Coastal communities nationwide grapple with current and future flood impacts from rising seas and extreme precipitation. Overland impacts from fluvial and high tide flooding are immediately visible; yet these sources also contribute to the less visible rise of shallow groundwater tables. Rising groundwater is expected to flood communities from below, damaging buried infrastructure, mobilizing contaminants, and impacting mobility networks even before coastal floodwaters overtop the shore. Increased extreme precipitation will also overload natural and engineered drainage systems, limiting groundwater drainage. Compound flooding will exacerbate as all three hazards converge and disadvantaged communities do, and will, bear the disproportionate burden of these compound impacts.

Researchers, government agencies, and community partners in the San Francisco Bay Area (Bay Area) are actively assessing risk from the confluence of these sources of flooding. Four Bay Area counties - Alameda, San Mateo, San Francisco, and Marin - worked together to map the impact of rising seas on groundwater tables. Gleaning water table data from soil boring logs, water quality monitoring records, well observations collected at contaminated sites, and review of local precipitation records, researchers developed GIS layers of existing shallow groundwater surfaces that identify the highest, winter water table elevations from historic extreme precipitation events. These surfaces were then modified under eight sea level rise scenarios to represent a range of potential futures.

In parallel, the impacts of climate change on individual storm events, at the spatial scales required for decision making, were quantified using high-resolution (3km) regional climate model simulations (Weather Research and Forecasting model). These outputs simulated how the magnitude of an extreme precipitation storm can change if a similar event occurs in a warmer climate. Model projections show that the most common storm type to impact the Bay Area may increase up to 37% by 2100, further increasing the shallow groundwater table. Recognizing that these outputs are highly technical, a “guidebook” was developed that translates this forward-looking information into actionable science for practitioners to use in climate adaptation planning.