“One thousand years” means that a meteorological event has a 0.1% chance of occurring each year, but with climate change, the frequency of flooding increases.
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It’s not every day that you see emergency rescue boats cruising North East Philly to rescue residents on flooded streets like they did last week.
But maybe every few decades.
The massive floods that hit the far northeast and parts of Lower Bucks County July 13 was expressed in various unique terms. Meteorologists called it a 100-year-old storm. Philadelphia officials went further. Citing the intensity of the rains, the city said On Thursday, the weather event was “on the order of what would be considered a 1,000-year storm.”
In the summer of 1982, the region found itself in similar straits.
Then, as now, “100-year storm” was the most common title. These nicknames are based on probabilities, the experts explained. A 1000-year-old storm means it has a 0.1% chance of occurring each year. That’s not to say it couldn’t happen more often, and with climate change the frequency is increasing.
Ray Kruzdlo, senior hydrologist with the Mount Holly National Weather Service, said the tri-state area is affected by two to three major floods per year, spread across southeastern Pennsylvania, New Jersey and Delaware. Where the rains fall – and how fast – makes all the difference.
Typically, the short window of heavy rain like those in northeast Philly, accumulating 6 inches in a few hours, can be the most dangerous. Some precipitation can cause flooding even in the most porous parts of New Jersey.
“Even in the sandy soils we had 10 inches of rainfall and that can’t stand it no matter what,” Kruzdlo said.
Last week’s flood damaged more than a thousand homes and businesses, displacing hundreds of residents. In the city, more than 500 properties have been affected. As recovery efforts continue with the intervention of the federal government to help, this might sound familiar to residents of Northeast Philly over a certain age.
In June 1982, it rained for nearly eight hours as the sky dropped millions of gallons of water over parts of the city. Nowhere has been hit harder than the Red Lion Village apartment complex along Byberry Creek.
A creek that flows into the larger Poquessing creek is the usual drainage route, but Byberry was not up to this event, and water spilled over its banks and into the complex residential.
The Red Lion Village apartments were located at a particularly low elevation – so low that some downstairs residents reported “about 8 feet of water in their homes during the height of the flooding,” according to the Philadelphia Daily News.
“The people of Red Lion Village, in fact, were virtually guaranteed to be inundated at some point, even without century-old rains,” the Daily News reported in 1982.
But the damage was no less devastating. As with the July Flood, images of lifeboats pulling people out of submerged buildings and rescuing dogs from rain tides grabbed the headlines.
Other parts of northeast Philly suffered a similar wet shock. Hundreds have been displaced. Two people died.
Rising sea levels contribute to the possibility of damaging flooding. Two years ago, Philly had one of the wettest winters in a century and a half, a National Weather Service spokesperson told Billy Penn at the time, with nine flood warnings issued in just six weeks.
So, especially nowadays, massive flooding does not necessarily mean it was a 100 year storm, or even a 1000 year storm.
“The federal government as well as the USGS are moving away from calling it 100-year flood or rainfall,” said NWS specialist Kruzdlo, “because it is confusing.”