Assessment of groundwater inundation as a consequence of sea-level rise

Strong evidence on climate change underscores the need for actions to reduce the impacts of sea-level rise. Global mean sea level may rise 0.18-0.48 m by mid-century(1,2) and 0.5-1.4 m by the end of the century(2). Besides marine inundation, it is largely unrecognized that low-lying coastal areas may also be vulnerable to groundwater inundation, which is localized coastal-plain flooding due to a rise of the groundwater table with sea level. Measurements of the coastal groundwater elevation and tidal influence in urban Honolulu, Hawaii, allow estimates of the mean water table, which was used to assess vulnerability to groundwater inundation from sea-level rise. We find that 0.6 m of potential sea-level rise causes substantial flooding, and 1 m sea-level rise inundates 10% of a 1-km wide heavily urbanized coastal zone. The flooded area including groundwater inundation is more than twice the area of marine inundation alone. This has consequences for decision-makers, resource managers and urban planners, and may be applicable to many low-lying coastal areas, especially where groundwater withdrawal is not substantial.