One foot at a time: Taking measures of rising seas

The School of Ocean and Earth Science and Technology at the University of Hawaii has an extensive section of its website devoted to the issue of rising sea levels (http://www.soest.hawaii.edu/coasts/sealevel/).

Part of the discussion on that page contemplates how Oahu might function based on scenarios of sea-level rise sketched out in a paper presented to the National Academy of Sciences in 2009. It projects the experience of 1-, 2- and 3-foot rises in sea level, forecast to arrive, respectively, sometime in 2040-2050, 2050-2070 and 2070-2090.

Here are some of the observations, culled from the 2009 report, summarized on the UH site:

» A 1-foot rise — Expect "high water tables, standing pools of rainwater that stubbornly refuse to dry and backed up storm drains when it rains and tides are high."

Erosion rates will vary, but "many areas that formerly had beaches will be lined by large seawalls unless communities make specific plans otherwise," according to the summary. "High value streets may have installed drainage gates and one-way flow vents that close the storm drain system to saltwater intrusion on a schedule tied to the tides."

» A 2-foot rise — Most coastal buildings will likely be inhabited, with people adapting their comings and goings to the road conditions, as businesses in low-lying Mapunapuna already do, on occasion. Some key road intersections and arteries will be adapted with raised roadbeds or low-profile bridges.

Because of flooded conditions, according to the summary, "Waikiki is in decline as the gateway for tourism. New investment by the visitor industry will probably be focused on any remaining healthy beaches on Oahu such as the Nanakuli, Waianae, Mokuleia, Kailua and Waimanalo districts.

"In McCully and Makiki, residents won’t see seawater; they will see the wetlands of the 19th century re-emerging as the water table rises above ground level in some areas."

» A 3-foot rise — Abandonment of specific buildings and entire blocks will be made, according to the UH website.

"Highly valued regions such as central Honolulu and portions of Waikiki may still be traversable because the groundwater table will be pumped by state-sponsored efforts funded by user fees and assessments," the summary continues. Beaches may still exist but are likely only in places where coastal development has been specifically formulated to capture sand and promote beach accretion."

The combination of flooding and groundwater rise "makes the makai quarter-mile of the coastal plain an unsafe location for development."