Florida Flood

A new paper (press release here) about sea level rise in southeast Florida concludes that locally, sea level rise has accelerated recently. What particularly caught my eye was the magnitude; from the abstract:


The average pre-2006 rate is 3 ± 2 mm/yr, similar to the global long-term rate of SLR, whereas after 2006 the average rate of SLR in Southeast Florida rose to 9 ± 4 mm/yr.

I must confess, I’m somewhat skeptical.


Naturally I went to get some data myself; there are a number of tide gauge stations in the area:

FLstations

The green arrow marks the station at Virginia Key, which (at 20 years) is the longest record which is also in the Miami Beach area. Some of the records don’t cover the time of interest, and a couple are outside the region of interest (Naples and Ft. Myers aren’t southeast Florida); I ended up with 7 stations which might inform how sea level has changed in the area.

This is a place where the annual cycle is pronounced, so that had to be removed. Then the station records had to be aligned. When I do so, the individual stations line up nicely:

SEFLstations

Now we can form a composite estimate of the sea level in southeast Florida, and smooth that data for a better image of what the trend looks like:

SEFLsealev

There has indeed been a recent uptick, you can certainly call it an acceleration, but I doubt that it’s the kind of acceleration in the long-term trend that will persist. After all, we’ve seen upticks like this before which didn’t last (late 1940s, early 1970s). It’s the nature of local sea level to show those ups and downs — and global sea level can, too, given the short-term impact of things like el Niño, and its consequent shifting of water from continents to ocean and back.

I’m also highly skeptical about their identification of 2006 as the time at which the sea level trend changed. I can fit a continuous piecewise linear trend to the data with its trend break at 2006, but I can also find a better trend break time by changepoint analysis, which suggests 2011:

SEFLbreaks

This is borne out by looking, not at monthly data, but annual averages:

SEFLannual

The pre-2011 rise rate is 2.4 mm/yr, while the post-2011 rate is a whopping 19.2 mm/yr! We certainly don’t expect that kind of sea level rise to continue in the near future, in southeast Florida or anywhere else (at least, not unless something drastic happens like the breakup of the ice sheets). As for later this century …

Despite my misgivings about some of the stated numerical results, the problem of flooding in Miami Beach is undeniable. I estimated the ocean height due to sea level trend and tidal variations alone (removing the weather influence of storms, wind, rain), and got this for the Virginia Key location:

TidalHeight

The red line, marking the flood level (due to tide alone), is a crude approximation but it gives the right idea. Starting around 2006, southeast Florida entered the regime in which flooding not only can, but will occur at high tide in October. As the sea has continued to rise, the amount by which high tide exceeds this (crude) threshhold has gotten bigger:

FloodHeight

The key question of course is: what will the future bring? Tidal variation suggests 2016 will be another bad year for flooding in southeast Florida, 2017 won’t be so bad, then the bad years will resume. Sea level fluctuation is an open question in the short term; it may actually decline (oh so slightly) in the next year or two. But in the long term, the sea will continue to rise and make the problem worse, far worse, and extraordinarily worse. As for the rate at which that happens, it may be faster in southeast Florida (with weakening of the ocean’s overturning circulation) but it’s too early to tell, and I very much doubt the stated 9 mm/yr rise rate is the proper forecast.


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15 responses to “Florida Flood

  1. Not cheerful news for my brother and sister-in-law in Kendall, at about 6 ft. above sea level–or any of the other 2.6 million living in Miami-Dade, for that matter.

  2. The interesting question is why the increase in the last few years? It might relate to large-scale circulation as exhibited by the Gulf Stream, or is there any land subsidence around Miami?

    • Dunno for sure about the land subsidence, but I doubt it, at first blush. Reason: the leading cause of quick subsidence issues in lowish latitudes seems to be groundwater depletion by human pumping. But that AFAIK isn’t what happens in south Florida, because the porous limestone bedrock allows ocean water to seep in to replace the fresh. So the concern around Biscayne Bay is mostly about saltwater intrusion, not subsidence.

      Open to comment by those more knowledgeable than I am…

  3. Hay and Mitrovica indicated above average SLR for the east coast.

  4. Mike Ellims

    Possibly gulf stream being blocked by all the cold water coming off Greenland and backing up of AMOC. I seem to remember a comment by RADM David Titley in one of his youtube talks to that effect. Possibly this… https://www.youtube.com/watch?v=7udNMqRmqV8

  5. Part of that acceleration may be due to changes in the Gulf Stream, such as slowing (which has been happening). Plus, the long-term climate change should be moving the interface between tropical trade winds and temperate westerlies farther north; for areas near the boundary, that means more frequent easterlies, hence higher seas for cities on any east coast of a continent at the critical latitudes.
    So I wouldn’t necessarily discount the possibility that the change is short-term.

  6. michael sweet

    I live in Florida so this data is very interesting to me. I have noticed in some of your last posts on sea level rise that it seemed that sea level had risen faster in Florida recently. I presume that the increase is not statisticly significant since you do not give confidence intervals and the time period is so short. We can only hope that it slows down again soon.

    Your tabulation of tidal flood height is very interesting. Regualr floods of 100 mm are significant in a location as flat as Miami. A storm surge of even a few inches at high tide would be very serious. Septic tanks, storm water ponds and other drains lose their gravity head. I have heard of a restaurant where the toilets do not flush at high tide. Many locations (like the Turkey Creek nuclear power station) have slopes of less than one foot per mile of run inland.

    I have also seen suggestions that a slowdown of the Thermohaline circulation would cause sea level to rise on the US East coast. Perhaps this is a sign of that slowdown. We will have to see if the effect is permanent or just a tempory fluctuation. If Greenland is melting it may persist.

  7. John McCormick

    From here on out, our future and that of next generations will be controlled by the expanding North Atlantic cool pool. More heat, more Greenland melt, larger cool pool. The feedback some feared most. No way to prepare and no way to reverse the stalling Gulf Stream. This is the onset of abrupt climate change in Eastern Northern Hemisphere.

  8. The press release you cite points out:

    “The study also provides new evidence that connects the weakening of the Gulf Stream with sea-level rise along the US Atlantic coast”

    Evidence that the Gulf Stream is weakening, not predicted by modelling, i.e. we don’t know why, has been pouring in.

    Apparently if there were no Gulf Stream sea level on the US east coast would be 1 1/2 meters higher.

    9 mm per year sea level rise may be lower than what’s in the cards for the next few decades….

  9. As others have commented, there have been numerous studies in recent years connecting the melting of Greenland, the slowing down of the Atlantic Overturning Circulation, and increased sea level rise on the US East Cost.
    The gist of Krasting et al is that the Atlantic may see up to 3x the global rate of sea level rise for several centuries. A sample of recent papers (some modelling, some observational):

    Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation, Rahmstorf et al
    http://www.nature.com/nclimate/journal/v5/n5/full/nclimate2554.html

    Enhanced Atlantic sea-level rise relative to the Pacific under high carbon emission rates, Krasting et al
    http://www.nature.com/ngeo/journal/v9/n3/full/ngeo2641.html

    Enhanced warming of the Northwest Atlantic Ocean under climate change, Saba et al
    http://onlinelibrary.wiley.com/wol1/doi/10.1002/2015JC011346/full

    North Atlantic meridional overturning circulation variations from GRACE ocean bottom pressure anomalies, Landerer et al,
    http://onlinelibrary.wiley.com/doi/10.1002/2015GL065730/full

    A couple of recent blogs:
    https://www.wunderground.com/blog/lpaocean/us-east-coast-sea-level-and-the-gulf-stream (March 9 2016)

    https://robertscribbler.com/2015/08/26/signs-of-gulf-stream-slowdown-sea-level-more-than-a-foot-higher-off-us-east-coast/

  10. Then there’s the whole ‘extreme precipitation’ thing. Seems to be biting Houston just now:

    http://www.cnn.com/2016/04/18/us/houston-texas-flooding/index.html

    Oh, well, at least Texas isn’t in drought.

  11. michael sweet

    Does anyone know how 9 inches of sea level rise affects areas several miles inland that are still less than 10 feet above sea level? It seems to me that severe rain like Houston got yesterday (20 inches in some areas and still raining) would drain more slowly when the sea into which it drains is higher. This would affect inland areas long before they were inundated by sea level rise. For areas less than 10 feet above sea level they have lost 10% of their gravity head. How much sea level rise is needed before the draining effect is noticable? How far inland can it propagate? (Sacramento, capitol of California, is 90 miles inland and still has 3 feet of tide.) Has anyone seen a paper on this?

    • I haven’t, and don’t know much about this. However, I do note that the Manning equation, which is used “to estimate the average velocity of water flowing in an open channel”, has no term for hydraulic head. The flow rate depends upon roughness of the surface over which flow occurs, and of the slope.

      https://en.wikipedia.org/wiki/Manning_formula

      Which would suggest that your idea may not be correct until the head begins to approach zero.

      • Michael Sweet

        Doc,
        Thank you for referencing the Manning formula. Reading your link, the formula includes slope-1/2 power. The slope includes the head (h/l) so the flow will be affected immediately. If the head changed from 6 feet to 5 feet that would be about a 10% lower flow. Since tidal changes affect the head in low areas and the depth of the water increases the head as the flood gets worse a trained engineer is in order.

        [Response: If h changes due to sea level rise, so will l. The slope won’t change.]

      • michael sweet

        In urban areas the drains usually have a fixed ending at a larger body of water so they cannot retreat in length. As head decreases the slope decreases. Look at Fort Lauderdale images. Houses are built up to the ends of the drains. The drains stay the same length as sea level rises until the water floods the houses. There are some areas where the land can retreat, but in urbanized areas the drains do not get shorter as the sea level rises.

        This post at Wunderground: https://www.wunderground.com/blog/lpaocean/houston-floods-and-water-levels–no-downhill-in-houston shows the drains in Houston rising during recent rains. It does not comment on sea level rise affectiong the drains, but since they are tidal the water is deeper from sea level rise.

        Much of Florida has retention ponds where water drains during rain. If sea level rises, the water level in ponds near sea level rises and their capacity for water decreases. If the rain is hard enough to fill the ponds that would cause flooding. Less rain is needed to overtop the ponds with sea level rise.

        I am not a water engineer, perhaps I misunderstand the situation.