What’s a 100-year flood in the times of Climate Change? A wetter Philadelphia means more historic events.

An 100-year flood is not exactly what it sounds like. The reality says a lot about the effects of climate change.

Philadelphians and everyone in the surrounding area are no strangers to flooding.

Philadelphia’s long history includes water main breaks, like the one that happened last week on 6th and Bainbridge, the sink holes that accompany them, and sewer water rising up through basement toilets.

The severe storm that hit Northeast Philadelphia and Bucks County with flooding  in July was deemed an 1,000-year storm by the City of Philadelphia and the flooding that followed was described as an 100-year flood by the National Weather Service.

This does not mean these events happen every one hundred years as it might seem.

Defining the 100 year flood

“We’ll get these really intense storms and tornadoes and other nasty things.”

An 100-year flood is a climate event that has a 1% probability of occurring within a year which means an 100-year flood does have a chance of occurring every year, or even more than once in a year. Similarly, a 500-year flood has a 0.2% chance of occurring and a 1,000-year storm or flood has a 0.1% probability.

According to Senior Advisor for Watershed Management and Policy at the Academy of Natural Sciences, Carol Collier, this somewhat confusing probability measurement was created for federal flood insurance.

If a house is built in the 100-year floodplain, the homeowner is required to have federal flood insurance.

Despite Philadelphia’s long history of water problems, Mayor Jim Kenney recognized that this was a storm with “little historical precedent” while encouraging residents affected to file claims for damages and to acquire flood insurance for the future.

The city has already had “more than 506 preliminary damage reports have been submitted” to the Office of Emergency Management for damage sustained during the flooding.

As summers in Philadelphia get hotter and wetter due to climate change, the city is likely to experience these 1% percent chance events much more often due to moisture in the warmer atmosphere.

Collier said, “you can’t say any one event is due to climate change, but when you start looking at the record and the increase in precipitation annually that we’re getting… it’s [the moisture] just sort of sitting up there waiting for the right time.. And we’ll get these really intense storms and tornadoes and other nasty things”

Unsurprisingly this July was above average for precipitation in Philadelphia, as well as the hottest. The storm that flooded the Northeast was “roughly double the average amount of rain for all of July” alone and this trend is likely to continue in future years.