Compound Inundation Impacts of Coastal Climate Change: Sea-Level Rise, Groundwater Rise, and Coastal Precipitation

The importance of considering the compound effects of multiple hazards has increased in recent years due to their catastrophic impacts on human lives and property. Compound effects correspond to events with multiple concurrent or consecutive drivers, e.g., heavy storms, coastal flooding, high tides, and sea level rise (SLR). There is a recent evidence on inundation caused by SLR-driven groundwater rise, and there is a distinct knowledge gap in understanding the compound inundation effects of this phenomenon considering the important hydrologic and hydraulic considerations under compound events. To fill this knowledge gap, we developed a novel analytical framework to understand the movements of the surface flow under typical precipitation events considering their interaction with uprising groundwater and SLR in a coastal watershed located in Oakland Flatlands, CA, USA, home to several disadvantaged communities. This modelling approach simulates the dynamics of compound flooding in two dimensions of the earth surface in a fine resolution, which is critical for devising proper flood management strategies. The reason to focus on disadvantaged coastal communities is that such communities typically encounter disproportionate environmental injustices due to the lack of sufficient drainage capacity in their infrastructure. Our results show that by considering the compound effect of SLR, groundwater inundation and precipitation flooding, the drainage capacity of infrastructure will be substantially exceeded, such that over 700 acres of the built infrastructure could be flooded. This is a considerable increase compared to scenarios that do not consider compound effect, or scenarios that consider inappropriate combinations of driving factors. In sum, our results highlight the significance of considering compound effects in the coastal inundation analyses, with a particular emphasis on the role of groundwater rise.