No, that’s not a photo of coastal New Jersey, post-Hurricane Sandy, below. It’s just another wet day in Mapunapuna, one of the Honolulu districts that is highly vulnerable to increased flooding due to sea-level rise (SLR), according to new research from UH Manoa (UHM).

The reason: It’s not just sea water encroaching inland, but the simultaneous upwelling of groundwater from aquifers during high waves and tides, that contributes to flooding.

By the end of this century, sea level may rise one meter. This would inundate 10 percent of a one-kilometre-wide stretch of coastal southern Oahu, scientists from the UHM Water Resources Resource Center reported in a study published Nov. 12 in the journal Nature Climate Change.

In the case of urban Oahu, up to 58 percent of the total flooded area would be groundwater from our coastal Honolulu caprock aquifer, said Kolja Rotzoll, postdoctoral researcher and lead author of the study. The role of groundwater was not factored in by resource managers, urban planners and decisionmakers trying to predict and adapt to SLR, said Chip Fletcher, study co-author and professor and associate dean of the UHM School of Ocean Sciences and Technology.

“It turned out that groundwater inundation poses a significant threat that had not been previously recognized,” Rotzoll stated in a press release announcing the study’s publication. “Finding that the inundated areas double when including groundwater inundation in coastal flooding scenarios [should alert] other coastal communities [to] use our research as the basis for conducting their own localized analysis,” Rotzoll added.

As sea level rises, so, too, does the groundwater table. The result: In some low-lying areas, groundwater “lies above mean sea level at some distance from the shoreline,” the scientists explained. When marine flooding occurs with a storm event, the watery stage has thus been readied for inland floods.