US coastal flooding seeing 'extraordinary' rise with some areas projected to get 6 days of high tides by 2021
This type of coastal flooding will continue to grow in extent, frequency and depth as sea levels continue to rise over the coming years and decades, warns the NOAA
Flooding of US coastlines due to sea-level rise is happening currently and it is becoming more frequent each year — with high-tide flooding more than twice as likely now as it was in 2000. According to researchers, the increase in high-tide flooding along the Atlantic and Gulf Coasts since 2000 has been "extraordinary", with the frequency of flooding in some cities growing five-fold. This type of coastal flooding will continue to grow in extent, frequency and depth as sea levels continue to rise over the coming years and decades, warns the National Oceanic and Atmospheric Administration (NOAA).
The report documents changes in high-tide flooding patterns at 98 NOAA tidal gauges along the US coast, and provides a flooding outlook for these locations for the coming meteorological year, May 2020 to April 2021, and projections for the next several decades.
Coastal communities across the US continued to see increased high tide flooding last year, forcing their residents and visitors to deal with flooded shorelines, streets, and basements — a trend that is expected to continue into 2021. The elevated water levels affected coastal economies, tourism and crucial infrastructure like septic systems and stormwater systems, says the report.
The authors project that from May 2020 to April 2021, the national high tide flood frequency is expected to accelerate, with US coastal communities seeing an average of 2 to 6 days of flooding in the coming year. Communities along the northeast and western Gulf coasts are projected to see even more days of flooding. "America’s coastal communities and their economies are suffering from the effects of high tide flooding, and it’s only going to increase in the future," says Nicole LeBoeuf, acting director of NOAA’s National Ocean Service, in the analysis.
According to long-term projections made by scientists, by 2030, there could be 7 to 15 days (2- to 3-fold higher) of high-tide flooding (HTF) for coastal communities nationally. By 2050, that rises to 25 to 75 days (5- to 15-fold higher), and potentially in some locations reaching nearly 180 days per year, suggesting high-tide flood levels "may become the new high tide". These long term outlooks are based on the range of relative sea-level rise, using two scenarios of the fourth national climate assessment considered more likely to occur by 2030 and 2050 – intermediate low and intermediate.
High-tide flooding, often referred to as "nuisance" or "sunny day" flooding, is increasingly common due to years of relative sea-level increases. It occurs when tides reach anywhere from 1.75 to 2 feet above the daily average high tide and start spilling onto streets or bubbling up from storm drains. As sea-level rise continues, damaging floods that decades ago happened only during a storm now happen more regularly, such as during a full-moon tide or with a change in prevailing winds or currents.
"High-tide flooding is called by many names: sunny-day, nuisance, recurrent, king-tide, tidal, or sea-level rise flooding. One thing they all have in common is that the cumulative toll of impacts is becoming disruptive and damaging within many coastal communities. HTF impacts are increasing in frequency and spatial extent and threaten a myriad of coastal infrastructure," write authors.
According to the analysis, US coastal communities saw a median flood frequency of four days in 2019, just shy of the record set last year. However, 19 locations along the East and Gulf coasts set or tied records where rapidly increasing trends in high tide flooding have emerged. In 2019, the Southeast saw a three-fold increase in flooding days compared to the year 2000.
Charleston in South Carolina, for example, had 13 days where flooding reached damaging levels, compared to only two days typical in 2000. Along the Western Gulf, percentage increases were the highest, greater than five-fold. In Texas, Sabine Pass and Corpus Christi had 21 and 18 flooding days in 2019, and in 2000 those locations would typically only experience about one and three days, respectively.
"Compared to high-tide flooding frequencies typical in 2000, HTF in 2019 was extraordinary. Flood days occurred 100-150% more frequently than in 2000 along the Northeast Atlantic and Eastern Gulf coastlines (for example, 14 HTF days in 2019 at Norfolk is over 150% higher than the trend value of about 5 days in 2000). Even higher percentage increases (over 300%) occurred along the Southeast Atlantic (over 500% increase in Charleston with 13 HTF days in 2019 compared to about 2 days in 2000). Percentage increases compared to 2000 were the greatest in the Western Gulf (over 500%)," says the report.
Regionally, the 2020 high-tide frequency outlook is 6 to 11 days along the Northeast Atlantic, 5-11 days along the Western Gulf, 3-6 days along the Southeast Atlantic, 2-5 days along the Eastern Gulf, 0-7 days along the Northwestern Pacific, and 0-3 days along the Southwestern Pacific.
"As a Chesapeake Bay resident, I see the flooding first hand and it is getting worse. Records seem to be set every year as communities are straddled with this growing problem. Fortunately, NOAA’s tide gauge network is keeping a close watch and helping us provide guidance about the disruptive flooding that is likely next year and for decades to come," says lead author Dr William Sweet, an oceanographer for NOAA’s National Ocean Service.