Atlantic Coast communities face higher flood risk this fall as unusually high tides push seawater into low-lying coastal areas, threatening tens of millions of people and billions of dollars in infrastructure.

Officials with NOAA confirmed that this year’s “king tides,” which occur between September and November, are contributing to increased “sunny-day flooding” from Florida to Maine as rising seas overtake beaches and shorelines, a condition attributable to climate change.

In the Florida Keys, the effects of a dayslong king tide in September have lingered for more than six weeks, exacerbating concerns about inundated roads, fouled drinking water and salt from seawater damaging cars.

In Boston, high tides swept over piers and sea walls twice last week as Boston Harbor swallowed the city’s iconic Long Wharf, impeding commuters along Atlantic Avenue and prompting tourists and locals to ogle the phenomenon known as sunny-day flooding.

The flooding, which extended to the edge of downtown, caused Mayor Martin Walsh to warn that the dramatic king tides are a harbinger of climate-related impacts like more frequent flooding that await Boston by midcentury.

The same is true for communities along the middle and south Atlantic seaboard, where tidal flooding is “becoming a way of life” in Annapolis, Md.; Norfolk, Va.; Charleston, S.C.; and South Florida, said William Sweet, author of NOAA’s annual high-tide flooding report.

According to NOAA, high-tide flooding is accelerating in more than 40 coastal communities, and 25 of those are seeing linear growth in such events, meaning tidal flooding will become “more chronic than sporadic.”

“When the tides start flooding you regularly like this, it’s a telltale sign that there are bigger problems ahead,” Sweet said in a telephone interview. “And it speaks to the vulnerability of [coastal areas] where people are living. It’s unfortunate, but the reality is that sea levels are going up and our lives and livelihoods are being affected by it.”

Scientists say a “king tide”—caused by gravitational pull when the moon draws close to the Earth and aligns with the sun—is part of the lunar cycle. Under normal conditions, a king tide can swell the near-shore ocean by a couple of inches before weakening over several days.

Such tides are common in spring and fall, and they tend to be strongest during full and new moon phases from September to November, experts say. King tides also occur on the Pacific Coast, but they have less impact on coastal communities because of the Pacific’s steep drop in ocean depths, Sweet said.

Less predictable tides

The most recent Atlantic king tide arrived just before Halloween and lasted about three days. The next should arrive around Thanksgiving, based on lunar timetables. But experts say today’s king tides don’t always adhere to schedule, and like other climate change phenomena, they are less predictable than ever.

Roy Coley, public works director for the city of Miami Beach, has witnessed such unpredictability firsthand. The city has long been a real-world laboratory for sea-level rise and adaptation strategies, including a $500 million program to raise roads and other critical infrastructure by several feet.

As a result, Miami Beach has fared better than many of its neighbors, Coley said, and the city of 90,000 has not seen the kind of long-duration tidal flooding that others have. But it has seen more days when high tides did not match NOAA’s projections, sometimes by significant amounts.

During a recent event, Miami Beach had “a predicted tide level of somewhere around half a foot, and it stayed at 2 feet for the peak of the tide cycle, which was about a week,” Coley said. While Miami Beach was resilient enough to absorb the higher-than-expected tide, other communities facing the same tide conditions would have been badly flooded, experts say.

“We’re finding that this year, in particular, we’re above the recent trend” for high-tide elevations, Sweet said. As a result, nuisance flooding is becoming more consequential and requiring a higher level of response from public works and emergency management agencies.

Sweet also noted that above-average high tides are occurring more often in the September-to-November period, when oceans are still warm, seas are generally higher and storms can be more severe.

“In several areas we’re already seeing records broken, and we’re only five months into the [meteorological] year,” Sweet said. “Fall is usually the time you start racking up the numbers, and indeed they have been historic thus far.”

Meanwhile, more communities are pondering the consequences of increased tidal flooding and the costs of countering it.

Officials in Rhode Island are asking residents to take pictures of the next king tide as it overtakes their neighborhoods. Local and state agencies will use the photos and other information to plan for future development, according to the state’s Sea Grant and Coastal Resources Management Council.

“We know we’re never going to overcome climate change, and we’re going to have to take incremental steps as long as we exist to manage climate change,” said Coley, adding that Miami Beach is “just a little bit ahead of the curve” compared with other coastal cities.

Boston has developed a “Climate Ready Boston” blueprint to help the city become more resilient to sea-level rise. Walsh has touted the plan as Boston’s best chance to mitigate the worst effects of climate change in the nation’s second oldest major seaport city.

“What we are experiencing today with #KingTide demonstrates the urgency in which we need to act on climate change,” Walsh said in a tweet last week. “Climate Ready is how we build a resilient shoreline that protects all of us from flooding.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news atwww.eenews.net.