ecoRI: 'Nuisance' coastal flooding on the rise

By ecoRI News staff By 2050, much of U.S. coastal areas are likely to be threatened by 30 or more days of flooding annually because of dramatically accelerating impacts from sea-level rise, according to a new National Oceanic and Atmospheric Administration (NOAA) study.

The findings appear in a paper entitled “From the Extreme to the Mean: Acceleration and Tipping Points for Coastal Inundation due to Sea Level Rise” and follows an earlier study by the report’s co-author, William Sweet, Ph.D., a NOAA oceanographer.

Sweet and fellow NOAA scientist Joseph Park established a frequency-based benchmark for what they call “tipping points,” when so-called “nuisance flooding,” defined by NOAA as between 1 and 2 feet above local high tide, occurs 30 or more times a year.

“Coastal communities are beginning to experience sunny-day nuisance or urban flooding, much more so than in decades past,” Sweet said. “This is due to sea-level rise. Unfortunately, once impacts are noticed, they will become commonplace rather quickly. We find that in 30 to 40 years, even modest projections of global sea-level rise will increase instances of daily high-tide flooding to a point requiring an active, and potentially costly, response.”

Based on that standard, the NOAA team found that these tipping points will be met or exceeded by 2050 at most of the U.S. coastal areas studied, regardless of sea-level rise likely to occur this century. In their study, Sweet and Park used a 1.5- to 4-foot set of recent projections for global sea-level rise by 2100 — similar to the rise projections of the Intergovernmental Panel for Climate Change, but also accounting for local factors such as the settlement of land, known as subsidence.

Boston could start experiencing nuisance flooding by 2021, and Providence and New London, Conn., a decade later (NOAA) . These regional tipping points will be surpassed in the coming decades in areas with more frequent storms, according to the report. These tipping points also will be be exceeded in areas where local sea levels rise more than the global projection. This also includes coastal areas such as Louisiana where subsidence is causing land to sink below sea level.

NOAA tide gauges show the annual rate of daily floods reaching these levels has drastically increased and are now five to 10 times more likely today than they were 50 years ago.

“The importance of this research is that it draws attention to the largely neglected part of the frequency of these events,” said Earth’s Future editor Michael Ellis in accepting the paper for the online journal. “This frequency distribution includes a hazard level referred to as ‘nuisance‘ — occasionally costly to clean up, but never catastrophic or perhaps newsworthy.”

Ellis also noted that the authors use observational data to drive home the important point that nuisance floods, from inundating seas, will cross a tipping point over the next several decades and significantly earlier than the 2100 date that is generally regarded as a target date for damaging levels of sea-level rise.

He also said the paper raises the interesting question of what frequency of “nuisance” corresponds to a perception of “this is no longer a nuisance but a serious hazard due to its rapidly growing and cumulative impacts.”

The scientists base the projections on NOAA tidal stations where there is a 50-year or greater continuous record. The study doesn’t include the Miami area, as the NOAA tide stations in the area were destroyed by Hurricane Andrew in 1992 and a continuous 50-year data set for the area doesn’t exist.

Based on that criteria, the NOAA team is projecting that Boston; New York City; Philadelphia; Baltimore; Washington, D.C.; Norfolk, Va.; and Wilmington, N.C., will soon make, or are already being forced to make, decisions on how to mitigate these nuisance floods earlier than planned.

In the Gulf, NOAA forecasts earlier than anticipated floods for Galveston Bay and Port Isabel, Texas. Along the Pacific coast, these earlier impacts will be most visible in the San Diego/La Jolla and San Francisco Bay areas.

Mitigation decisions could range from retreating further inland to coastal fortification, or to a combination of “green” infrastructure using both natural resources such as dunes and wetland, along with “gray” manmade infrastructure such as seawalls and redesigned stormwater systems.