Sea-level changes modulate beach face slope in coastal upwelling zones

Abstract

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.