Encroaching Tides: How Sea Level Rise and Tidal Flooding Threaten U.S. East and Gulf Coasts Communities over the Next 30 Years

Today UCS is releasing a report that outlines steep increases in the frequency, extent, and duration of tidal flooding for communities along the U.S. East and Gulf Coasts. Riding on higher seas over the next 15 and 30 years, the tides alone have the potential to start reshaping 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.

Many of us, including many of us actively concerned with sea level rise, tend to be of this mindset: as long as major storms miss us, or else strike at low tide as Hurricane Sandy did in Boston, doing little damage, then we won’t have to deal with the impacts of sea level rise for many decades.

But thousands of others up and down our coasts – people like my uncle, who lost his truck to repeated trips through tidal flooding; my mother, who sees the main road near her Massachusetts home flood with the tides; my friend, who helps document extreme tides in Portland, Maine – people who see the increased reach of the tides up close, would tell us differently.

Scan a map of the East Coast and there will be communities on the tidal flooding “front line” in every state. Miami, Savannah, Charleston, Norfolk, Annapolis, Atlantic City, Jamaica Bay, all have stories to tell, and many more. This week, as they witness some of the highest tides of the year, some of these communities are bracing for disruption.

Along our highly developed coasts (to loosely quote the National Climate Assessment, no other region but our coasts concentrates so many people and so much economic activity on so little land, while also being relentlessly affected by the sea) the sea is closing the gap between harmless high tides and tides that cause flooding, both minor and more extensive flooding. Cities familiar with flooding may appreciate this threat, but it represents a new chapter for many others – and all of these communities need to prepare for the changes in store.

Tidal flooding: A daily cycle gains disruptive force

For a quick overview of the mechanisms of tidal flooding, see the report, or this blog post. The important thing to note here is that there is usually a lot of variability in the height of the tides, and as we increase sea level, we nudge the higher tides higher still. Increasingly, those tides can reach levels that cause coastal flooding. (Our friends at NOAA helped draw attention to this with a great report this summer about the sharp increases in tidal flooding over the last 40 years.)

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 5 inches globally in the next 15 years, and nearly a foot in 30 years’ time (with local and regional differences), you can see where this is going.

Tidal Flooding in the Next 15 to 30 Years: Frequent, Disruptive, Widespread

Our new report does the following:

• Looks at 52 locations (using 52 NOAA tide gauges) on the U.S. East and Gulf Coasts
• Identifies the height of the tide, locally, that can cause flooding, both minor and moderate
• Uses local sea level rise projections for 2030 and 2045 for a mid-range scenario of sea level rise, (NOAA’s intermediate-high scenario)
• Projects the frequency and duration with which future tides would exceed flooding thresholds.

Pretty straightforward. See the report and supporting technical document for more information.

What we found is 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.

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.
• For 20 of those communities, this will represent a tripling in flood frequency in 15 years. (In all, 36 communities can expect to see a tripling in flood frequency in this timeframe, though some of those increase from just a couple 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.
• In places where the difference between the minor flooding threshold and the moderate flooding threshold are relatively small, extensive flooding will start to occur from tides alone. Eight of our 52 communities fall into this category.

By 2045:

• Most of the towns analyzed (38) would see at least 48 floods a year, the equivalent of four floods a month.
• 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 five in the mid-Atlantic area are projected to be flooded 10 percent of the time.
• Beyond the tidal flooding front line, around a dozen communities that today face five or fewer tidal floods per year, including several on the Gulf Coast, could see a 10-fold or greater increase. See the chart for more.

And there’s much more to this story…

Beyond the stats, there are qualitative aspects of our results worth calling out. For example:

(1)   Though flood-prone areas would not be permanently inundated by 2045, some could be under water enough of the time to make them, for all intents and purposes, ceded to the sea. We say in the report “If a neighborhood floods each day, it will be considered unlivable without major investments designed to protect it.” And it would probably take much less flooding than that.

(2)   The East and Gulf Coasts are vast, and we could look at only 52 locations. The fact that the rise in flood frequency is nearly ubiquitous across these locations suggests that many places in between will need to brace for similar changes, depending on local factors like topography, coastal defenses in place, and measures they take to prepare.

(3)   Our analysis projects the future frequency of conditions that cause flooding today, but whether and to what extent that future flooding actually affects a community depends in no small part part on how we respond, both as communities and as a country.

(4)   The projected frequency, extent, and duration of these tidal floods, though, have the potential to cause major and presumably costly disruption, or require major and presumably costly adaptation measures, and in either case, need to be taken seriously.

(5)   And though sea level rise and some level of tidal flooding in the next several decades are pretty much guaranteed, we have a say in the coastal changes we set in motion for late this century and beyond, if we get serious about mitigating global warming and slowing the rate at which the sea is rising. Ultimately, this may be the most reliable way of protecting coastal communities over the long term.

But these are just some highlights. Please check out the executive summary and full report here. Also, we’ll be unpacking the report in this month’s blog series, so please tune in for more commentary on our findings and on solutions.

Lastly, on behalf of the report team, we hope that this report serves to focus attention on the real and present challenges facing many coastal communities, that we’ve told their stories well, and that we will resolve locally and nationally to take these challenges on.