Decentralized stormwater management strategies, including rain barrels, water-efficient fixtures, and greywater reuse, can reduce combined sewer overflows and surface flooding by 11-13%, according to new research from Drexel University published in the journal Urban Climate.
Researchers from Drexel’s College of Engineering modeled stormwater movement in Cramer Hill, a Camden, N.J., neighborhood located in the state’s coastal hazard zone. The neighborhood is highly susceptible to tidal and storm-surge flooding and sits within a combined sewer system, where stormwater and sewage share the same treatment infrastructure. During extreme precipitation events, that type of system can discharge untreated sewage into nearby waterways when overwhelmed.
The study is among the first to model combinations of decentralized strategies and establish a baseline for incorporating their effects into stormwater models used by municipalities and urban planners.
“This marks one of the first pieces of research to extensively model combinations of decentralized stormwater management strategies,” said lead researcher Amanda Carneiro Marques, PhD, an assistant professor in the College of Engineering.
The team structured its model to measure the distinct effects of outdoor strategies, such as cisterns and rain barrels, and indoor strategies, including water-efficient fixtures and reusing sink water for toilet flushing. Researchers simulated 16 different combinations of these strategies using precipitation and tidal data from 2014 as a baseline, measuring flooding at locations across the neighborhood and at five stormwater outfall areas that drain into the Delaware River.
Results showed that retrofitting homes with water-efficient fixtures, installing rain barrels, and practicing greywater reuse, when adopted by 75% of households, reduced combined sewer overflow volume by up to 11% and floodwater volume by up to 13%.
To test how well these strategies hold up under future climate conditions, the team adjusted the model to account for precipitation increases of 10%, 20%, and 30%, as well as sea-level rises of 30 centimeters, 90 centimeters, and 1.8 meters. Both conditions increased sewer overflow and surface flooding. However, the decentralized strategies continued to reduce those increased volumes by 11 to 13%, suggesting they remain effective under more severe conditions.
Higher sea levels, the researchers noted, create backpressure on sewer systems that prevents regular drainage and compounds surface flooding, in addition to the effects of increased rainfall.
Philadelphia and Camden together report approximately 16 billion gallons of combined sewer overflow annually, according to their respective water utilities. Both cities are among the older coastal urban areas the researchers identify as particularly vulnerable due to their proximity to large bodies of water and their aging infrastructure.
The research was supported by the Consortium for Climate Risks in the Urban Northeast. In addition to Marques, contributing researchers included Fernanda Cruz Rios, PhD; Meghna Rajbhandari; Katelyn Singh; Franco Montalto, PhD; and Ahmad Haseeb Payab, PhD.
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