Flooding is one of the most common and destructive disaster types in the United States.  As climate change intensifies, urban areas are becoming increasingly vulnerable to flooding and face an urgent need for adaptive urban planning and mitigation.  A Philadelphia-focused case study conducted by Princeton researcher Sina Razzaghi Asl explores the relationship between high flood risk communities and green spaces.

Green spaces, which include forests, parks, gardens, and other areas with vegetation, mitigate flooding by enhancing water absorption and decreasing runoff from heavy rainfall and storms.  As rapid urbanization increases, environmental justice researchers are growing increasingly concerned with the lack of green spaces and flood protections in vulnerable urban communities.  Using social and geophysical data from the National Risk Index (NRI), the U.S. Geological Survey (USGS), Web Soil Survey (WSS), and the American Community Survey (ACS), Razzaghi Asl analyzed census block groups in eight townships and boroughs within the Philadelphia Metropolitan Area to examine the relationship between  flood vulnerability and green space density.

“Philadelphia was chosen for this study due to its moderate-to-high projected flood risk, high population growth, and diverse green spaces,” says Razzaghi Asl, a postdoctoral researcher at C-PREE.  “These factors make it an ideal area to examine flood vulnerability and green space density.”

In the study, Razzaghi Asl used advanced spatial analysis tools in ArcGIS Pro to create two indices: 1) a social vulnerability index based on indicators from existing flood research and 2) a geophysical vulnerability index that includes indicators such as soil type, hydrological parameters, topographic indices, and elevation.  He then combined the two indices to composite a flood vulnerability score for each census block group.  

According to the flood vulnerability scores, the most affected areas are located in Plymouth, Norriton, and Lower Merion, which have high population densities and property exposure.  However, despite their high flood risk, these areas have relatively high income levels and property values, especially near the Schuylkill River.  

“As the study results reveal, geophysical factors can significantly influence flood risk, sometimes to a degree where social advantages alone cannot mitigate the threat,” explains Razzaghi Asl.  “Conditions such as low soil filtration rates and high drainage density may exacerbate flood risk, making it difficult for even well-resourced communities to fully shield themselves from potential impacts.”

The study also uses spatial clustering analysis to examine the overlap between flood vulnerability and green space density. Using high-resolution land cover datasets, Razzaghi Asl quantitatively measured green space density from tree canopies and shrubs/grass.  The author then coupled green space density with flood vulnerability scores to reveal clusters with high flood vulnerability and low green space density (“High-Low” clusters).  

The study found that High-Low clusters are predominantly found in densely urbanized areas like Norristown and are the most at risk.  Notably, these clusters have poverty levels that are nearly 25% higher than low-risk areas and significant older populations (15%) aged 65 years or older.  Racial minorities were also overrepresented in these vulnerable areas, especially Black and Asian residents. Even with higher levels of income, high flood risk in more affluent Black and Asian communities still persisted.  

“The results reflect how systemic and historical inequalities can leave certain communities, particularly Black and Asian residents, more exposed to flood risks, even when their median household incomes are above the national average,” says Razzaghi Asl. “Factors like long-standing underinvestment in infrastructure, high-density urbanization, and limited access to green space disproportionately impact these areas, creating vulnerabilities that income levels alone cannot offset.”  

The results emphasize the urgent need for targeted flood interventions for urban areas,  especially those with limited green spaces.  Prof. Eric Tate, a C-PREE faculty member and flooding expert, highlights the need for integrated flood management and the policy implications moving forward.     

“The study adds to the empirical evidence that intersections of environmental hazards and lack of amenities often occur in low-income communities,” explains Prof. Eric Tate, a faculty member at Princeton’s School of Public and International Affairs.  “Focusing adaptation investments on places with such compounding burdens is likely to yield environmental and social co-benefits.”

The paper, “Green spaces and flood vulnerability in urbanising regions: a case study in the Philadelphia metropolitan area,” was authored by Sina Razzaghi Asl (School of Public and International Affairs, Princeton University). The paper was published in Environmental Hazards on October 9th, 2024.  This research was supported by the Department of Geography, Environment, and Urban Studies and the Public Policy Lab at Temple University, Philadelphia.