Tree trenches can be used in parking lots where a walkway system is being developed along shade trees. A tree trench is a system of trees connected by an underground infiltration structure. Photo: Thomas McCann, Penn State
There are so many terms out there now, and they are all generally describing similar scenarios that are innovative approaches to water management. But what does it all mean?
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 including macadam, concrete, roofing materials, sidewalks, or generally most paved surfaces. Surface areas that once allowed rainwater to drain through them (before the concrete for example), and are now impervious, place an additional burden on every location where water is still able to drain.
Since the industrial revolution or so (circa 1880), our landscapes have been able to absorb that additional burden relatively easily. Fast forward to the 2000’s, 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?
Every major city in the United States has 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 sewer systems combined both water from the streets with water from toilet discharges. As technologies progressed, sewage was eventually treated at a localized sewage treatment plant. The first vestiges of sewage treatment plants began in the United States as far back as 1890 and slowly progressed into the complicated systems we see today. These systems can use a combination of technologies to remove contaminants and clean the water from the sewer systems for reuse by each city or municipality. This is an amazing step forward towards the goal of sustainability, but not easily accomplished.
Every sewage treatment plant has a built-in maximum capacity, and all have varying capacities for the volume of water that they can accommodate. Predicting what water capacity needs would be in 2017 back in the late 1800’s would have been impossible. The same can be said from 1950 to 1990, and so forth. Upgrades are made to improve and repair systems over time, but it is a constant balancing act with water volumes. 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. Especially those systems with the old piping systems from the time period where all water from streets, roofs, and toilets went through the same pipes. Defining the Green Stormwater Infrastructure (GSI) movement in its simplest terms, engineers and designers are looking at alternative strategy and technology solutions 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 Environmental Protection Agency (EPA) recognizes that these large storm events are negatively impacting the vast sewer systems around the country and constantly seeks ways to support cities and municipalities in reducing this burden. As part of this recognition of sewer plant water capacity issues, the EPA permits systems to be put in place for the extra volume of water created in large storm 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. With the allowance of these permits however, also come significate permit fees to encourage cities and municipalities to reduce the occurrence of CSO events.
The bigger issue of CSO events during major storms is that contaminates collected from streets, toilets, and roof tops get directly expelled into major water bodies. This can affect wildlife, down-stream communities, erosion, sedimentation, and more. This is where the whole Green Stormwater Infrastructure (GSI) movement comes in—the need to reduce CSO events. 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 would be to separate toilet pipes from surface water pipes, which would reduce the overall water burden. However, this would be a monumental hurdle for any city to achieve.
Green Stormwater Infrastructure (GSI)
The question then becomes, ‘How else do we reduce the volume of water entering sewer treatment plants?' The answer 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 a CSO event. This is a major change in traditional thinking and is leading a whole new industry.
The main goal of GSI is to reduce the amount of water entering sewage treatment plants and ultimately reduce or eliminate the amount of untreated sewer drainage entering major water bodies during high volume storm events. By "going green" additional benefits include the reduction of contaminants entering water treatment plants, lower costs than traditional piping or gray infrastructure, and the direct site containment with zero or very little discharge to conventional sewers during smaller rain events. There are also the aesthetic and social benefits to local communities which are often more difficult to quantify.
You may be used to seeing 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. Recently the focus has shifted from sole reliance on finding large water detention areas to also seeking small areas, particularly in cities, to try and collect water in any location possible. All of these small areas are adding up and making an impact on reducing the burden on the sewer systems.
Philadelphia Water, the entity who is in charge of stormwater issues for the city of Philadelphia, is now seen as a leader in this emerging field of Green Stormwater Infrastructure. The city is five years into a 25-year plan where 2 billion dollars is being spent to reduce the volume of water detained before 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!
Permeable pavement (also known as pervious or porous concrete) is a specific type of pavement with a high porosity that allows rainwater to pass through it into the ground below. Pervious concrete mimics the natural process that occurs on the ground’s surface, reducing runoff, and returning water to underground aquifers. It also traps suspended solids and pollutants, keeping them from polluting the water stream. Photo: Thomas McCann, Penn State
A rain garden is a garden of native shrubs, perennials, and flowers planted in a small depression, which is generally formed on a natural slope. It is designed to temporarily hold and soak in rainwater runoff that flows from roofs, driveways, patios or lawns. Photo: Thomas McCann, Penn State