The Green Infrastructure Leadership Exchange, in partnership with Earth Economics, recently completed a Green Stormwater Infrastructure (GSI) FAQs to help Exchange members have effective conversations with agency leadership and colleagues about the adoption of GSI, with intent to preventing GSI budget cuts in the short term and expand funding in the long term. This is the second of several deep dives that aims to help members better navigate and internalize this comprehensive document. 

The first two sections of the document describe its purpose and the key terms used. The third and fourth sections – “Performance Considerations” and “Cost Considerations” – were already summarized here.

The fifth section of the document – summarized below – includes 5 case studies that demonstrate examples of utilities that have opted to implement some type of green stormwater infrastructure (GSI), typically as part of a hybrid approach, to achieving water quality and volume reduction goals.

Camden County, NJ: Camden County Municipal Utilities Authority (CCMUA) and the two city permittees (City of Camden and Gloucester City) face combined sewer overflow (CSO) problems and need to meet updated National Pollutant Discharge Elimination System (NPDES) permit regulatory requirements from the State of New Jersey, while also addressing street flooding and water quality optimization. The goal is to capture 85% of the annual combined sewage generated during precipitation events for treatment by optimizing current grey infrastructure systems with expanded treatment capacity at a water pollution control facility (WPCF). In addition, they are committed to mitigating 10% of all system-wide directly connected impervious areas (DCIA) from contributing runoff to the combined sewer system by using GSI. While they know they’ll be unable to meet all concerns with GSI, results thus far indicate that hybrid annual and life cycle operations and maintenance (O&M) costs will be lower than a solely grey approach.

Metropolitan Milwaukee, WI: Milwaukee’s urban core consists of combined sewer systems (CSS) that experience approximately two CSO events each year, but many residents continue to experience basement backups. In addition to these concerns, updated discharge permit conditions required MMSD to annually install one million gallons of green infrastructure (GI) capacity to the region. To meet these goals, a Green Infrastructure Plan was established with the goal of capturing the first half inch of rainfall on impervious surfaces by 2035 – an equivalent of 740 million gallons of stormwater storage. To address stormwater concerns and meet permit requirements, MMSD analyzed 10 GI alternatives, from greenways to green parking lots, for performance and costs, and also, a system-wide triple-bottom line analysis was conducted on the economic, social, and environmental benefits of the GI strategy. Most GI options were estimated to be more cost-effective than the deep tunnel alternative per gallon of storage, and the benefits included $44 million in cost savings by avoiding the need for deep tunnel storage; creating over 500 maintenance jobs at full implementation; $1.4 million in public health benefits, and $1.5 to $2.1 million in energy savings.

City of Lancaster, PA: Historically, about 1 billion gallons of polluted stormwater flowed into the Conestoga River from Lancaster’s CSS annually. The city was placed under a US Environmental Protection Agency (EPA) consent decree for CSO)and total maximum daily load (TMDL) violations within the Chesapeake Bay watershed. Despite repeated efforts and substantial investments to mitigate the concerns, the city still struggles. Numerous green infrastructure (GI) investments have sought to address this long-standing concern. A 2011 plan considered additional GI options, comparing various GI types with overflow storage tanks and the city implemented multiple GI pilot projects to monitor the effectiveness. The city are finding the best approach is to make GI a core part of its Public Works activities as roads, alleys, parks, and other public infrastructure are improved, relying on opportunities for available funding and integrating with other infrastructure and planning efforts.

City of Vancouver, BC: The City of Vancouver has a rapidly growing population and with urbanization, the natural infiltration ability of the land decreases, leading to greater runoff. In areas with CSS, this can lead to more frequent and intense overflow events. At the same time, the increasing development to meet growing population needs creates competition for land that often impedes GI implementation. Vancouver discovered that Stormwater Tree Trenches (STT) fit well into many developments and transportation projects being implemented through the citywide Integrated Rainwater Management Plan (IRMP). A literature review was conducted for four STT designs in conjunction with bioswales and grey infrastructure approaches, in addition to a life cycle cost analysis of these various scenarios within a boulevard reconstruction project. STT were found to be 20% – 30% cheaper than bioswales and conventional grey infrastructure. And although GI options were 28% – 59% more expensive over the project life cycle than a pave only approach, that approach alone will not meet the City’s sustainability goals. Furthermore, the capital expenditures were the costliest for conventional approaches with water quality treatment devices.

City of Gresham, OR: The City of Gresham requires new and redevelopment projects that add or replace at least 1,000 square feet of impervious surface to manage stormwater quality and quantity, in accordance with requirements in the City’s Stormwater Management Manual. To identify the most cost-effective approaches, the City conducted a lifecycle cost analysis of commonly used green and gray infrastructure alternatives. Streetside planters were found to be the most expensive option due to their relatively high maintenance costs, followed by the Contech filters. Pervious asphalt, despite its initial high cost, was found to be comparable on an annual basis to Stormwater Tree Wells, because it doesn’t require additional maintenance beyond what is already conducted on other conventional streets. Ponds/swales were the most cost-effective, with low capital and maintenance costs. Finally, a “hybrid” option that blends street-side stormwater planters with large downstream centralized vegetated facilities is almost half the cost of street-side planters alone.

City of Harrisburg, PA: The City of Harrisburg, PA has a combination of CSS and MS4 . Capital investments are needed to meet National Pollution Discharge Elimination System (NPDES) permit baseline levels of control, capturing 85% of CSS volume in a typical year. Capital Region Water conducted an analysis to determine the most cost-effective approach to meet regulatory requirements while avoiding unreasonable ratepayer burdens over the next 20-years. They assessed alternatives at both the systemwide scale and locally in 10 catchment planning areas. At the systemwide level, rehabilitation and replacement of treatment and conveyance components were more feasible than the deep storage tunnel option. Among the localized alternatives, the decentralized green/grey hybrid approach was predominantly the most cost-effective option. In the few areas where it did not meet the baseline levels of control, it was found that adding targeted localized sewer separation alongside it was most effective. In addition, a Triple Bottom Line evaluation found the hybrid approach provides the most benefits.

To review all of the details of each case study, along with related data tables, check out the full GSI FAQ document.


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