Wednesday, April 23, 2014

Coastal inundation: it's the surge, not just the tide.

What’s clear to emergency managers and may not be clear to the public is this: when worrying about sea level change, it’s not the highest tide but the storm tides that should concern you.

In low-lying parts of Honolulu, as at Mapunapuna, high tides already flood streets. Annoying but not catastrophic.

Put a storm surge on top of that, and you start having real problems. Put a storm surge on top of the anticipated sea level rise in the next few decades, and low-lying areas can become uninhabitable. And finally, put a storm surge on top of sea level rise, PLUS regional sea level oscillations, and you have something even worse.

A recent study of New York’s experience shows that flooding threat has increased significantly. 

The American Geophysical Union reports that “Maximum water levels in New York harbor during major storms have risen by nearly two and a half feet since the mid-1800s, making the chances of water overtopping the Manhattan seawall now at least 20 times greater than they were 170 years ago.”

The study on which that statement is based is here

The abstract says: “Three of the nine highest recorded water levels in the New York Harbor (NYH) region have occurred since 2010 (Mar. 2010, Aug. 2011, and Oct. 2012), and eight of the largest twenty have occurred since 1990.”

There, they’re talking about a combination of a regional sea level oscillation, plus climate change-related sea level rise, plus storm surge—the terrible trio for coastal zones.

The result is that in any given year, the chance of water overtopping Manhattan’s seawall is up from 1 percent to between 20 and 25 percent.

Those kinds of conditions can occur in the Hawaiian Islands as well, threatening coastal roads, coastal homes, coastal businesses, coastal utilities and potentially turning coastal lowlands into saltwater swamps.

© Jan TenBruggencate 2014

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