Sea Level Rise and Tidal Flooding: Forthcoming Report on Encroaching Tides Signals a New Chapter for Many Coastal Communities

October 2, 2014 | 2:28 pm
Erika Spanger-Siegfried
Director of Strategic Climate Analytics

There are things we know pretty well about the effects of sea level rise. Today it is making damage from storm surge worse. In the second half of this century, it will permanently inundate certain places. In between, it is rising, and at an accelerating rate. But there are things we don’t understand nearly well enough for a country with a third of its population living in coastal counties. Things like: What effect will it have on coastal communities over just the next several decades?

Next week, UCS is releasing a new report that takes a hard look at this question for the U.S. East and Gulf Coasts, and outlines in detail the central and disruptive role that tidal flooding is poised to play.

16_a Profile_Charleston, SC

A tide-flooded roadway in Charleston, SC. This city saw just two or three days a year with tidal flooding in the 1970s, and now sees 10 or more annually. Photo: Sean Bath

Act One — What we’ve already seen: steady creep, punctuated effects

If this were all a play, I’d say we have seen the opening act of sea level rise. It included global sea level increases of 8 inches since 1880, and much steeper increases in certain places — like Atlantic City, with 8 inches since 1970 alone. It included storm surge from storms like Hurricane Sandy, which was aided by higher seas in doing catastrophic damage. And on a limited but growing basis, it included tidal flooding, which has gone from a rarity to a familiar sight in locations scattered up and down our coasts.

Those rates of sea level rise and, as a result, the frequency and reach of tidal flooding are both increasing rapidly today.

Figure 1: Tidal flooding can occur when high tide exceeds the normal level by about one to three feet (white arrow), depending on the location. Minor flooding, as determined by the National Weather Service, can disrupt local transportation and daily life. Moderate flooding is more extensive and can threaten life and property. As sea levels continue to rise, tides will exceed these thresholds more often. In this example from Lewes, DE, in October 2011, high tides crossed the minor flooding threshold on four consecutive days, until the pull of the new moon began to diminish. (Sources: Iowa Environmental Mesonet 2014; NOAA 2013.)

Here’s the deal with tidal flooding: Tides rise and fall twice daily along the East Coast. Twice a month, during new and full moons, when the Earth, sun, and moon align, the combined gravitational pull causes tides to rise higher than normal. Several times a year, the tides rise higher still, often called “king tides.” In the report, we call both of these types “extreme tides.”

With extreme tides, tidal flooding can result. That flooding is typically categorized as “minor flooding,” and often has minor impacts. In some locations, though, it can overtop roadways; back up sewers and storm drains; fill neighborhood streets, parks, and parking lots; flood lawns; and seep into basements. Less frequently, more extensive “moderate flooding” can result from tides alone, though today the combined effect of wind or rain on top of tides is typically required for flooding to become extensive (see Figure 1).

All of this is changing. 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 30 years’ time (with local and regional differences), you can see where this is going.

So, now for Act Two — The next several decades: tides gain disruptive force

Much of the script for what happens with SLR in the next several decades is written. Increases have been largely set in motion by past heat-trapping emissions. The biggest uncertainty in that time frame is how fast land-based ice melts.

The new UCS study uses one of several NOAA sea level rise scenarios (see Figure 2) – the one that assumes ice melts at a modest but increasing rate. (Another scenario assumes more rapid ice melt; another assumes almost no ice melt; still another assumes a continuation of historic rates of sea level rise (but requires fairy dust).) With this scenario, we can expect global sea level rise on the order of 5 inches above 2012 levels by 2030, 11 inches by 2045, and higher rates in specific locations.


Figure 2: The 2014 National Climate Assessment used several different assumptions about how oceans and land-based ice will respond to future warming to project global sea level rise. We based projections for sea level rise at our 52 locations on the assessment’s intermediate-high scenario, which factors in moderate rates of ice sheet loss. (Sources: Climate Central N.D.; Walsh et al. 2014; Parris et al. 2012.)

This is the point of departure for the new UCS study, which spells out how high tides in 52 locations up and down the U.S. East and Gulf Coasts (see map) will respond to this additional water.


Figure 3: We analyzed information recorded by 52 tide gauges from Portland, ME, to Freeport, TX. The forthcoming report includes profiles of starred locations. (Source: NOAA Tides and Currents 2014)

In many locations, the difference between a normal high tide and a high tide that causes flooding is a matter of inches. And in many places, sea level rise is rapidly closing this gap. The report:

  • Outlines projected changes in tidal flood frequency, extent and duration as that happens, for specific locations (see Figure 3).
  • Explores some of the implications of these changes for communities.
  • And, in a nod to a key wildcard at play in Act Two – our efforts to adapt – it also discusses our options for building local coastal resilience.

Act Three — What’s in store longer term?

We’re all still writing Act Three, which features changes in sea level rise and their effects on our coasts late this century and beyond (centuries beyond). Here, outcomes for our coastal communities depend, not only on the rate of ice melt, but on our efforts today to reduce greenhouse gas emissions, and to adapt to unavoidable changes.

So, don’t let up on that. And please tune in next week for the report release!

19_Profile_Norfolk_Colonial Place flooding

UCS’s forthcoming report will outline changes in the frequency, reach, and duration of high-tide floods, and the potential impact on locations on the front line of sea level rise, like this neighborhood in Norfolk, VA. Photo: Stephen M. Katz/Virginian-Pilot