As extreme storms become more ubiquitous, Philadelphia is among numerous cities grappling with flooding issues against the backdrop of aging infrastructure, rising sea levels and more extreme precipitation events.
In recent years, Philadelphia experienced several extreme precipitation events, sometimes called cloudbursts, where isolated parts of the city experienced 500-year and 1,000-year storms, which have an estimated 0.2% and 0.1% chance of happening in any given year, respectively. These rare storms are expected to become more frequent with climate change as a warmer atmosphere can hold (and during storms release!) more moisture.
For every one degree Celsius that the temperature increases, the atmosphere can hold 7% more water, a thermodynamic law of physics called atmospheric holding capacity. More intense rain events, especially those coupled with coastal storms and sea level rise, will exacerbate the coastal, riverine and infrastructure flooding we experience in Philadelphia.
Over the course of Philadelphia’s history, city officials and engineers took drastic measures to create an infrastructure network of roughly 3,700 miles of sewers to remove stormwater and wastewater, systematically changing the city’s topography and hydrology forever. Natural watersheds were utilized to facilitate drainage, and most of Philadelphia’s surface creeks and streams became the framework from which conveyance pipes for the sewer system were planned.
As the city grew, the landscape was flattened, soils were capped over, preventing rainwater from seeping into the ground, all while more and more buildings and roadways were created, increasing demand on existing drainage pipes. While urbanization has been instrumental to sustaining periods of rapid growth in Philadelphia, there are repercussions to modifying the landscape to this degree.
Some of the most challenging impacts from development have to do with changes to the water cycle. Urbanized areas are highly impervious, resulting in less infiltration, more stormwater runoff, localized flooding and source water quality degradation.
Today, Philadelphia is a densely populated city with a significantly different landscape from when the Philadelphia Water Department was established over 200 years ago. Some parts of the drainage system that were once historic streams and creeks can now quickly become overwhelmed by heavy rain events — creating flood risk in areas far from the visible waterways. In addition to surface flooding, sewer lines in areas with older combined sewer systems can back up into basements through plumbing fixtures (If this is an issue for you, check out PWD’s Basement Backup Protection Program.) and untreated wastewater from combined sewers can overflow into the riverways.
While increased infrastructure flooding is a major concern in urban areas, riverine and coastal flooding are also exacerbated by development and likely to be amplified by climate change. As a city located at the bottom of two large watersheds and between two tidally influenced rivers, Philadelphia is already vulnerable to flooding from creeks and rivers overtopping their banks.
The removal of wetlands and the hardening of the coastline through development also make Philadelphia more vulnerable to coastal storms and sea level rise in tidal areas along the Delaware and Schuylkill rivers. Philadelphia’s history of urbanization has accommodated its growth since the 19th century, but existing and future development will have to adapt to a changing climate and be more resilient to extreme weather events.
Climate change compounds the issues already present in urban environments. The vulnerability of urban dwellers multiplies when the effects of climate change interact with pre-existing urban stressors, such as deteriorating infrastructure, areas of intense poverty, and high population density. If not directly addressed, climate change will continue to disproportionately impact marginalized and historically under-resourced communities that are the most affected by these compounding issues.
While flooding issues, and in particular infrastructure flooding, may seem like a Water Department issue, the solutions must be coordinated across departments. Citywide policies and regulations for sustainable development — especially development in the floodplain — and flood mitigation solutions will require coordination with the Philadelphia City Planning Commission, License and Inspections, the Department of Public Property, the Philadelphia Parks and Recreation Department and more.
This coordination happens through Philadelphia’s Flood Risk Management Task Force and Flood Management Program. This coordinated approach to flood risk management followed the American Society of Civil Engineer’s call for a National Flood Risk Management Strategy (2014), which in the wake of Hurricane Katrina and other catastrophic storms, urged all levels of government in the United States to develop and align sustainable flood risk management strategies.
Today the Flood Management Program combines the efforts of more than 15 City agencies as well as state and federal partners, and it is led by the City’s Floodplain Manager who was hired in 2018. The program coordinates with Philadelphia’s Office of Emergency Management, Office of Sustainability and the newly hired Chief Resiliency Officer to address climate change impacts — flooding and sea level rise in particular — with a focus on environmental justice and equity.
The goals of the Flood Management Program include providing public information, mapping and regulations, flood damage reduction, and flood preparedness for Philadelphia. Among other things, the Flood Management Program creates education and outreach materials for residents like the Guide to Flooding in Philadelphia and is developing neighborhood-specific materials for the areas hardest hit, such as Germantown and Eastwick. The City is also working with community members to create community-led flooding stakeholder groups, such as the newly formed Germantown Community Flood Risk Management Task Force.
The Philadelphia Water Department is an active member in the FRMTF, but within the organization there are groups dedicated to addressing flooding and climate change. PWD’s Flood Risk Management conducts assessments to understand and address flooding. Their studies — sometimes in partnership with the United States Army Corps of Engineers — look for mitigatory options to combat flooding in some of the hardest hit areas of the city like South Philadelphia, Eastwick, Germantown, and Northern Liberties where sewers are being upsized.
PWD also works to manage stormwater runoff through the Green City Clean Waters Program. The program, which launched in 2011, is a 25-year effort to install stormwater runoff-absorbing green infrastructure and improve traditional infrastructure, easing the burden on our combined sewer system and reducing combined sewer overflows.
The Department also has a dedicated team working on reducing PWD’s carbon emissions and making the utility more energy efficient to mitigate climate change. Their work is an important part of the City’s long term commitment to reduce citywide carbon emissions 80% by 2050.
PWD’s Climate Change Adaptation Program was created in 2014 to understand and address climate change impacts on drinking water, wastewater, and stormwater systems. The program aims to have climate science and projections mainstreamed into the planning and design of critical assets to ensure the longevity and quality of these essential services in the face of climate change. Risk assessments using climate projections from Global Climate Models guide CCAP’s work and help the program prioritize cost-effective adaptation measures for the Department.
More extreme precipitation events coupled with sea level rise and storm surge are among the climate change impacts that pose the greatest risk to cities like Philadelphia. As development continues in the city, there is a strong need to incorporate the risks associated with a warmer and wetter climate in land use and policy decisions and infrastructure designs.
Coordinated efforts across city departments and with community members today will help Philadelphia take the necessary steps to build a more resilient and just city for future generations.
By Allison Lau and Abby Sullivan, Climate Change Adaptation Program, Philadelphia Water Department