Norfolk, Virginia East Coast Flooding
An aerial view of a naval station in Norfolk, Virginia. Norfolk is among the top 10 U.S. cities to see increased nuisance flooding because of sea level rise and climate change. Reuters/U.S. Navy/Mass Communication Specialist 1st Class Christopher B. Stoltz

The U.S. East Coast is bearing the brunt of the rise in “nuisance flooding” – minor events that shut down roads and clog storm drains but aren’t particularly dangerous. Such floods have increased by more than fivefold in the past 50 years as coastal sea levels rise because of climate change, federal scientists say.

Of the top 10 U.S. cities to see more nuisance flooding, eight are on the Atlantic. Maryland has suffered the most, with two cities – Annapolis and Baltimore – experiencing more than 920 percent more flood days since 1960, according to a new report by the National Oceanic and Atmospheric Administration (NOAA). San Francisco and Port Isabel, Texas, saw the most flooding on the West and Gulf Coasts, respectively.

The study defines nuisance flooding as a daily rise in water level above a particular area’s minor flooding threshold, a number that is set locally by NOAA’s National Weather Service. Scientists focused on 25 coastal cities that are especially vulnerable to flooding and have records dating back before 1950.

It found that in the 1950s, nuisance flooding occurred once every one to five years. By 2012, the events happened about once every three months. The main reason? In those 25 cities, sea levels have risen by nearly half a foot since 1963, according to the report.

“As relative sea level increases, it no longer takes a strong storm or a hurricane to cause flooding," William Sweet, an oceanographer at NOAA and the report’s lead author, said in a statement. He added that flooding now occurs in many places due to climate-related sea level rise, the rampant draining of groundwater – which causes land to sink – and the loss of natural barriers like wetlands and coral reefs.

The northeast Atlantic coast in particular saw a “significant increase” in nuisance flooding because of the combination of rising sea levels and sinking lands.

NOAA’s Margaret Davidson said that while scientists and local governments focus largely on major events like hurricanes, the minor floods affect people’s daily lives the most. “It’s the stuff that keeps you from conducting your business or picking up your kids from school,” Davidson told the Associated Press. “It is clear that changing climate and weather patterns will cause us to be increasingly inconvenienced and challenged in our everyday lives.”

Frequent storms and heavy rain events are also projected to increase flooding-related hazards in the coming decades. A White House report last month listed more frequent and severe flooding as one of the six main health risks posed by climate change because it creates immediate safety risks and increases outbreaks of waterborne diseases.

The NOAA report, released on Monday, upholds recent research on minor flooding and sea level rise. Last week, Old Dominion University researchers said they discovered that the East Coast is “a hotspot of accelerated flooding,” and that flooding unrelated to storm surges has increased in frequency and duration, Reuters noted. The news agency published its own independent analysis on July 10, which found that the number of days a year that tidal waters reached or exceeded flood thresholds more than tripled in many places.

Global average sea levels remained virtually unchanged for about 2,000 years but in the 20th century began to steadily climb. In recent years, that pace has accelerated, rising at a rate of about 0.12 inches per year – roughly twice as fast as the long-term trend – due in large part to manmade climate change, according to federal data.

Climate change can ramp up sea level rise in two ways: One, the increasing global temperature heats up the oceans, causing seawater to expand and take up more space in the ocean basin, leading to higher water levels. Two, the melting of ice over land – such as in Greenland – adds water to the ocean, similarly pushing levels up.

Sweet, the oceanographer, said that additional sea level rise this century will further intensify the impacts of nuisance flooding and will reduce the time between flood events. “The effects of rising sea levels along most of the continental U.S. coastline are only going to become more noticeable and much more severe in the coming decades, probably more so than any other climate-change-related factor,” he said.