Eco-Explainer: What is Tidal flooding, and why does it happen in Philly?

Philadelphia is not directly next to the ocean…but still floods with high tides. How and why does this happen? What can we do to prevent and prepare for it?

The National Weather Service issued a Coastal Flood Advisory for Philadelphia and surrounding counties on February 5th. It had been a dry weekend, even sunny at times in the days prior. A flood warning for the city seemed out of the ordinary for these mild weather circumstances.

Yet, Philadelphia is not immune to flooding on sunny days like this.

What is tidal flooding?

Tidal flooding, also called high tide or sunny day flooding, occurs when high tides raise water levels in coastal or tidal waterways.

Both the Delaware River bordering Philadelphia’s east and the Schuylkill bordering the west are tidal rivers; their water levels rise and fall with those of the Atlantic Ocean, channeled through the Delaware Bay.

Future tidal flooding of the rivers will likely increase in frequency primarily due to two factors that are exacerbated by the climate crisis: rising sea levels and stronger coastal storms. As global sea levels rise, Philadelphia’s Flood Risk Management Task Force reports that the sea levels around Philadelphia specifically have risen approximately 12 inches over the past century. This number is “higher than the global average by around eight inches due to land subsidence [sinking of land from underground earth removal] in our region.” That, coupled with a Category 1 hurricane like Philadelphia has experienced in the past, could “significantly increase flooding impacts.”

How can we prevent and prepare for tidal flooding?

“Flooding in Pennsylvania is the most frequent and damaging natural disaster that occurs throughout the Commonwealth,” according to PA Flood Risk.

Fortunately, information is accessible for both the public and public leaders to address tidal flooding.

How to stay updated on floods

Philadelphia and the state of Pennsylvania have several resources available to stay informed about floods and stormwater preparation.

• ReadyPhiladelphia is Philadelphia’s emergency alert notification system. Members may receive alerts through both text messages and emails. To receive text alerts, text ReadyPhila to 888-777.
• A Guide to Flooding in Philadelphia is Philadelphia’s Citywide Flood Risk Management Task Force’s tool for property owners and residents. The guide provides information on how to prepare houses for flooding and what to do after a flood. It also breaks down some key terms on what flooding looks like in Philadelphia, including tidal flooding.
• The PA Flood Risk Tool allows anyone to view their flood risk by mapping out an area according to a specific address or county. The data may be used to help individuals make an informed decision on flood risk and flood insurance.

Resources for civic and community leaders

• The EPA’s Environmental Justice Screening and Mapping Tool aggregates data from different areas of study to give a more full picture of the environmental impact on neighborhoods. For example, “flood risk” can be mapped onto the “low income” socioeconomic indicator to understand which neighborhoods could be at greater risk. Links to the tool can also be found on the Office of Sustainability’s request for public input on flood insurance access and affordability.
• The National Oceanic and Atmospheric Administration (NOAA) has a site dedicated to Adapting Stormwater Management for Coastal Floods. The NOAA breaks it down into four steps: Understand, Assess, Analyze and Take Action.
• Research continues to surface regarding methods of addressing tidal flooding. A recent overview of the efficacy of three coastal flood mitigation methods was published through the Institute of Physics and is open access.

Even on sunny days, parts of Philadelphia and surrounding counties may experience tidal flooding. Staying informed and aware of oceanic activity can help all of our communities.

Photo by Dan Parlante on Unsplash