Detection, Classification, and Characterization of Compound Coastal Flooding along the Gulf and Southeastern U.S. Coasts

Samrin S Sauda , Manzhu Yu

Compound coastal flooding, driven by the concurrent or sequential exceedance of storm surge and river discharge thresholds, poses disproportionate risk to Gulf and Southeast US coastal communities, yet systematic observation-based characterization of its spatial distribution, typological structure, and joint intensity remains limited. This study investigates compound coastal flooding using a nonstationary threshold-based detection framework applied to 72 paired USGS-NOAA gauge stations over 2010-2024. Station-level flood events were identified and classified into five mutually exclusive types based on driver exceedance and surge-discharge timing lag: surge-only, riverine-only, compound-synchronous, compound-lagged, and compound-antecedent events. Results show that surge-only events dominate the regional flood record, accounting for 58.7% of events, followed by riverine-only events at 31.4%, while compound events represent 9.9%. Compound flood occurrence varies strongly across sub-regions, with the highest compound fraction observed in the Pascagoula-Mobile-Tombigbee region and distinct lagged and antecedent behavior in the Florida-Panhandle and Southeast Atlantic Coast, respectively. Annual compound fractions show pronounced interannual variability, with elevated values during active or impactful hurricane seasons. Compound events also persist substantially longer than surge-only events, indicating greater cumulative exposure. Overall, this study provides an observation-based, physically interpretable framework for identifying compound coastal flood typologies and demonstrates the importance of incorporating event timing, joint intensity, and regional hydrologic controls into coastal flood risk assessment, early warning, infrastructure design, and adaptation planning.