Compound Minor Flooding and Relative Sea Level Rise in the Lower Winyah Bay Watershed, South Carolina, USA

High-tide flooding is becoming an increasingly common phenomenon in coastal communities. This study characterizes the driving mechanisms and temporal variability of minor flood events in the Lower Winyah Bay Watershed, South Carolina, USA caused by a combination of river discharge, tides, non-tidal residuals (NTR), and relative sea-level rise (RSLR), using a physics-based regression model of daily high-water (DHW). Results showed that all three mechanisms contributed to 534 floods between 2017 and 2024 in Georgetown (23 river km; rkm), either as single-driver floods (2–10% of flood days; range based on associated model error) or compound floods (90–98% of total flood days). Tides dominated 237–244 (45%) floods, river discharge drove 162–186 (33%) floods, and NTR was the primary mechanism contributing to 111–128 (22%) floods. Further upstream at Conway (91 rkm), for the 128 identified flood days, ~ 87% were single-driver river discharge flood days and only 4–11% were compound flood days. The model further shows frequent flooding within the estuary is driven by high RSLR rates of 11.3 ± 1.7 mm/yr at Georgetown and 14.4 ± 1.9 mm/yr at Conway. By comparison, the rate of DHW rise at the coast is 8.6 ± 4.3 mm/yr. We project that by 2041, the flood frequency will increase by 249% in Georgetown and ~ 160% in Conway. Our results demonstrate the increasingly chronic nature of flooding and the need for low-lying communities along tidal rivers to account for both marine and fluvial processes, including RSLR, in future adaptation planning.