Saturday, October 13, 2018

The Tide's They are a Changin'

(apologies to Bob Dylan)

Sea levels are rising. That’s nothing new.

They rose over 400 feet at the end of the last glacial epoch over 10,000 years ago. Over the last 2,000 years, sea levels increased about 1.5 feet, or about 0.00025 mm per year. A really small number. Over the last two decades though, the rate increased to 3.2 mm per year, about 1,300 times faster than it was just a century ago. 

Still these numbers seem really, really tiny and you are tempted to say, so what?

Well, overall, U.S. sea level has risen about 0.7 feet since 1920. During this same interval, coastal tidal flooding frequency has gone from 1 or 2 a year to 20 per year. It’s double what it was just 30 years ago. That’s what.

Sea levels are actually increasing faster along the east coast of the US than elsewhere. The reasons include a slowing Gulf Stream, which causes water to pile up along the eastern seaboard; thermal expansion of warming waters, and changes in the distribution of gravity around the planet due to melting ice sheets which alone may add about a foot to east coast sea level rise by 2100.

This information comes not from a science fiction movie, but from a detailed 2017 report produced by NOAA (NOAA Technical Report NOS CO-OPS 083).  It’s a no kidding, here’s what’s gonna happen folks, report.

An axiom of engineers is that any problem can be solved with the proper application of money. Of course, there’s the flip side. Is the solution worth the cost? Boston recently decided the answer is no.

A study released last month recommended that the city NOT build a flood protection barrier at the outer edge of Boston Harbor. The barrier, which would have been closed to protect the city from storm surges caused by Nor’easters or hurricanes, would have cost over $10 billion and taken about 30 years to build.

The reason for not building the barrier was not because the study’s authors thought sea levels are not rising - they do - but because they thought it wouldn’t be cost effective and not help with the sorts of day-to-day nuisance flooding that Boston and other cities are now coping with.

Another big issue with the barrier was that it could very well be obsolete by the time it was completed. If we could slow or reverse greenhouse gas emissions quickly, sea level rise may be limited, so the barrier would be a waste. On the other (and more likely) hand, if trends continue as they are, the barrier might be ineffective by the time it was completed.

Last, if Boston built a barrier, any deflected storm surge would just go somewhere else, like Beverly, Cohasset, Manchester, or Scituate, for example. I imagine that the legal battles alone could prevent the first load of concrete from even being poured for 30 years.

Instead of a barrier, Boston has decided to address this issue using strategies right along the coast line, detailed in a plan called Climate Ready Boston, which, if implemented, would protect critical infrastructure and housing in low lying areas using a variety of strategies, including local berms and floodwalls. In East Boston, protecting against a 100 year flood (1% chance in a given year) combined with 21 inches of sea level rise by 2050 would prevent $1.3 billion in losses . . . from just one flood.

Boston is taking the issue of sea level rise very seriously. In the year ending April 30th, the city experienced 22 days of tidal flooding, breaking the record set in 2009. During last January’s Nor’easter, the city had the highest tide ever recorded, which flooded downtown streets. The city expects these types of floods will be the norm in coming years.

Boston is not the only city trying to cope with this issue. In Miami, Florida, and Norfolk, Virginia (home to the US Navy’s largest base), residents now routinely wade through knee deep water during peak high tides. They weren’t 20 years ago.

NOAA is projecting that this year the frequency of such events will be even higher as we head into another El Nino year. The last one ended barely 2 years ago.

There is old saying which goes, you can’t hold back the tide. It’s truer than ever these days.

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