Abstract
Although waves are commonly regarded as the primary drivers of beach profile evolution, observations and experiments show that the slope of the beach face does not always directly reflect the conditions of the incident waves. The mechanisms governing transitions between equilibrium states are unclear due to feedback loops and delayed sediment responses. In this study, we analyze 3.5 years of daily beach profile data from two tropical low-tide terrace sites in order to characterize nearshore transient periods between seasonal equilibria. Our results reveal episodes in which the traditional classification of equilibrium beaches fails to predict slope evolution. We demonstrate that, at these beaches, transient events can be influenced by water-level variations linked to coastal upwelling, thereby altering sediment redistribution across the swash–surf continuum. Complementary laboratory experiments confirm that the observed dynamics can be reproduced by considering both the Dean number (\(\Omega _0\)), representing wave energy, and nearshore water-level variability. Our findings emphasize that transient sea-level modulations, such as those induced by upwelling, mesoscale eddies or atmospheric systems, can reshape beach profiles across multiple timescales. This highlights the need to extend equilibrium frameworks to account for non-wave-driven processes.
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Data availability
Field and laboratory data supporting the conclusions of this study are made available on Zenodo (Dataset). Sea surface temperature data are extracted from ODYSSEA Global Sea Surface Temperature Gridded Level 4 Daily Multi-Sensor Observations (https://doi.org/10.48670/mds-00321). Sea level anomaly data are extracted from Global Ocean Gridded L 4 Sea Surface Heights And Derived Variables Nrt (https://doi.org/10.48670/moi-00149). Surface wind directions and intensities are extracted from Global Ocean Physics Reanalysis (https://doi.org/10.48670/moi-00021)
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Acknowledgements
MA and LL would like to thank Jean-Dominique Barron, Sébastien Cazin, Hervé Ayroles and Quentin Amati for the crucial technical support they provide to our work. MA, LL and RA are thankful to Ivana M. Mingo for her expertise and her work on which they have relied.
Funding
This study ha been funded by Université de Toulouse and Région Occitanie (project “ADSL”, \(n^\circ\)J452IMF23121). This study has been partially supported through the grant EUR TESS \(N^\circ\)ANR-18-EURE-0018 in the framework of the Programme des Investissements d’Avenir.
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R.A, L.L, J.M.A and M.A conceived, realized and analyzed the experiments. All authors reviewed the manuscript.
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Aparicio, M., Lacaze, L., Almar, R. et al. Sea-level changes modulate beach face slope in coastal upwelling zones. Sci Rep (2026). https://doi.org/10.1038/s41598-026-40630-3
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DOI: https://doi.org/10.1038/s41598-026-40630-3
