EGU2020-18730, updated on 12 Jun 2020
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Characteristics and dynamics of crescentic bar events at an open, Mediterranean beach

Rinse de Swart1, Francesca Ribas1, Daniel Calvete1, Gonzalo Simarro2, and Jorge Guillén2
Rinse de Swart et al.
  • 1Universitat Politècnica de Catalunya, Department of Physics, Barcelona, Spain (
  • 2Institut de Ciències del Mar (ICM-CSIC), Department of Marine Geosciences, Barcelona, Spain.

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.

Garnier, 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).

How to cite: de Swart, R., Ribas, F., Calvete, D., Simarro, G., and Guillén, J.: Characteristics and dynamics of crescentic bar events at an open, Mediterranean beach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18730,, 2020


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displays version 2 – uploaded on 04 May 2020
Some minor issues were fixed and one figure was updated.
  • CC1: Comment on EGU2020-18730, Timothy Price, 08 May 2020

    Hi Rinse, nice work!

    Why do you think that crescentic bars don't form when bars are too close to shore? Are the wave conditions similar to the cases that crescentic bars do form when the bar is further form the shore?

    Cheers, Tim

    • AC1: Reply to CC1, Rinse de Swart, 08 May 2020

      Hi Tim,

      Thanks for your comment!

      That is a very good question and I am not yet sure about the answer. I think it could be related to the sandbar being kind of obstructed in its cross-shore movement (particularly the onshore movement). What I observe at Castelldefels is that generally first the horns (shoreward perturbations) develop in the sandbar, whilst the rest of the sandbar will at first stay more or less at the original cross-shore position. The bays (seaward perturbations) generally only start developing after the formation of the horns. Furthermore, I think it is quite difficult to get a developed rip current circulation when the bar is close to shore, because some space is needed between a horn and the shoreline to develop this circulation pattern. However, this space is probably not sufficiently large when the bar is located close to shore and thus the circulation will be weak or absent. Finally, when the bar is close to shore it can in fact be part of a terrace, which can also hamper its cross-shore movement. Unfortunately, whether a terrace exists or not is often not clear from the video images, but I know that they occur quite often.

      Regarding the wave conditions, I have taken a quick look at the years 2011-2014. As you can see in the figure on slide 11, there are very few crescentic bar events in 2011 and 2012, whilst there are many in 2013-2014. The average wave conditions during these years are identical (Hm0 = 0.5 m, Tm02 = 4.0 s, mean direction = 15 degrees), which makes me believe that the big difference in crescentic bar events cannot be attributed to a difference in wave conditions.

      Regards, Rinse

      • CC2: Reply to AC1, Timothy Price, 12 May 2020

        Thanks for the insights, Rinse!

        Interesting that you observe the formation of horns to characterize the initial development of the crescentic pattern, that's something I saw in the video images of the Gold Coast too.  
        You say that crescentic bars formed more frequently in 2013-2014, following a period where the terraced morphology hampered this. Did you identify a specific event where the terrace bar seperated from the beach and migrated offshore, making way for a trough? Or was it a more gradual process?


        • AC5: Reply to CC2, Rinse de Swart, 13 May 2020

          Hi Tim,

          Interesting to hear that the formation of horns also seem to characterize the initial development of the crescentic pattern at the Gold Coast.

          Regarding your question about the bar separating from the beach at Castelldefels: a shift in bar position at Castelldefels generally occurs very suddenly. For this, it is important to know that the wave conditions at Castelldefels are mostly low-energetic (i.e. Hm0 below 1.0 m and during summer often below 0.5 m). When Hm0 is above 1.5 m, it is already classified as a storm by the Spanish Ports Authority and storms with Hm0 above 2.0 m occur only a few times a year. Offshore bar migration generally occurs during these storms, but interestingly not all storms cause offshore bar migration. However, when offshore migration occurs then it can be quite strong (several tens of metres in one single night). Hardly any offshore bar movement is observed during the lower-energetic wave conditions, which means that once the bar has moved to a certain cross-shore position it can stay there for a long time. Of course, there can still be alongshore variability in the morphology (crescentic bars will form if the conditions are right), but the alongshore averaged bar position stays more or less the same.


    • AC2: Reply to CC1, Francesca Ribas, 12 May 2020

      Dear Tim, thank you very much for your question!

      Just a short extension of Rinse's answer, with which I completely agree.

      The strong influence of the pre-existing morphology on the characteristics of emering crescentic bars has been studied in previous articles (see for instance ). 

      You were also wondering if part of the reason why crescentic bars did no form in 2011-2012 were interannual variations in wave conditions. We do not think this is the case, as Rinse explained to you, because the averaged conditions were not different. However, just in case, he will also plot the annual histograms to see if there are differences. 

      Thank you very much for the interaction! See you soon!



      Francesca Ribas

      Associate professor, Physics Department

      Universitat Politècnica de Catalunya

      • AC3: Reply to AC2, Francesca Ribas, 12 May 2020

        Sorry! The link to the article was deleted! It is:

        • AC4: Reply to AC3, Francesca Ribas, 12 May 2020

          Waw! The link does not appear.... sorry! I wite the full citation:

          Calvete, D., G. Coco, A. Falqués, and N. Dodd (2007), (Un)predictability in rip channel systems, Geophys. Res. Lett., 34, L05605, doi:10.1029/2006GL028162.

          • CC3: Reply to AC4, Timothy Price, 12 May 2020

            Thanks Cesca! I'll have another look at that paper with your findings in the back of my mind.


      • CC4: Reply to AC2, Timothy Price, 12 May 2020

        Thanks for the further elaboration, Cesca!

        So would that mean the bar morphology is rather persistent at your site? Like I suggested to Rinse above, did you see any event or sequence of events that might have triggered a different bar state? Perhaps the difference in driving forces is not captured so much by interannual differences, but by a specific event of sequence of events.

        Hope to see you all soon!

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