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 (CSOs) to fill basements or discharge into local rivers through the city's 164 outfalls.
In 2009, mayor Michael Nutter of Philadelphia announced Greenworks, an ambitious plan to make his city the greenest in the country. Among the Greenworks initiatives was a stormwater management plan called "Green City, Clean Waters."
Rather than build multimillion-dollar tunnels to store excess stormwater, the city decided to use green techniques including stormwater tree trenches, stormwater curb extensions or "bump-outs," stormwater planters, pervious pavement, green roofs, rain barrels, rain gardens, and flow-through planters.
Mott MacDonald was retained in 2009 to prepare final design plans and specifications for several Green Streets projects in the Northern Liberties section of Philadelphia. Stormwater control measures included bump-outs, infiltration trenches that water trees with stored stormwater, and sidewalk planters that collect drainage from the street.
We selected native species for planters and sidewalk trees, modeled stormwater devices, resolved constructability issues where new measures were near existing structures, and considered durability and maintenance in the design.
Mott MacDonald also retained in 2010 and 2011 to assist with Phases IV and V of a Green Streets program in West Philadelphia. Each phase included measures applied to schools, parks, playgrounds, and other locations in the area.
For Phase IV in West Philadelphia, we redesigned the standard Dual-Trap inlet to eliminate the need for three separate inlets at a low-point intersection. We provided efficient modeling of stormwater devices, developed construction documents, and resolved constructability issues. Piezometer wells were used to collect data and analyze the groundwater recharge effects of the project.
For Phase V, Mott MacDonald redesigned a traffic island into a multi-pool rain garden planted with native grasses and shrubs. Runoff is conducted into the rain garden through decorative trench drains connected to the street through curb cuts.
We also designed tree trenches within the sidewalks, including waterproof barriers where trenches were close to buildings and utilities. Raised planter boxes captured street runoff, enhanced the streetscape, and provided a teaching tool for a nearby school.
"Our commitment to sustainability is making Philadelphia a green city, attracting clean tech companies and increasing quality of life in our neighborhoods," said Mayor Nutter in 2011. "Our great progress toward Greenworks goals not only benefits us today, but also sets the stage for Philadelphia to thrive in the future."
According to the Philadelphia Water Department, the Green Streets program offers a variety of benefits:
- Reducing flooding and combined sewer overflows
- Reducing the amount of polluted runoff reaching sewers
- Recharging the groundwater table
- Improving pedestrian and bicycle safety
- Improving air quality
- Alleviating the urban heat island effect
- Making streetscapes more attractive