Sea Level Rise and the March of King Tides: Why We Need to Get Familiar with This Week’s Tidal Flooding

September 30, 2015 | 11:23 am
Erika Spanger-Siegfried
Director of Strategic Climate Analytics

This week, many residents of the U.S. East Coast communities are witnessing flood waters rise with each high tide. These people are getting a taste of the future. Almost like being picked to try out some futuristic device for a few days — only this is messy, costly, and, if you realize it’s a taste of things to come, unnerving. Unwilling pioneers, in a way, these people are living on the front line of sea level rise and experiencing the periodic soaking that others don’t yet know, but will.

It started in some places on Saturday – salt water creeping onto roads and sidewalks, into basements and businesses. It continued under the supermoon eclipse. And over the last several days, we’ve had repeated demonstrations of the new reach of the tide – and we should expect it in places for several days to come.

Yes, this is a “king tide” – one of those instances when the moon exerts a slightly stronger tug on the tides than normal. It happens several times each year. This one even has a cool astronomical twist that you can read about here. But in certain affected places it is some of the biggest tidal flooding in memory. We should get used to it, says the latest science. With sea level rise, the highest tides are only getting higher, and the flooding they bring is only getting more frequent.

A sample of this week’s notable flooding

Here’s a round-up of some of the flooding seen, so far:

  • At the writing of this post, coastal flood advisories and flood watches due to “astronomical tides” were in effect in locations from Key West to Maine, including Baltimore, Washington, DC, and others in between.
  • In Miami, Fort Lauderdale, and other parts of South Florida, a third night of flooding has closed roads, flooded sidewalks and yards, threatened homes and businesses, literally brought fish onto flooded streets (see 0:58), and led some to call this the most extensive tidal flooding they’d ever witnessed. Says Miami Beach spokeswoman Nannette Rodriguez, “The tides have been higher than we predicted, so we are taking preventive measures.”
  • In Georgia, tide waters flooded Highway 80, the single access to Tybee Island.
  • In South Carolina, tide waters have already flooded coastal neighborhoods for hours at a time, covering roads, reaching into buildings, and leaving behind a rime of debris. Charleston has seen consecutive days with road closures due to flooding. (Which, though a big deal, is notably duller than last month’s king tide, which saw alligators swimming flooded streets.)
  • In North Carolina, homes in one neighborhood were completely surrounded by tidewaters that washed away fences and stairways.
  • Further up the coast, tidal flooding is expected to continue in places into the latter part of the week, a time when the soaking rains could exacerbate the situation.

This photo from Murrells Inlet, SC, was taken on Tuesday, two hours after high tide. Credit: Dave Fuss

What’s going on here?

One vital piece of this phenomenon – the astronomical piece – is just doing what it does. Stronger tides twice a month with each full and new moon. Stronger still a few times a year with the lunar “perigee” (when the moon is closest to the earth and exerts a stronger gravitational tug). And there’s that additional celestial factor, too, which amplifies the tides further every 18 or so years (Thanks, EarthSky for the great coverage).

The other vital piece though – the ocean – is simply taking up more space, and the tides have nowhere else to go but onto roads and into backyards. Here are the basics:

  • Global average sea level rose roughly eight inches from 1880 – 2009.
  • The U.S. East Coast and Gulf of Mexico experienced some of the world’s fastest rates of sea level rise in the twentieth century due to local and regional factors.
  • The frequency of tidal flooding has jumped dramatically in many locations in the past several decades, quadrupling in some places since the 1970s.
  • And with sea level rise accelerating and projected to rise roughly 6 inches in the next 15 years, and roughly a foot in the 30 years’ time (globally, with local and regional differences), you can see where this is going.



What next?

UCS released a report last fall illustrating how the tides alone, riding on higher seas over the next 15 and 30 years, have the potential to reshape how and where people in affected areas live, work, and otherwise go about their daily lives. And by causing certain areas to be regularly flooded, sea level rise has the potential to effectively claim land decades before that land is projected to be permanently underwater. We need to understand what we’re dealing with and start responding.

We found that, as sea level rises, the number of tidal floods, their extent, and their duration, all increase steeply in the overwhelming majority of our locations. In addition, new locations that are currently unfamiliar with tidal flooding join the tidal flooding front line. Here’s a sample:

By 2030:

  • Most of the communities analyzed (30 of 52 total) are projected to experience at least 24 floods per year in exposed areas, the equivalent of flooding twice a month.
  • 15 of the 52 communities can expect to see at least 48 floods a year, the equivalent of flooding four times a month. And some can expect much more flooding than this.

By 2045

  • Half of the towns (26) are expected to face more than 100 floods a year, the equivalent of eight or more floods a month.
  • 17 towns would face at least 180 floods a year, on average, or 15 floods each month, and nine towns could see tidal flooding 240 times or more per year.
  • Nearly half of our communities (23) can expect normal tidal fluctuations to bring extensive flooding.
  • And larger floods will last longer. More than one-third of our 52 locations can expect flood-prone areas to be underwater 5 percent of the year, and 5 in the mid-Atlantic area are projected to be flooded 10 percent of the time.

Fair warning

Plenty of communities are no stranger to this flooding. The video below introduces us to their experience. And plenty of others will be joining their ranks as sea level rises. This is not a matter of speculation.  It’s a matter of the steady (and accelerating) march of high tide, and it’s really only a question of what’s in its path.  With the increasing frequency and reach of tidal flooding in recent years, we’ve had fair warning.  So this week, as we trade supermoon photos and gripe about the limitations of smartphone cameras, keep an eye on those tides. It’s not every day you get a glimpse of the future.