Accumulating climate change influences on extreme coastal, fluvial, and compound flooding in the upper transition zone

The increasing frequency and magnitude of flooding driven by climate change requires a thorough understanding of future flood hazards to inform comprehensive mitigation strategies. Traditional analyses often study coastal and fluvial flooding in isolation, not enabling understanding of compound flooding, nor the accumulation of climate change influences (CCIs) that affect multiple flood drivers. In this study we introduce a simplified hazard and climate change assessment framework and apply it to study flooding for Eastwick, a neighborhood in southwest Philadelphia at the inland limit of estuarine-riverine systems, termed here the upper transition zone (UTZ). Utilizing a validated coupled watershed model and two-dimensional flood model, we assess the impacts of individual and combined changes in flood drivers (changes to mean sea level, precipitation, and storm surge). Climate change effects on flood hazard are quantified through flood modeling for 100-year coastal, fluvial and compound events with present-day, mid- and late century time horizons. Our results demonstrate how the present-day distinctiveness of flood characteristics across the three flood events declines as sea level rise becomes prominent later in the century throughout the UTZ. Our results also demonstrate future increases in flood extent and depth can be significantly underestimated if combined CCIs are not considered. Moreover, CCIs accumulate in depth and area in the floodplain where Eastwick lies, instead of traveling further up the adjacent steep tributaries. This study presents a simple, conservative framework to study extreme flood hazards with multiple drivers and demonstrates how multiple CCIs can combine to worsen future flooding.