Like many older US cities, more than half of Philadelphia is served by combined sewers that carry both sewage and stormwater. Heavy rain or snow can cause combined sewer overflows to discharge into local rivers through the city’s 164 outfalls. A sustainable stormwater management plan was launched that would not only improve the city’s ailing infrastructure but also support regeneration through green improvements to the urban environment and social fabric. The programme is reducing the number and volume of combined sewer overflows in the average rainfall year and the amount of polluted run-off reaching sewers.
The Philadelphia Water Department decided to invest in green infrastructure to collect and treat stormwater run-off at source as part of an ambitious programme to reduce the amount of stormwater entering the combined sewer system. We prepared final design plans and specifications for several projects including stormwater curb extensions, infiltration trenches that water trees with stored stormwater, and planters that collect drainage from the street. We modelled these and other stormwater devices, resolved constructability issues where new measures were to be built near existing structures, and optimised the designs to improve durability and maintenance.
The programme has reduced the number and volume of combined sewer overflows in the average rainfall year and the amount of polluted run-off reaching sewers. In neighbourhoods, air quality has been improved, and streetscapes are now more attractive, enhancing the quality of life for residents. Trees and rain gardens create a cooling effect that helps to reduce heat stress-related fatalities during heatwaves. Carbon emissions have been reduced in two ways: capturing and treating stormwater at source has reduced the energy needed to store, pipe and treat it compared with traditional systems; and the cooling effects of trees and plants shade and insulate buildings from wide temperature swings, decreasing the energy needed for heating and cooling. Greenery also absorbs carbon dioxide from the air, while the green stormwater infrastructure has improved ecosystems by providing a natural water quality filter and limiting erosion of waterways caused by high flows, both of which have benefitted aquatic species. The restoration of streams and wetlands has created habitats to also support rich and diverse aquatic life. Clean tech companies have been attracted by Philadelphia’s greener city ambitions, creating jobs and improving economic prosperity.