High Water
Posted on October 27, 2015 |
We’ve already mentioned that Miami is suffering more flooding because of sea level rise. The overall elevation there is so low, and the sea level rise big enough, that you don’t need a storm to get local flooding — a very high tide (the kind that occurs annualy, sometimes called “ perigean spring tide“) can do it.
Multiple studies have shown that this isn’t just happening in Miami, it’s happening all over the U.S. The effect is strongest along the Atlatic coast and Gulf of Mexico. The most common type is called “nuisance flooding,” which can close roads, overload storm drains, and threaten infrastructure. In Miami, it also brings about saltwater intrusion into the water table, which is more than just a nuisance, it’s a very serious problem.
Nuisance flooding generally occurs when sea level rises a certain level above mean higher high water, also called MHHW. MHHW is the mean, over some reference period (usually the 19-year period from 1960 through 1978), of the higher high tide, i.e. the daily high water mark. The level above MHHW required to bring about local flooding varies from place to place, but is generally around 0.3 to 0.5 meters (about 12 to 20 inches).
It’s close to twice the standard deviation of daily high water level, therefore I decided to take hourly tide gauge data from the Joint Archive for Sea Level, then compute the mean and standard deviation of daily high water level from 1960 through 1978. This enabled me to compute a “minor flood” threshhold as the mean plus twice the standard deviation. Finally, I tallied the high water level for each day of record, in order to estimate how often various locations in the eastern U.S. have been suffering local flooding, and take a look at how that has been changing over time as well as how it’s likely to change due to future sea level rise.
Here, for example is the number of days per year above the flooding threshhold for Atlantic City, NJ:
The red line is a smooth (by “modified lowess smooth”), the blue line is a Poisson regression fit. Statistical significance of the regression is so great (p-value less than 10-15) that there’s no doubt whatever, flooding in Atlantic City is on the rise. Do be aware that these counts (of days above flooding threshhold) include major floods, and those caused by storms, as well as nuisance flooding from large tides.
The bottom line is that what used to happen quite rarely during a typical year in Atlantic City, then started happening a half dozen times a year or so, and is now occurring dozens of times a year on average. And this is just the beginning; as sea level continues to rise (which it will), the number of flood days in Atlantic City will increase even further.
Don’t let the name “nuisance flooding” fool you. It’s more than just a nuisance, making roads impassable, backing up storm drains, seriously hurting local businesses, actually threatening important infrastructure, and in some places (like Miami) it’s a major threat to the local water table. Just because it doesn’t threaten to cause local chaos or immediate loss of life, doesn’t mean it won’t cost. Big. In Miami, they’re already spending hundreds of millions of dollars trying to deal with it.
Hundreds of millions — and that’s just the beginning. When they’re done with their hundreds-of-millions-of-dollars pumping station installation it won’t be enough, because sea level will already be higher than it is now.
Sea level is already so high, that in places extreme tides can bring more than just nuisance flooding. You might wonder, for instance, how often sea level has been rising, not two, but four standard deviations above MHHW. In most locations that’s not happening very often yet. But the time is comming — within a few decades — when it will. That’s the kind of thing that brings major disruption to a metropolitan area, especially one at low elevation like Miami.
But, some places don’t even have to wait. In Galveston, TX, they’re already seeing water levels rise four standard deviations above MHHW nearly three times a year:
Yes, the water level is getting that high almost three times a year on average.
Anybody who thinks that sea level rise isn’t a major, extraordinarily costly threat from man-made climate change, is a fool. For that matter, anybody who thinks it isn’t a major problem already is sorely mistaken.
UPDATE:
More-than-just-nuisance flooding seems to have struck again.
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