OCT 2020/Brett Walton, Circle of Blue/
Build a wall?
The biggest source of flooding linked to sea-level rise for Hawaii’s capital city comes not from the sea itself. It comes from underground.
It seems like an intuitive response for protecting a coastal city from rising seas. Just raise your external defenses. But the intuitive response, in certain cases, is also the wrong response, says Shellie Habel.
If the plan to prevent flooding in a city like Honolulu were to be simply block the ocean, “it’s not going to work,” Habel told Circle of Blue. Water has other, less obvious ways of invading, and that stealth movement has implications for water pollution and transportation in Hawaii’s largest city.
Habel is a coastal geologist with the Hawaii Department of Land and Natural Resources and Hawaii Sea Grant. She is the lead author on a study that investigated three flooding pathways in Honolulu, all of which are a consequence of rising seas. The study is the first to attribute the percent of flooded area in the city that is due to each factor individually and in combination. The likelihood that floods will come from multiple pathways simultaneously — and cause more infrastructure damage — rises along with the seas.
In Honolulu’s case, a sea wall would be a losing strategy against sea-level rise because a wall would not address the most serious flooding problem. The largest source of inundation for the city’s roughly 350,000 residents is not the ocean. It’s groundwater.
Habel and her colleagues found that little more than 2 to 3 percent of flooded area in Honolulu’s urban core is a result only of overland marine flooding. That range holds for the four sea-level elevations that the study looked at.
Groundwater flooding, by contrast, is the predominant individual source of flooding. How does this happen? Groundwater in the coastal region is hydrologically connected to the ocean. When the Pacific swells, so does the inland water table.
A hypothetical wall on the Honolulu waterfront would fail to prevent flooding, Habel said, because water would still bubble up from underground and come through backed up storm drains, which are the third flooding pathway. Groundwater flooding today swamps basements and roadways in low-lying areas and is noticeable in underground parking garages.
“So even if you put in a sea wall, even if you change the type of drainage management, you still have flooding,” she said, adding that a solution that only addresses marine flooding will not work because 97 percent of the total area that is projected to flood will still flood from rising groundwater and storm drain backups. Places like New Orleans and cities in the Netherlands, which also have groundwater flooding problems, employ pumps for that purpose. “In order to mitigate all the flooding you have to adapt to all three pathways.”
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