High Water

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.


More-than-just-nuisance flooding seems to have struck again.

26 responses to “High Water

  1. How clumpy is the flooding? In the 1950’s Atlantic City had about 6 days per year, or every two months. Now it is around 50, or almost once a week. But I would guess it doesn’t really flood once a week. Rather I suspect they now get a few successive days of flooding where they once had single day floods. How have the frequency and duration of flood events changed?

  2. The variability in your graphs appears to increase with time as well, I see this most clearly in the Atlantic City graph. Any ideas why? Perhaps I’m seeing something that is not really there.

    [Response: As sea level rises, the probability of flooding increases. As long as it remains low, the number of days will *roughly* follow a Poisson distribution, which shows greater variance when the mean is greater.]

  3. W Scott Lincoln

    Were no NWS-surveyed elevations for “minor” and/or “major” flood available at any of the coastal locations?

  4. More than just a nuisance. Driving through saltwater rusts everything on the car. Simple maintenance procedures like bleeding brakes becomes a nightmare due to rusted valves. And there’s the body damage. I would guess the damage to cars and infrastructure from corrosion is very costly.

  5. Atlantic City is the inspiration for the board game Monopoly. The real-world Boardwalk district might depreciate in value as ocean level continues to rise.

  6. Good for GDP then :) we can spend our way out of this. New pumps, new dykes, new water infrastructure, mechanics will go gangbusters with all those flood damaged cars etc. You guys might have unwisely saved otherwise… now you’ll have to work and spend more, which is great news for the economy. What can go wrong, pumping the Atlantic into the Atlantic ! /s

    Time to go long pump manufacturers and civil contractors if this is the response :)

  7. A skeptic may argue that some or much of the sea level rise is due to subsidence, which we can do little about (some negative vertical land movement is apparently caused by anthropogenic draining of aquifers in the Atlantic City area, too). But they don’t seem to be able to recognize that rising sea level exacerbates increased inundation due to these factors.

  8. If my quick data download, import into MATLAB and simple linear regression didn’t play me wrong, local sea level rise of ~42 cm in a century accounts for almost all of the increase in Atlantic City ‘nuisance’ flooding. That said, I don’t fully trust my eyeballs to spot and quantify extremes and outliers in nearly 1 million hourly data points.

  9. David B. Benson

    Atmospheric carbon dioxide levels are now in the range of the late Miocene period. At that time the sea stand was over 40 meters higher than today.

    That sea level rise will occur although it will take some time.

    So do not just abandon Miami but all of Florida.

    • At Climate Central http://sealevel.climatecentral.org/ they have new sea level rise maps. These max out at about 5 meters of sea level rise. Why don’t they mention the possibility of these much higher rises?

      We are left betting big that Hansen is incorrect. That has not been a good bet in the past.

    • “That sea level rise [40+ meters] will occur although it will take some time.”

      Note that “some time” in this case means centuries to millennia. So while our very distant descendants will presumably be living with 40+ meter sea level rise, many generations of Floridians could be born, grow old, and pass away before that happens.

      Sea level rise is an urgent issue for communities located right on the coastline. It’s not particularly urgent for communities further inland.

  10. I like that analysis. There will probably be a lot of effects that begin in a somewhat analogous way to this–nothing spectacular, just incremental changes that seem small at first, and ‘sneak up’ as we get to steeper parts of the curve. And that cost progressively more and more.

    Hmm. Maybe I *don’t* like that analysis. Though I still think it’s a good one.

  11. Andy Lee Robinson

    See Florida, before Sea Floorida…

  12. “A skeptic may argue that some or much of the sea level rise is due to subsidence‚Ķ”

    They may, and they do. But in doing so, they ignore satellite altimeter data (and very, very basic physics.)

    • Not sure if they ignore the satellite data for global sea level, but when one mentions a particular location, they often google ‘subsidence’ with location X and tell you if the land has been sinking (as is the case with Atlantic City). Their blind spot is that they don’t perceive that rising sea levels worsens a condition we can do little about, and that this makes the problem we can do something about more urgent. All part of the ABC (anything but carbon) conga line: the myopia of AGW denial.

    • All sea level is local. Let’s see that again: All sea level is local. They hammered this into my head in the geology classes I took at UT-Austin in the Noughties.

      Factors lowering [local] sea level:
      – isostatic rebound of the crust (upward) from the mass loss since the last ice-age is ongoing (e.g. Scandinavia, US Atlantic coast down to about Delaware)
      – loss of gravitational pull from accelerating loss of ice mass lowers regional sea level (Greenland and Antarctic ice caps)

      Factors raising [local] sea level:
      – total expansion of ocean water from heating and accelerating melt
      – deltas slumping, especially after the fluvial silt supply is cut off (Louisiana coastline, Nile delta, lots more)
      – isostatic rebound (downward) of crust on the periphery of glacial extent (Maryland)
      – slowing of mid-sea currents (a strong Gulf Stream creates a warm ridge that draws water to it)
      – local water and fossil fuel extraction that causes subsidence (Venice, Houston)
      – acceleration of delta loss due to channeling of marshes (Louisiana, Nigeria)
      – sustained/increased wind flow across a long fetch (Western Pacific)

      – local geological behavior (Earlier confusion about historic sea level markers on ancient Roman pillars in Naples has been explained by the discovery of a regional magma chamber.)

      Maryland is hurting: https://www.youtube.com/watch?v=RCc3C89qxOM

      [Response: Changes in the gravitational field due to melting of the Greenland ice sheet lowers sea level around Greendland, but raises it at other regions, including the U.S. Atlantic coast.

      And the real reason that sea level is rising along *most* of the coastal regions of the world, and especially the continental U.S., is the thermal expansion of seawater and melting of land-fast ice, a.k.a. global warming.

      The reason oceanographers focus on the other factors is that they want to understand the process completely. The reason climate deniers focus on them is that they want to increase doubt, so they can blame sea level rise — both local and global — on “ABC” (Anything But Carbon).

      And as Doc Snow aptly points out, sea level rise is global. It might be useful to hammer “all sea level is local” into the heads of geography students, but in the context of global climate and impending danger (and reality), it just ain’t so.]

      • TIDES and various chart datums (e.g., usually MLLW on US charts). are local. The worldwide trend in sea level is global. You are simply conflating weather and climate in a new area.

  13. Frankeinstein

    How to boil a frog: put it in a pot of cold water and raise the temperature very slowly…

  14. While I agree that sea level rise is an issue I’m wondering whether it could play a factor that a city grows. So maybe part of the increase is due to to a bigger city could be flooded more often or more area is more directly exposed to the sea.

    • The parts of Miami Beach that are most flood prone were developed early in the city’s history. It’s been developed vertically and grown in population, but it’s a pretty small island, so the limits of the development haven’t really changed much since the early 1900s.

    • Bernard from Australia

      The extent of urban area affected and damages cost is directly tied to development – especially irresponsible development on land that is pretty much guaranteed to flood regularly – I’ve seen some shocking examples here in Australia, including one where single-story dwellings were approved for land that was more than 4m (13ft) *below* the 1-in-100 year flood level, in an area where the difference between ‘minor’ & ‘major’ flooding is about 2m (6.6ft) (and said dwellings went under 4-5m of water a couple of years of being built).

      Having said that – the analysis Tamino presents above is just looking at levels, not the extent of urban area flooded or the cost of damages, so it’s independent of human development.

    • In practical terms–and this is a general comment, not a specific response to Fabian’s question about impact on the analysis–it’s usually not a binary thing that SLR ‘is/is not’ responsible for whatever disaster occurs.

      Take Katrina, which AFAIK has not been formally attributed to climate change, but which nevertheless was observed to intensify rapidly upon passing over unusually warm surface waters. If you grant a role to climate change in that reality, you still have to acknowledge the failures of policy and engineering in creating the disaster that decimated New Orleans.

      And that’s typical. Accident analysis usually reveals multiple contributing factors–for a simple example, if you step out into a traffic lane without checking traffic, that’s an error that could be a contributing factor. But there won’t be an accident unless there’s an oncoming vehicle, which is a second contributing factor.

      The denialist tactic, of course, is to seize on the contributing factors that *aren’t* related to climate change and insist that they are the ones that are ‘really’ responsible. As usual, it’s ABC–“anything but carbon.”

  15. Minor quibble. Miami and Miami Beach are separate cities on opposite sides of Biscayne Bay that face different threats.

    Miami Beach is the low-lying barrier island whose streets and shops are flooded with seawater. They are the city that has spent hundreds of millions of dollars to deal with the problem.

    Miami is on the other side of the bay and generally at higher elevations. They don’t currently experience any significant seawater flooding. They do face threats from saltwater intrusion and the inability to drain away heavy rains, which are exacerbated by sea level rise, but responsibility (and the bill) for addressing those issues falls to other entities, so it’s not exactly true that Miami has spent hundreds of millions combating sea level rise.