Influence of Shoreface Morphodynamic Limits on Coastal Response under Sea-Level Rise Scenarios

The position of the shoreline, particularly along sandy coasts, is closely linked to sediment exchanges between the shoreface, the transitional zone with the continental shelf, and the beach-dune system, as well as to variations in sea level (SL). The study area, located along the coast of Rio Grande do Sul, was divided into two sectors: Santa Vitória do Palmar–Rio Grande (Cell I) and São José do Norte–Mostardas (Cell II). The aim was to quantify the influence of the active morphodynamic limits of the shoreface on coastal response under a projected SL rise scenario for 2100. To this end, the upper (hc) and lower (hi) shoreface closure depths were calculated based on wave data, and the lower limit was compared with the morphological hi limits of each sector. Differences in coastal response were evaluated using the behavioral-morphodynamic Random Shoreface Translation Model (RanSTM), in independent simulations for hi variations: calculated limits (Case 1) and morphological limits (Case 2). The results indicate that Cell I, characterized by a deeper hi, is more susceptible to SL rise, with shoreline retreats ranging from –580 to –491 m, depending on the considered morphodynamic conditions. In contrast, Cell II, with a shallower hi, showed lower vulnerability, with retreats between –276 and –265 m. The Kruskal-Wallis test indicated statistical significance (p < 0.01) only for Cell I, confirming that deepening of the lower shoreface leads to relevant differences in shoreline recession. This study highlights the importance of characterizing the active limits of the shoreface, based on wave climate, to support coastal planning and adaptation under sea-level rise.