Green Stormwater Infrastructure (GSI) – What's it all about?

Green Infrastructure. You may have heard this term before, but what does it mean?
Green Stormwater Infrastructure (GSI) – What's it all about? - News

Updated: September 17, 2018

Green Stormwater Infrastructure (GSI) – What's it all about?

An example of pourous paving in North Philadelphia (Photo by: Tommy McCann, Penn State)

A Brief History

Have you been seeing terms like GI, GSI, raingarden, stormwater management, infiltration basin, swale, bump out, raised dropped inlet or tree trenches? There are so many terms out there now, and they are all generally describing a similar scenario that are innovative approaches to water management. But what does it all mean?

Well, one aspect that I think we can all relate to is the volume of impervious surfaces that exist in virtually every landscape in the United States, and around the world. When we discuss impervious surfaces, we are referring to areas where water cannot drain and include: macadam, concrete, roofing materials, sidewalks, or generally most paved surfaces. Every time it rains, the areas that are now impervious, that once allowed water to drain through them (before the concrete for example), additional burden is placed on every location where water is still able to drain.

For a long time, since the industrial revolution or so (circa 1880), our landscapes have been able to absorb that additional burden relatively easily. Fast forward to say the 2000s, and two things are happening, we are seeing more and more areas being covered by surfaces that do not allow water to penetrate, and we are observing more intense volumes and frequency of rain events. So, the big question becomes, where does the water go?

In virtually every major city in the United States, cities historically constructed infrastructure to accommodate the volume of rain they typically saw, and then some. The older the city, the older that infrastructure. The first thinking of sewer systems combined both water from the streets with water from toilet discharges, to then eventually treating all of this water at a localized sewage treatment plant as technologies progressed.

The first vestiges of sewage treatment plants began in the United State as far back as 1890, and slowly progressed into the complicated systems we see today. These systems are able to use a combination of technologies to take and remove contaminates and clean water from the sewer systems for reuse by each city or municipality. This is an amazing step forward towards a goal of sustainability. This is not as easy as it all sounds however.

Every sewage treatment plant is built with a maximum capacity. These systems have been built over a 100-year period and all have varying capacities for the volume of water that they can accommodate. As we discussed above, it was difficult to predict in the late 1800s what water capacity needs would be in 2017. The same can be said from say 1950 to 1990, and of course, upgrades are made to improve and repair systems over time, but it is still a constant issue of balancing water volumes. We also discussed the idea that the number and frequency of major storm events has increased over this period as well.

With the increase of surfaces where water cannot penetrate along with larger and more frequent storms comes an increased burden on existing sewage treatment plants. Then, the old piping systems have been constructed over that long period of time where all water from streets, roofs and toilets go through the same pipes. Although this is not the entire story, to break the movement on Green Stormwater Infrastructure (GSI) down to its simplest terms, engineers and designers are looking at alternative technologies to reduce the impact that large storm events have on existing sewerage infrastructure. Not only are these professionals seeking a cost-effective solution, but also looking at the longer-term community benefits as well.

Combined Sewer Overflows (CSOs)

The EPA (Environmental Protection Agency) recognizes the difficulty that these large storm events pose on the vast sewer systems around the country and are constantly seeking ways to support cities and municipalities to reduce this burden as well. As part of this recognition of capacity issues of these plants, the EPA allows, or permits systems to be put in place for the extra volume of water in large events to overflow into major water bodies. These permits are called CSOs, or Combined Sewer Overflows. The EPA reports that there are over 850 municipalities across the nation that have issues related to CSOs.

The bigger issue of CSO events during major storms, is that the contaminates collected from streets, toilets and roof tops get directly expelled into major water bodies. The consequence of which can affect wildlife, down-stream communities, erosion, sedimentation and more. With the allowance of these permits, however, also come fees that are significant enough to encourage cities and municipalities to reduce the occurrence of CSO events.

This is the crux of where the whole GSI (Green Stormwater Infrastructure) movement comes in. The need is to reduce CSO events. The issue is that at peak times, there is too much water flowing into sewer systems, which needs to be released when the systems are taxed beyond capacity. One approach that has been looked at is to separate toilet pipes from surface water pipes, which would reduce the burden. This would be a monumental hurdle for any city to achieve, however.

Green Stormwater Infrastructure (GSI)

Then comes the discussion of, ‘How else do we reduce the volume of water entering the treatment plants’? The answer that is gaining national and global attention, is to go green, and collect water anywhere you can, and detain it for approximately 72 hours. This detainment gives the sewage treatment plants enough time to accommodate the additional volume, and not cause CSO events. This was a major change in traditional thinking and is leading towards a whole new industry.

In this thinking, the goals are multifaceted and layered, however. The overarching goal is to reduce the amount of water entering sewage treatment plants, and ultimately reducing the amount of untreated drainage entering major water bodies during high volume storm events. By going ‘green’, there are added benefits that include the reduction of contaminates entering water treatment plants, lower cost than traditional piping or gray infrastructure, the direct site containment with zero or very little discharge to conventional sewers during smaller rain events and the aesthetic and social benefits to local communities which are often more difficult to quantify.

Traditionally, you may be used to detention basins in a housing development. These basins were created to detain all of the water associated with a particular development, and then slowly release that water into the ground. These structures are often very large and can look unsightly if not maintained well. What is happening now is to try and collect water in any location possible. So, no longer is the focus solely on finding large detention, but also seeking small areas, particularly in cities, to detain water. All of these small areas are adding up and making an impact on reducing the burden on the sewer systems.

Philadelphia Water, the organization that is in charge of stormwater issues for the city, is now seen as a leader in this field across the country and internationally. They are 5 years into a 25-year plan where $2 billion is being spent to reduce the volume of water that is detained prior to entering sewer systems during rain events. To raise these funds, the city of Philadelphia has enacted a Stormwater fee for commercial properties. The fee is based on the impermeable square footage of a particular property. There are now over 700 of these GSI units throughout the city.

For more information contact the Extension Green Industry Team and Tommy McCann at