News•February 8, 2013

Blizzard of 2013 Brings Another Threat: Coastal Flooding

While the blizzard bearing down on New England has understandbly brought national attention to what could be record snowfall, the storm brings another dangerous threat to the region: coastal flooding.

The blizzard of 2013 may cause major coastal flooding along the Massachusetts coastline from a storm surge as high as 5 feet and waves greater than 30 feet tall. The storm also arrives just days after a report warned of the region’s growing vulnerability to such storm-surge events. The report, by the nonprofit Boston Harbor Association, found that coastal flooding of 5 feet above the current average high tide — similar to the surge seen during Hurricane Sandy and translating to the city’s 100-year flood level — would flood 6.6 percent of the city of Boston.

Storm surge and coastal flooding risk map from the NWS in Boston on Feb. 8, 2013.
Click to enlarge the image. Credit: NOAA/NWS.

Luckily for Bostonians, Hurricane Sandy’s surge hit its peak of 4.57 feet just as the tide was ebbing, preventing major flooding in the city. Residents of New York and New Jersey were not so fortunate.

At 7.5 feet above the current average high tide, more than 30 percent of Boston could be flooded, the study found. “More than 50 percent of 12 Boston neighborhoods would be flooded; East Boston would have the largest flooded area” with more than 140 million square feet of flooded real estate, the report said. This week, Boston Mayor Thomas Menino directed city agencies to assess actions they could take to adapt to and mitigate risks from sea level rise and future coastal storms.

Sea levels are rising worldwide due to warming ocean temperatures, melting polar ice caps, and sinking land masses, along with other factors. Higher sea levels provide a higher launching pad for storm surges from hurricanes and nor’easters, making it possible for relatively weak storms to cause major damage. In Boston, the top five highest storm tides, which is the combination of the tide level and storm surge, all occurred during nor’easters.

According to research by Climate Central scientists, the sea level trend in Boston Harbor from 1959 to 2008 has seen an increase of 2.31 milimeters per year, which is slightly below the global average over the same period. In the past 50 years, the Boston Harbor water level has risen by about 4.5 inches, although it has increased much more in other spots along the northeastern coast.

On Nantucket Island, where coastal flooding is also anticipated from this storm along with hurricane-force winds, the sea level has risen by about half a foot during the past 50 years.

Computer model forecast for early Saturday morning, showing a powerful storm off Cape Cod and very strong northeasterly winds, which will drive a storm surge into the coast.
Click to enlarge the image. Credit: Weatherbell.com.

During the region’s benchmark blizzard and coastal flooding event, which occurred in February 1978, the storm tide in Boston rose to an all-time high, and the roiling ocean waters swallowed coastal homes from the North Shore to Cape Cod.

The blizzard currently bearing down on the region is not expected to produce quite as high a storm surge, but if the greatest surge occurs at the time of high tide on Saturday morning, damage to coastal property and flooding of coastal roads, especially between Boston and Cape Cod, is likely to occur.

The National Weather Service has issued a coastal flood warning for the east coast of Massachusetts for the high tides on Friday evening and Saturday morning. The NWS said the storm surge on Friday evening should be in the 2-to-3 foot range, resulting in “a minor to moderate impact.”

But astronomical tides will be higher on Saturday morning, and seas will be far higher as well given winds gusting to hurricane force. The NWS is predicting a storm surge of 4-to-4.5 feet along the north side of Cape Cod, and seas of up to 35 feet are likely offshore. “WE ARE ESPECIALLY CONCERNED WITH THE IMPACT OF LARGE BREAKERS ON TOP OF AN ELEVATED TIDE. THERE IS A RISK OF SOME STRUCTURAL DAMAGE ALONG THE IMMEDIATE COASTLINE SATURDAY MORNING . . . ESPECIALLY [between the towns of] HULL TO SANDWICH,” the NWS said in a technical discussion posted online.

According to the NWS, major coastal flooding means that “at least scattered structural damage,” and widespread flooding of vulnerable shoreline areas is anticipated.

Damage from coastal flooding during the 1991 Halloween nor'easter, known as “The Perfect Storm.”
Credit: NOAA.gov.

In Scituate, Mass., which sticks out into the sea on Boston’s South Shore making it vulnerable to coastal flooding from northeasterly winds, the NWS is predicting a total storm tide of 14.2 feet relative to mean lower low water (which is the average of the lowest low tides at that location), including a storm surge of 3 feet. In Sandwich Harbor on Cape Cod, the NWS is predicting a storm tide of 14.7 feet above mean lower low water, including a surge component of 4 feet, which would cause major coastal flooding on Saturday morning.

Coastal flooding is also a concern in southeastern New York, where strong winds may funnel waters from Long Island Sound into flood-prone areas of northern Long Island and New York City. Up to 2 to 4 foot tidal departures from astronomical high tides are possible during the times of high tide Friday evening along western Long Island Sound and the Twin Forks of Long Island, which along with high waves, could cause moderate coastal flooding. Only minor coastal flooding is expected in New York Harbor, but the impacts will depend on the wind direction at the time of high tide.

The coastal flooding threat for this storm in New York pales in comparison to what it was during Hurricane Sandy, when large parts of the city’s iconic subway system flooded in the face of a record storm surge, and many New Yorkers drowned in flood waters.

According to the draft National Climate Assessment report released in January, even without any changes in storms, the chance of what is now a 1-in-10-year coastal flood event in the Northeast could triple by 2100, occurring once every 3 years, due to rising sea levels. Climate Central research shows that the odds of a 100-year flood or worse in Boston Harbor by 2030 would be 23 percent with sea level rise from global warming. Without global warming-related sea level rise, that figure would be 9 percent. The bottom line is sea level rise multiplies the odds by 2.5 times.