Green Infrastructure Overview

Understanding the issue

The City of Pittsburgh is largely served by a combined sewer system, which collects wastewater and stormwater. During dry weather, the system conveys wastewater to the Allegheny County Sanitary Authority (ALCOSAN), where it is treated and returned to the Ohio River. During storms when, the combined sewer system experiences more water than it can handle, overflows of combined sewage and stormwater discharge into the waterways and can pollute our rivers and streams. During very large storm events, the pipes fill up too quickly and cause flooding in some parts of the city. These overflow events are water quality violations, and municipalities in the region must take action to reduce their volume and frequency.

Green Infrastructure (GI) combines natural and engineered systems to reduce and treat stormwater at its source through infiltration (absorption of water into soil), evapotranspiration (evaporation of water from plants), and/or detention. GI uses soil, plants, and trees to manage and filter stormwater where it falls to recharge our groundwater, or slowly release the stormwater back into the sewer when it is not overloaded. GI also reduces the volume of stormwater entering storm drains, thereby reducing stress on the sewer system and minimizing combined sewer overflows (CSOs)—ultimately improving the water quality of our local rivers and streams. 

Examples of green infrastructure practices:

Rain Gardens: A rain garden uses native plants to infiltrate water into the ground, reduces the water volume entering the sewers, filters stormwater by capturing suspended solids with their roots, and ultimately recharges the groundwater supply.

Bioswales: Bioswales are vegetated depressions that are often placed between streets and sidewalks to slow, absorb, and filter stormwater.


                                                                                             Rain Garden at 3 Rivers Wet Weather (Photo from 3RWW)

Permeable Pavement: This type of pavement allows rain or melting snow to permeate through the surface to the ground (or storage tank) below.  Examples include pervious concrete, porous asphalt, or permeable pavers. They can be installed as alternatives to asphalt or cement low traffic areas such as parking lots, bike lanes, or traditional sidewalks.  

Green RoofsGreen roofs are building roofs that are either entirely or partially covered with vegetation. The roof contains layers of growing media to hold thewater so that the vegetation can absorb it through evapotranspiration.  In addition to managing stormwater, green roofs may also reduce the cooling demands of a building during summer months.

 Rainwater HarvestingRain barrels or cisterns are a simple way to collect and store rainwater for later use. These can be installed adjacent to a home or business and collect rain that falls on the roof through downspouts.                                                                          Green Roof at County Building with monitoring equipment                                                                                                                                                                                              (Photo from 3RWW)

Detention Tank: This is an underground tank designed to hold stormwater runoff during rainstorms.  It can be installed under any of the systems mentioned above to hold stormwater and allow for infiltration into the ground, or detain and slowly release back into the sewer if infiltration is not an option.

Rain Garden at East Liberty Presbyterian Church (Photo by Katherine Camp)

Pittsburgh has been implementing and monitoring GI systems to determine their effectiveness at managing stormwater. As the number of GI projects increase, PWSA will document their effectiveness to show the tangible benefits.  In addition to using GI to reduce the number and volume of overflow events and the likelihood of neighborhood flooding and backups, GI can provide environmental, economic and social benefits that would not be provided with traditional grey infrastructure. GI can drive neighborhood revitalization and economic development,   and improve public health. Many studies document the physical and mental health benefits of living near trees and other vegetation in urban areas. GI is also visible compared to traditional infrastructure, which is mostly found undergroud, so each project can serve as an opportunity to educate people about stormwater.

If you have additional questions about GI, please contact us at