Nearshore sandbars are important features in the surf zone of many beaches because they strongly influence the mean circulation and the evolving morphology. Due to variations in wave conditions, sandbars can experience cross-shore migration and vary in shape from alongshore-uniform (shore-parallel) to alongshore rhythmic (crescentic). Sandbar dynamics have been studied extensively, but existing observational studies usually do not quantify the processes leading to crescentic bar formation and straightening. This study analyses the dynamics of crescentic bar events at the fetch-limited beach of Castelldefels (northwestern Mediterranean Sea, Spain) using 7.5 years of hourly time-exposure video images and detailed wave conditions. The results show that, despite the generally calm wave conditions, the sandbars were very dynamic in the cross-shore and longshore directions. They often migrated rapidly offshore during storms (up to 70 m in one day) and more slowly onshore during post-storm conditions. Crescentic bars were often present at the study site (48% of the time), but only when the sandbar was at least 10 m from the shoreline. They displayed a large variability in wavelengths (100–700 m), alongshore migration speeds (0–50 m/day) and cross-shore amplitudes (5–20 m). Wavelengths increased for larger bar-shoreline distances and the alongshore migration speeds were strongly correlated to the alongshore component of the radiation stresses. Crescentic patterns typically developed during low-medium energetic waves with limited obliquity (θ ≲ 20° at 10-m depth), whilst bar straightening occurred during medium-high energetic waves with strong oblique angles of incidence (θ ≳ 15°). Overall, this study provides further proof for the important role of wave direction in crescentic bar dynamics and highlights the strong dependence of crescentic bar development on the initial bathymetric configuration.

The sensitivity of a 2DH coastal area (XBeach) and a reduced-complexity (Q2Dmorfo) morphodynamic model to using different forcing sources is studied. The models are tested by simulating the morphodynamic response of an embayed beach in the NW Mediterranean over a 6-month period. Wave and sea-level forcing from in situ data, propagated buoy measurements, and hindcasts, as well as combinations of these different data sources, are used, and the outputs are compared to in situ bathymetric measurements. Results show that when the two models are calibrated with in situ measurements, they accurately reproduce the morphodynamic evolution with a “good” Brier skill score (BSS). The calibration process reduces the errors by 65 %–85 % compared with the default setting. The wave data propagated from the buoy also produce reliable morphodynamic simulations but with a slight decrease in the BSS. Conversely, when the models are forced with hindcast wave data, the mismatch between the modelled and observed beach evolution increases. This is attributed to a large extent to biased mean directions in hindcast waves. Interestingly, in this small tide site, the accuracy of the simulations hardly depends on the sea-level data source, and using filtered or non-filtered tides also yields similar results. These results have implications for long-term morphodynamic studies, like those needed to validate models for climate change projections, emphasizing the need to use accurate forcing sources such as those obtained by propagating buoy data.

Alongshore changes in shorelines along sandy beaches are a result of variations in wave conditions and nearshore morphology, which can cause straight shorelines to develop large‐scale (100–1000 m wavelength) undulations, called megacusps, which cause local shoreline accretion and erosion. Megacusps are often morphologically coupled to alongshore variability in the nearshore sandbar (crescentic bars). Sandbar rhythmicity has been studied extensively, whilst megacusp dynamics and their coupling with crescentic bars have received less attention. This study uses a long‐term dataset of hourly time‐exposure video images and detailed propagated wave conditions to investigate megacusp dynamics and the corresponding sandbar–shoreline coupling at the low‐energetic, tideless beach of Castelldefels (northwestern Mediterranean Sea, Spain). Megacusps were observed during 24% of the study period. Crescentic bars were present during 91% of the days with megacusps, whilst megacusps were not observed during 50% of the days with crescentic bars. Megacusp wavelengths (100–700 m) were comparable to those of crescentic bars, whilst cross‐shore amplitudes (3–8 m) and alongshore migration speeds (0–15 m/day) were smaller than those of crescentic bars. No clear relation was observed between wave conditions and megacusp formation, migration and disappearance. However, megacusp presence was strongly linked to crescentic bar presence, as megacusps mostly developed some days after the formation of a crescentic bar. Coupling between shoreline and inner sandbar was significant during 74% of the time with simultaneous presence of megacusps and crescentic bars. No dominance of one particular coupling pattern was observed, although the pattern depended on the wave height and the wave angle. It is hypothesised that the wave height and angle determine the type of flow pattern over the inner crescentic bar (single or double rip cell circulations versus meandering currents) and thus control the type of coupling.

Crescentic sand bars have attracted significant attention from coastal scientists during the last decades, which has lead to comparatively good understanding of their formation mechanism, as well as their characteristics and dynamics (e.g. Van Enckevort et al., 2004; Price and Ruessink, 2011). However, the effect of wave obliquity on crescentic bar formation is not yet clear, and processes like coupling of crescentic bars with megacusps deserve further attention. Furthermore, the mechanisms leading to crescentic bar straightening are not well understood. Previously, this was mainly linked to high-energetic wave conditions, but more recent studies (e.g. Price and Ruessink, 2011; Garnier et al., 2013) indicate that this is not always the case. Instead, those studies have found that bar straightening predominantly occurs when the waves are obliquely incident. Finally, there are not many studies of crescentic bars in fetch-limited environments with insignificant tides (such as Mediterranean beaches). Therefore, the objective of the present work is to increase our knowledge on the dynamics of crescentic bars (including bar straightening) using data from an open, Mediterranean beach (Castelldefels beach, 20 km southwest of Barcelona) with hardly any tides and limited fetch. Crescentic bar dynamics have been analysed using a nearly 8-year dataset of time-exposure video images (October 2010 to August 2018). The crescentic bar events, including formation and destruction moments, have been detected using visual analysis. Wave conditions in front of the study site have been collected by propagating 2D spectra (measured by a permanent wave buoy in front of Barcelona harbour) using the SWAN spectral wave model. The first results indicate that there is a lot of morphodynamic variability at the study site, even for low-energetic wave conditions (Hm0 < 0.5 m). Tens of crescentic bar events, including formation, evolution and destruction, can be observed. The bars show a large variation in wavelength (ranging from 100 to 500 m), which is often related to splitting and merging of individual crescents. Furthermore, the results reveal a strong relation between crescentic bar formation and the initial configuration of the bathymetry. Crescentic bars develop often when the sandbar is located some distance from the shoreline, whilst they are hardly observed when the sandbar is located close to the shoreline. Further work (which will be presented at the conference) consists of a detailed analysis of bar characteristics, including their alongshore migration, and the quantification of the role of wave conditions (especially wave direction) on crescentic bar dynamics. ReferencesGarnier, R., Falqués, A., Calvete, D., Thiebot, J., & Ribas, F. (2013). A mechanism for sandbar straightening by oblique wave incidence. Geophysical Research Letters, 40(11), 2726-2730.Price, T. D., & Ruessink, B. G. (2011). State dynamics of a double sandbar system. Continental Shelf Research, 31(6), 659-674.Van Enckevort, I. M. J., Ruessink, B. G., Coco, G., Suzuki, K., Turner, I. L., Plant, N. G., & Holman, R. A. (2004). Observations of nearshore crescentic sandbars. Journal of Geophysical Research: Oceans, 109(C6).

Modelling the response of sandy beaches to sea level rise is a major scientific challenge and several types of models can be applied. Process-based models are useful tools for understanding beach responses on short time scales, but they are computationally expensive and tend to accumulate errors when resolving the many short-term processes they contain. Alternatively, reduced-complexity models can be an interesting option for long-term modelling. Furthermore, to simulate the long-term response of beaches, it is necessary to combine different model forcing sources (wave and sea-level, e.g., from buoys or hindcast models). The aim of this contribution is to quantify the effect of using different forcing sources in morphodynamic modelling with these model types.Two numerical models are applied to the embayed microtidal beach of El Castell (Palamós, Catalunya, NW Mediterranean). The XBeach process-based model, which solves the full 2DH nearshore hydrodynamics and the corresponding bed evolution, and the Q2Dmorfo reduced-complexity model, which simulates the bed level variations by calculating the sediment fluxes parametrically directly from the wave field without resolving the currents. Both models are first calibrated using two topobathymetric surveys conducted in January and July 2020 and wave and sea-level data measured from an AWAC deployed at 14.5 m depth during those 6 months. Calibration is performed using the most sensitive parameters, i.e., those related to cross-shore transport. In XBeach, the surfbeat mode must be used to obtain realistic results. It generates an aleatory spectral wave time series at the boundary that includes groupiness. To handle the effect of this randomness, a total of 5 realizations are made for each set of parameter values and a mean bathymetry is computed out of these realizations. To assess the model performance, the Brier Skill Score (BSS) is calculated both for the modelled bathymetry and its coastline during the 6-month period. Moreover, the Standard deviation (STD) of the 5 realizations is also computed. The chosen optimum parameter setting is the one maximizing the BSS (0.36 and 0.79 for the bathymetry and shoreline respectively) and minimizing the STD so that the result is robust and reproducible. In Q2Dmorfo, the BSS of the simulated final coastline is calculated for each set of parameter values. The optimum parameter setting also produces a maximum BSS of 0.79.Once both models are calibrated, the other potential forcing sources are applied. They include wave and sea-level datasets from a sea-level and wind-waves 72-years hindcast generated with the hydrodynamic-wave coupled SCHISM model, a wave dataset from an offshore buoy propagated to the AWAC position using SWAN model and a sea-level dataset measured by the tidal gauge at Barcelona harbour. The results show a very significant sensitivity to the wave forcing source and much less sensitivity to the sea-level source. By using the dataset propagated from the buoy by SWAN, both models represent well the observed beach rotation, whereas using the dataset obtained with SCHISM, the beach rotation is systematically under-predicted by both models, giving negative BSS values. This is because SCHISM predicts wave angles biased to the west.