Spanish Mediterranean coast). Wave conditions were propagated from the nearest wave buoy to the study site (10 m depth). Crescentic bars and megacusps were detected visually from video images, after which the barlines and shoreline were extracted to compute characteristics like wavelengths, amplitudes and alongshore migration rates. Finally, the bar-shoreline coupling was evaluated using cross-correlation. Crescentic bars were only observed when the sandbar- shoreline distance was at least 10 m. They developed during lower energetic waves with both oblique and shore-normal angles, whereas straightening occurred primarily during high-energetic obliquely-incident waves. Megacusps only developed in the presence of a crescentic bar and were less pronounced and dynamic compared to crescentic bars. During megacusps presence, a highly-significant coupling was present during 73% of the time but the relative phase varied, partly due to different alongshore migration rates of the two patterns.Peer ReviewedPostprint (published version

Sediment exchanges from the shoreline to the lower shoreface inferred from morphological changes (Llobregat Delta, Western Mediterranean)

Se realizaron 14 campañas topo-batimétricas desde la playa emergida hasta los 15 m de profundidad, a lo largo de 1 km en la playa de Castelldefels (Delta del Llobregat, Barcelona) durante el periodo 2011-2020. Se analizan las variaciones morfológicas que tienen lugar, los cambios volumétricos y sus posibles implicaciones en los intercambios sedimentarios en el perfil litoral. Los principales cambios morfológicos están causados por los desplazamientos transversales de las barras de arena, generalmente hacia tierra durante condiciones de buen tiempo y hacia mayor profundidad durante las tormentas más fuertes. Los cambios volumétricos sugieren que el retroceso de la línea de costa durante la última década aporta sedimento a la parte superior de la playa sumergida, mientras que los eventos más extremos producen una transferencia de sedimento hacia la plataforma continental, que podrían ser parcialmente irrecuperables para la dinámica litoral. El balance sedimentario resultante es negativoPostprint (updated version

Characterization of bottom sediment resuspension events observed in a micro-tidal bay

In this study we investigate the variability in near-bottom turbidity in Alfacs Bay (in the northwestern Mediterranean Sea). The bay is characterized by a micro-tidal environment and seiching activity that may lead to flow velocities of more than 50 cm s−1. A set of current meters and optical sensors were mounted near the sea bottom to acquire synchronous hydrodynamic and optical information from the water column. The time-series observations showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node–antinode location of the fundamental and first resonance periods of the bay. The implementation of a coupled wave–current numerical model shows strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible for the resuspension of fine sediment within the bay. This is confirmed by suspended sediment concentration maps derived from Sentinel-2 satellite imagery. We suggest that the sequence of resuspension events plays an important role in the suspended sediment concentration, meaning that the occurrence of sediment resuspension events may increase the suspended sediment in subsequent events. The suspended sediment events likely affect the ecological status of the bay and the sedimentary process over a long-term period.Peer ReviewedPostprint (published version

Breeze influence on waves and vertical current profile in the coastal area based on EOF analysis (Vilanova i la Geltrú, Barcelona)

This article presents some preliminary results on the breeze influence on waves and vertical current profiles during eleven months of data recorded by an Acoustic Wave and Current Profiler installed in the OBSEA platform in the coast of Vilanova i la Geltrú (NW Mediterranean). The collected data has been analyzed using Empirical Orthogonal Functions and spectral analysisPostprint (published version