How important are sediment-budget estimates for shoreline protection under sea-level rise?

Abstract Sediment-budget (SB) estimates are critical for shoreline conservation because sea-level rise (SLR) alone does not dictate coastal recession; sediment deficits or surpluses can amplify or buffer its impacts. The interaction between SB, shoreface morphology, and SLR controls shoreline evolution across multiple timescales. This study evaluates how annual, decadal, and millennial-scale SB estimates influence shoreline response along the southern-central coast of Rio Grande do Sul, Brazil, using the probabilistic Random Shoreface Translation Model (RanSTM). The wave-dominated coast was subdivided into four SB-control coastal cells: Cell I (Albardão–Verga), Cell II (Verga–Sarita), Cell III (Cassino), and Cell IV (Mar Grosso), which differ in sedimentary behavior and shoreface morphology. SB estimates were derived from remotely piloted aircraft surveys (2023–2024), satellite imagery (2010–2024), and stratigraphic records and combined with SSP5–8.5 SLR projections for 2040, 2050, 2075, and 2125. Annual observations showed larger shoreline displacements than decadal records. Cell I exhibited short-term erosion and medium-term accretion, Cells II and IV were accretionary at both scales, and Cell III remained stable in the short term and accreted over the decadal period. Future projections showed the greatest retreat in Cell I, while all cells shifted toward retreat at longer horizons as SLR intensified. Overall, SB exerted stronger control over short- to medium-term shoreline behavior, whereas SLR became increasingly dominant toward centennial horizons, with shoreface morphology modulating the magnitude of the response. Integrating multitemporal SB estimates with morphological differences provides a physical basis for understanding coastal change and supports targeted monitoring, conservation strategies, and long-term climate adaptation planning.