About the Exchange

The Green Infrastructure Leadership Exchange is a practitioner network that supports communities seeking to grow green stormwater infrastructure programs by accelerating implementation through peer learning, innovation, partnerships and advocacy. The Exchange develops, hosts and shares resources to advance the benefits and viability of green infrastructure.

About Green Stormwater Infrastructure (GSI)

Green stormwater infrastructure includes a range of soil-water-plant systems that intercept stormwater, infiltrate a portion of it into the ground, evaporate a portion of it into the air, and in some cases release a portion of it slowly back into the sewer system.

Types of Green Stormwater Infrastructure


A bioswale or vegetated swale is a form of bioretention used to partially treat water quality, attenuate flooding potential and convey stormwater away from critical infrastructure. The function of these open-channel (broad) drainageways is to convey stormwater runoff. They are often used as an alternative to, or an enhancement of, traditional stormwater piping. Bioswales are often integrated into parking lot and road medians and parallel to roadways to infiltrate and treat a portion of the stormwater volume. These systems can often be integrated into existing ditch and swale systems to increase their treatment function. Where soils are well drained, infiltration can also be facilitated in the swale by placing ditch blocks or weirs perpendicular to the flow path, causing small volumes of water to be captured in the swale and allowing more time for infiltration.


Credit: http://buildgreen.ufl.edu/Fact_sheet_Bioswales_Vegetated_Swales.pdf
Philadelphia, PA
Womrath Park Swale
Photo by: Louis Cook for Philadelphia Water Department

Blue Roofs

Blue roofs are non-vegetated source controls that detain stormwater. Weirs at the roof drain inlets and along the roof can create temporary ponding and gradual release of stormwater. Blue roofs are less costly than green roofs. Coupled with light colored roofing material they can also provide rooftop cooling.

Credit: http://www.nyc.gov/html/dep/html/stormwater/green_pilot_project_ps118.shtml


A stormwater bump-out is a vegetated curb extension that protrudes into the street either mid-block or at an intersection, creating a new curb some distance from the existing curb. A bump-out is composed of a layer of stone that is topped with soil and plants. An inlet or curb-cut directs runoff into the bump-out structure where it can be stored, infiltrated, and taken up by the plants (evapotranspiration). Excess runoff is permitted to leave the system and flow to an existing inlet. The vegetation of the bump-out will be short enough to allow for open sight lines of traffic. Aside from managing stormwater, bump-outs also help with traffic-calming, and when located at crosswalks, they provide a pedestrian safety benefit by reducing the street crossing distance.


Philadelphia, PA
Queen Lane Stormwater Bumpout
Photo by: Philadelphia Water Department

Green Streets

Green streets are streets that act as local stormwater management systems, capturing stormwater runoff and reducing the amount of stormwater that would otherwise make its way into Philadelphia’s sewer collection system. Green Streets utilize green stormwater management infrastructure such as tree trenches, vegetated bump-outs, stormwater planters and permeable pavement to manage the runoff from streets, sidewalks and other paved surfaces.

Green Roofs

A green roof is a roof or section of roof that is vegetated. A green roof system is composed of multiple layers including waterproofing, a drainage layer, an engineered planting media, and specially selected plants. Green roofs can be installed on many types of roofs, from small slanting roofs to large commercial flat roofs. Two basic types of green roofs have been developed: extensive and intensive. An extensive green roof system is a thin (usually less than 6 inches), lighter-weight system planted predominantly with drought-tolerant succulent plants and grasses. An intensive green roof is a deeper, heavier system designed to sustain more complex landscapes. A green roof is effective in reducing the volume and velocity of stormwater runoff from roofs by temporarily storing stormwater, slowing excess stormwater release into the combined sewer system, and promoting evapotranspiration.


Philadelphia, PA
PECO Green Roof
Photo by: Paul Rider

Pervious Paving

Pervious pavement is a specially designed pavement system that allows water to infiltrate through the pavement and never become runoff. This system provides the structural support of conventional pavement but is made up of a porous surface and an underground stone reservoir. The stone reservoir provides temporary storage before the water infiltrates the soil. There are many different types of porous surfaces, including pervious asphalt, pervious concrete, and interlocking pavers. Interlocking pavers function slightly differently than pervious concrete and asphalt. Rather than allowing the water to penetrate through the paving, pavers are spaced apart with gravel or grass in between to allow for infiltration.


Philadelphia, PA
Percy Street
Photo by: Philadelphia Water Department


A stormwater planter is a specialized planter installed in the sidewalk area that is designed to manage street and sidewalk runoff. It is normally rectangular, with four concrete sides providing structure and curbs for the planter. The planter is lined with a permeable fabric, filled with gravel or stone, and topped off with soil, plants, and sometimes trees. The top of the soil in the planter is lower in elevation than the sidewalk, allowing for runoff to flow into the planter through an inlet at street level. These planters manage stormwater by providing storage, infiltration, and evapotranspiration of runoff. Excess runoff is directed into an overflow pipe connected to the existing combined sewer pipe.


Philadelphia, PA
Columbus Square
Photo by: Philadelphia Water Department

Rain Barrels/Cisterns

A rain barrel or cistern is a structure that collects and stores stormwater runoff from rooftops. The collected rain water can be used for irrigation to water lawns, gardens, window boxes or street trees. By temporarily holding the stormwater runoff during a rain event, more capacity can be added to the city’s sewer system. However, rain barrels and cisterns only serve an effective stormwater control function if the stored water is used or emptied between most storms so that there is free storage volume for the next storm. Rain barrels are designed to overflow into the sewer system through the existing downspout connection in large storm events. Although these systems store only a small volume of stormwater, collectively they can be effective at preventing large volumes of runoff from entering the sewer system.


Philadelphia, PA
Photo by: Philadelphia Water Department

Rain Gardens

A rain garden is a garden designed to collect runoff from impervious surfaces such as roofs, walkways, and parking lots, allowing water to infiltrate the ground. The garden is normally moderately depressed (lower than the surrounding ground level), with the bottom layer filled with stone so runoff can collect and pond within it. The site is graded appropriately to cause stormwater to flow into the rain garden area from the nearby impervious area. The water ponds on the surface, is used by the vegetation in evapotranspiration, and infiltrates into the subsurface stone storage and soil. Rain gardens can be connected to sewer systems through an overflow structure, but usually they are sized to infiltrate the collected stormwater runoff within 72 hours. Flexible and easy to incorporate into landscaped areas, rain gardens are suitable for many types and sizes of development and retrofits. Rain gardens are effective at removing pollutants and reducing stormwater runoff volume.


Philadelphia, PA
Greenfield School
Photo by: Philadelphia Water Department

Tree Trenches

A stormwater tree trench is a system of trees that are connected by an underground infiltration structure. On the surface, a stormwater tree trench looks just like a series of street tree pits. However, under the sidewalk, there is an engineered system to manage the incoming runoff. This system is composed of a trench dug along the sidewalk, lined with a permeable geotextile fabric, filled with stone or gravel, and topped off with soil and trees. Stormwater runoff flows through a special inlet (storm drain) leading to the stormwater tree trench. The runoff is stored in the empty spaces between the stones, watering the trees and slowly infiltrating through the bottom. If the capacity of this system is exceeded, stormwater runoff can bypass it entirely and flow into an existing street inlet.


Philadelphia, PA
Photo by: Philadelphia Water Department


Wetlands can be constructed to treat stormwater runoff in an effort to improve water quality and minimize the impacts of storm-related flows on the aquatic and structural integrity of riparian ecosystems.


Credit: http://chesapeakestormwater.net/training-library/stormwater-bmps/constructed-wetlands/
Philadelphia, PA
Saylor Grove
Photo by: Louis Cook for PWD