Economical CSO Management

Combined sewer overflows (CSOs) represent major water-quality threats to hundreds of cities and communities in the US that are served by combined sewer systems. CSO events cause the release of untreated stormwater and wastewater into receiving rivers, lakes, and estuaries, causing a host of environmental and economic problems. Costs associated with CSO management are expensive. The EPA estimates the costs of controlling CSOs throughout the country are approximately $56 billion (MacMullan 2007).

The traditional approach to CSO management involves the development of a separate drainage system to convey stormwater flows or the use of grey infrastructure and conventional stormwater controls for enhancing the storage and conveyance capacity of combined systems. These approaches can include the construction of large underground storage tunnels that store sewage overflows during rain events for later treatment, as well as necessary improvements and upgrades to municipal treatment facilities to handle increasing volumes. Both approaches, while effective for CSO controls, are very expensive.

Integrating green infrastructure strategies and low-impact development (LID) designs into a CSO mitigation plan can help communities achieve CSO management requirements at lower costs. In addition to many benefits including groundwater recharge, water-quality improvements, and reduced treatment costs, the use of LID can help minimize the number of CSO events and the volume of contaminated flows by managing more stormwater onsite and keeping volumes of runoff out of combined sewers (MacMullan 2007).

Using a combination approach of grey and green infrastructure strategies can be a considerably more cost-effective method for CSO management as compared to a traditional grey infrastructure approach alone. Indeed, LID methods can cost less to install, can have lower operations and maintenance (O&M) costs, and can provide more cost-effective stormwater management and water-quality services than conventional stormwater controls (MacMullan 2007). Some LID alternatives are also being initiated by the private sector. While municipalities may provide oversight and consultation, as is the case with the city of Portland, OR, private sector projects are not controlled by municipalities in regard to implementation, operation, and maintenance.

The purpose of this article is to show the costs and benefits of integrating green infrastructure strategies with traditional grey infrastructure. Although communities rarely attempt to quantify and monetize the avoided treatment costs from the use of LID designs, the benefits of these practices for decreasing the need for CSO storage and conveyance systems should be factored into any economic analyses (USEPA 2007).

The following examples are presented to develop an economic context for the use of green infrastructure and LID designs as a strategy for CSO compliance. The examples also identify and contrast historical grey infrastructure approaches to CSO management using store, pump, and treat with approaches using green infrastructure and LID designs that focus on reduced stormwater runoff volumes.

Portland, OR
The city of Portland is considered a national leader in the implementation of innovative stormwater management strategies and designs. Included among the city’s Sustainable Stormwater Management Programs is the Innovative Wet Weather Program, the Green Street Program, the Portland EcoRoof Program, and individual case studies and projects that include commercial and multifamily stormwater retrofits and porous pavement placements.

With Portland receiving an average of 37 inches of precipitation annually, creating roughly 10 billion gallons of stormwater runoff per year, these programs are very important for helping reduce flooding and erosion as well as minimizing CSO events.

Innovative Wet Weather Program. This citywide program encourages the implementation of stormwater projects that improve water quality and watershed health, reduce CSO events and stormwater pollution, and control stormwater runoff peaks and volumes. According to the Portland Bureau of Environmental Services, the program goals include:

• capturing and detaining stormwater runoff as close to the source as possible;
• reducing the volume of stormwater entering the combined sewer system;
• filtering stormwater to remove pollutants before the runoff enters groundwater, streams, or
  wetlands;
• using and promoting methods that provide multiple environmental benefits; and
• using techniques that are less costly than traditional piped solutions.

Green Streets Program. Portland’s Green Street Program promotes the use of natural aboveground and vegetated stormwater controls in public and private development to reduce the amount of untreated stormwater entering Portland’s rivers, streams, and sewers. The program is geared toward diverting stormwater from the city’s overworked combined system and decreasing the amount of impervious surface so that stormwater can infiltrate and recharge groundwater systems.

The program takes a sustainable and blended approach to finding the most optimal solution for storm and sanitary sewer management. This includes overlaying and integrating green and sustainable stormwater strategies with traditional gray infrastructure to maintain or improve the city’s sewer capacity (Dobson 2008).

Green streets have been demonstrated to be effective tools for inflow control of stormwater to Portland’s CSO system. Two such green street designs, the Glencoe Rain Garden and the Siskiyou Curb Extension facilities, were shown to reduce peak flows that cause basement sewer backups and aid compliance with CSO regulations by reducing runoff volumes sent to the CSO Tunnel system (Portland 2007). The city of Portland also conducted simulated storm event modeling for basement sewer backups and determined that two green street project designs would reduce peak flows from their drainage areas to the combined sewer by at least 80 to 85%. The city also ran a simulation of a CSO design storm and found that the same two green street project designs retained at least 60% of the storm volume, which is believed to be a conservative estimate.

Economic Benefit
The following sections communicate the economic context for both the application of LID strategies in Portland, as well as the city’s programs that promote the use of green infrastructure designs for stormwater management.

Green Streets Program. For the city of Portland, utilizing green streets is the preferred strategy for helping relieve sewer overflow conditions because it is the most cost-effective and eliminates the need for expensive belowground repairs, which often involve replacing infrastructure (Dobson 2008). As an example, a basement flooding relief project that was under design was projected to cost 60% less than what would have been the cost of a traditional pipe upsize and replacement project. This is because the solution, a mix of green streets and private system disconnects, intercepts and infiltrates the water before it enters the public storm system, thereby reducing the need to dig up and upsize the existing piped infrastructure (Portland 2007).

Cost Comparisons Between Grey and Green Infrastructure Strategies. In June 2000, before the implementation of the Green Street Program, the city of Portland was faced with the need to upgrade an undersized sewer pipe system in the Brooklyn Creek Basin, which extends from the Willamette River to Mt. Tabor between SE Hawthorne and SE Powell boulevards, and covers approximately 2.3 square miles. Upgrades were needed to improve the sewer system reliability, contain street flooding, stop sewer backups from occurring in basements, and help control CSOs to the Willamette River.

At that time, the city considered constructing a new separated stormwater collection system to support the existing undersize pipes in this basin. The original cost estimate for constructing this new system using traditional grey infrastructure was $144 million (2009 dollars). However, following this proposal, a second plan was developed that included a basin redesign using a combined grey and green infrastructure approach. Including a total of $11 million allocated for green solutions, the cost estimate for this integrated approach was $81 million, a savings of $63 million for the city (Portland 2009).

The combined grey and green approach was chosen as the 2006 Recommended Plan for the Brooklyn Creek Basin, and includes project objectives of reducing CSO events, improving surface and groundwater hydrology, protecting and improving sewer infrastructure, optimizing cost-effectiveness, boosting water quality, and enhancing community livability.

The approved basin improvement plan consists of 35 public and private sector projects over the next 10 to 20 years. Grey infrastructure upgrades include repairing or replacing 81,000 feet of combined sewer pipes, while the green infrastructure strategies include building green roofs, retrofitting parking lots with sustainable stormwater controls, planting nearly 4,000 street trees, and adding more than 500 green streets with vegetated curb extensions and stormwater planters.

Green Infrastructure for CSO Compliance: Cost Comparisons. Portland’s combined sewer system covers 26,000 acres and contains 4,548,000 linear feet (861 miles) of gravity-drained, combined sewer pipe. The city’s combined system also includes 42 separate basins connected via three major interceptor systems and served by three major pump stations.

The city of Portland, under federal and state requirements as well as stipulations from the Clean Water Act to comply with regulations regarding CSO management, initiated the construction of a new pump station and two CSO tunnels (West Side and East Side CSO Tunnels), which would serve as the primary means to protect the city’s receiving waters from future CSO events. However, in addition to these initiatives, more projects and programs were needed for providing additional CSO mitigation.

In December 2005, Portland’s Bureau of Environmental Services prepared a report (Portland 2005) charged with sizing of the East Side CSO Tunnel and providing recommendations for long-term operations and flow management of the Willamette CSO system. The city’s final recommendations (Table 1) included the following for the Willamette CSO tunnels and supporting infrastructure:

East Side CSO Tunnel- This storage facility will be constructed with a 22-foot diameter and will have a capacity of 83 million gallons. Total length is 29,145 linear feet; annual O&M costs are $0.78 per linear foot. Design life is 50 years.

Swan Island CSO Pump Station- This facility pumps approximately 500 million gallons per year with an annual O&M cost of $0.0002 per gallon for pump station operations and $0.006 per gallon for Columbia Boulevard Wastewater Treatment Plant treatment. Design life is 50 years.

Portsmouth Force Main- This infrastructure is 66 inches in diameter and 15,000 feet in length. Annual O&M costs are $0.80 per linear foot. Design life is 50 years.

Balch Consolidated Conduit- This infrastructure is 84 inches in diameter and 4,900 linear feet. Annual O&M costs are $0.80 per linear foot. Design life is 50 years.

Along with determining the final recommendations for the East Side CSO Tunnel and supporting infrastructure, the city considered a range of possible alternatives for additional CSO mitigation. This included 12 different stormwater separation projects as well as a number of watershed health initiatives, some of which involved green infrastructure strategies, including:

Eastside Curb Extensions- Curb extensions involved the use of vegetated swales at a cost of $50,000 per acre and O&M costs of $2,000 per year per acre.

Eastside Roof and Parking Inflow Control- Parking retrofits use vegetated infiltration basins at a cost of $90,000 per acre and O&M costs of $1,100 per year per acre. Rooftop stormwater controls use either stormwater planters ($40,000 per acre; O&M costs of $600 per year per acre) or vegetated infiltration basins.

Green Roof Legacy Project- The project would retrofit 20 acres of rooftop in an industrial district with ecoroofs. Project costs include $285,000 per acre per year for design/construction and $935 per acre per year for O&M activities.

Extended Downspout Disconnection Program (DDP)- This program continues the city’s successful existing DDP at the cost of $22,300 per acre and O&M costs of $7 per year per downspout. Depending on site conditions, this can include the use of LID strategies including rain gardens and soakage trenches built by private citizens with city of Portland consultation.

The city’s goal was to determine which project/program alternatives would be the most cost-effective for long-term CSO management. The basic metric common to the projects identified for CSO control was the amount of stormwater volume that could be removed from the CSO tunnel system. The city’s final evaluation was based on the relationship between project capital costs and stormwater volume that could be removed from the system. This analysis took into account cumulative capital costs, marginal costs for gallons removed, and cumulative volume removed from the system.

The stormwater separation and watershed health projects/programs considered by the city of Portland are sorted by dollars per gallons of stormwater that can be removed (marginal cost). Project staff agreed that cost-effectiveness was determined by an inflection point, or knee-of-the-curve point, on a graph that compared costs to stormwater volume that could be diverted from the CSO system. This inflection point was determined to be approximately $4 per gallon removed from the system.

Projects/programs costing at or below $4 per gallon were the ones recommended for further design and eventual implementation for long-term CSO control.

The projects/programs chosen on the basis of cost-effectiveness included the Eastside Curb Extension projects (vegetated swales), the Eastside Roof and Parking Inflow Control projects (vegetated infiltration basins and stormwater planters), three disconnection programs (which can include LID strategies), and two stormwater separation projects.

LID Avoidance Costs. The city of Portland recognizes two avoidance costs for incorporating LID strategies with combined sewer systems. One of these avoidance costs is annual O&M costs to pump and convey stormwater through the existing combined sewer system. The city measures this by applying a rate of $0.0001 per gallon treated and $0.0001 per gallon pumped. This equates to an annual O&M avoidance cost of $0.0002 per gallon.

Secondly, the city recognizes an avoidance cost that benefits the CSO system. This is based on the relationship between project capital costs and stormwater volume removed from the CSO system, which was described above. The cost-effectiveness point for projects/programs that remove stormwater volume from the CSO system ($4 per gallon) is also considered as the avoidance cost of constructing a larger CSO tunnel. In lifecycle-cost analyses, this “savings” can reduce the capital costs of other LID facilities that the city builds for objectives other than CSO control (e.g., water-quality improvements, basement flooding relief), but still removes stormwater from entering the CSO tunnels (Owen 2009).

Kansas City, MO
Kansas City, MO, has committed to implementing a green design initiative that will be considered a community amenity and will work to reduce the amount of water entering the city’s combined system.

Under a USEPA mandate, Kansas City is required to update its network of aging sewer infrastructure to address overflows from its combined and separate sewer systems. Kansas City’s 318-square-mile sewer system includes 58 square miles of a combined system and 260 miles of a separated system. The overall system serves 668,000 people and includes seven wastewater treatment plants with a total capacity of 153 million gallons per day.

Overflows in the combined system amount to 6.4 billion gallons in a typical year, and, on average, 12 rain events per year are responsible for 67% of this total overflow. This contributes to the poor water quality of Kansas City’s streams, urban lakes, and rivers.

The original planned improvements associated with upgrading the city’s combined system include 310 million gallons per day of additional treatment capacity, 25 million gallons of inline storage, 10 separation areas, neighborhood sewer rehabilitations, and pump station and treatment plant modifications. Three storage tunnels from 16 to 26 feet in diameter are also proposed, which would run between 1.4 and 3.4 miles in length and would be capable of storing 78 million gallons of overflow. The goals of the improvements in the combined sewer system are to capture 88% of flows, reduce the frequency of overflow events by 65%, and lower the 6.4 billion gallons of overflow per year down to 1.4 billion gallons (KCWSD 2009).

The original estimated capital costs associated with overhauling Kansas City’s total sewer system is $2.4 billion dollars, of which $1.4 billion would go toward the combined system. The yearly O&M costs of this total upgrade are estimated at $33 million per year.

Green Solutions. In developing a plan for the combined sewer system upgrade, Kansas City began exploring the possibility of incorporating green infrastructure strategies in combination with grey infrastructure improvements. The city formed a green solutions subcommittee and later developed a green solutions position paper, which eventually resulted in a city council resolution directing city staff to develop a plan to implement green infrastructure strategies.

Green Overflow Control Plan. In May 2008, the Kansas City Water Services Department proposed $30 million in green solutions during the first five years of the proposed $1.4 billion overflow control plan. This plan included language to allow green solutions to replace grey infrastructure. Upon review, however, the city council determined that additional green infrastructure strategies were needed in the overflow control plan and directed the Water Services Department to request a six-month extension for submittal of the plan. The extension was granted by the Missouri Department of Natural Resources and EPA Region 7.

The city moved ahead in developing a more green-orientated overflow control plan and conducted reviews of basins located within the combined system to identify areas where green solutions could replace grey infrastructure in whole or in part. High-altitude desktop analyses were performed to assess the potential for shifting from grey storage to green solutions for storage in three major basins. The types of green solutions considered included catch basin retrofits, curb extension swales, pervious pavement, street trees, green roofs, and stormwater planters.

Two principal assumptions were included with these considerations. Firstly, storage volume in green solutions would replace an equal volume in conventional storage facilities; and secondly, each one million gallons of green storage would result in a 0.5-million-gallon-per-day reduction in capacity of downstream pumping stations and treatment facilities due to infiltration and evaporation (KCWSD 2009). Following revisions, the city submitted a new plan that proposed a total of $80 million in green solutions programs.

Economic Benefit
Based on city analyses, it was determined that replacing grey infrastructure with green solutions would be cost-effective in portions of the Middle Blue River Basin (MBRB), a 744-acre region with 34% impervious surface. Based on calculations, the city estimated that it should be possible to completely replace two CSO storage tanks with distributed green solutions without increasing costs or reducing CSO control performance (Leeds 2009).

The original MBRB plan was based on a traditional grey infrastructure design with controls capable of providing 3 million gallons of storage. The capital costs associated with these upgrades were estimated at $54 million, an average of $18 per gallon, and would be capable of reducing overflows in the MBRB to fewer than six per year, on average.

The revised MBRB plan is a nontraditional design that includes grey infrastructure projects as well as green infrastructure strategies and will provide distributed storage of at least 3.5 million gallons. The revised plan would also eliminate the need for storage tanks while still achieving the goal of reducing the number of overflows to fewer than six per year. The projected costs associated with this revised plan are $35 million, potentially $19 million less than the original grey infrastructure plan. However, because of uncertainties, the green solutions project budget has been set at $46 million.

MBRB Green Solutions Pilot Project. A large-scale study was needed to test the city’s key assumptions regarding the performance of green solutions. As such, the city initiated a pilot project within a 100-acre area of the MBRB. The MBRB Green Solutions Pilot Project will help determine the effects of widespread implementation of distributed storage utilizing green solutions, infiltration, and inflow rehabilitation on combined sewer overflows and is potentially the largest green solutions-based CSO control project in the nation.

Green-based strategies in the pilot area will be installed on both residential and commercial areas and will need to provide at least 0.5 million gallons of distributed storage, replacing an equal amount of stormwater stored in conventional concrete tanks. Following implementation, post-construction monitoring will be conducted to determine functionality and performance.

Green Solutions Unit Costs. In developing unit costs for green solutions, the city used a number of assumptions including:

• Green roofs have incremental costs above normal roof replacements, with 3 to 4 inches of growth media providing 1 inch of storage. Incremental capital costs associated with green roofs are $14 per square foot.
• Deciduous street trees have interception storage of 0.032 inch, 20-foot crown radius, with 25 gallons per tree.
• Porous pavement provides effective storage for an area approximately three times its surface area.

Table 3 (Leeds 2009) presents unit costs, in dollars per gallon, used by the city for each type of green solution. The results of the pilot project will be used to guide work in the remaining 644 acres as well as other future green solutions projects.

Chicago, IL
The city of Chicago has implemented a number of innovative plans geared toward building community resiliency toward climate change, while promoting sustainability and conservation. The city is recognized as a worldwide leader in terms of its environmental initiatives. In addition to green building and energy efficiency, Chicago has implemented advanced citywide programs that address water quality, water efficiency, and stormwater management.

As part of the Chicago Water Agenda, the city is committed to managing stormwater more sustainably and encourages the use of BMPs that include a range of green infrastructure designs such as green roofs, permeable paving, filter strips, rain gardens, drainage swales, naturalized detention basins, and the use of rain barrels and natural landscaping. These measures are important strategies for facilitating infiltration, improving water quality, and minimizing the potential for basement flooding. BMP strategies that divert water away from the combined sewer system also reduce the energy demands associated with pumping and treating the combined sewage.

Chicago’s gravity-based combined collection system includes 4,400 miles of sewer main lines that flow to interceptor sewers owned and operated by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC). The interceptor sewers convey dry-weather flow to the MWRDGC’s treatment plants. During storm events, excess flows are diverted to the MWRDGC’s Tunnel and Reservoir Plan system for storage, which is intended to prevent combined sewer overflows to the city’s waterways. This tunnel reservoir system is the largest in the world and includes 109 miles of 8- to 33-foot-diameter pipes that are generally located 200 feet below the Chicago River system.

CSO events occur with regular frequency each year, causing untreated wastewater and stormwater to be released into the city’s river systems. Green infrastructure controls and other BMPs help limit inflow stormwater volumes to the system, thus reducing the frequency and intensity of CSO events.

Chicago Green Alley Program. One of the city’s more progressive green infrastructure initiatives is the Chicago Green Alley Program, developed to alleviate flooding in the city’s extensive alley network, which consists of approximately 1,900 miles of public alleys and roughly 3,500 acres of impervious surface. The program encourages the use of porous pavements to reduce the city’s quantity of impervious surface, as well as to filter runoff and recharge groundwater.

In addition to facilitating infiltration and diverting stormwater from Chicago’s combined system, the Green Alley Program brings environmental benefits such as heat reduction, material recycling, energy conservation, and glare reduction.

Economic Benefit
The city of Chicago actively records the ongoing number or coverage area of various green BMP designs that are added within city limits. This includes the year-to-date number of rain gardens and rain barrels added and downspouts disconnected, as well as the effective square footage of green roofs, green paving, turf to native grass, and stormwater management ordinance permits. Each of these BMPs has been assigned an equivalence factor by the city, which, when multiplied by the actual number or amount of square footage of each BMP, will calculate a more accurate shed of capture for each representative design.

To calculate the volume of stormwater that is diverted from the combined system, the city uses a conversion factor of 21.19 that is multiplied by the square-foot equivalence of each corresponding BMP design. Based on the above BMPs, equivalent factors, and calculations, a total of 70,182,236 gallons of stormwater is estimated to have been diverted from Chicago’s combined system in 2009 through November 10, 2009.

New York, NY
New York City, facing the need to improve the water quality of its waterways and coastal waters, has developed a multi-tiered, long-term plan that will draw upon green infrastructure strategies toward managing stormwater more sustainably. The NYC Green Infrastructure Plan, an extension of the city’s Sustainable Stormwater Management Plan and Mayor Bloomberg’s PlaNYC initiative for a cleaner, greener city, will employ a hybrid approach to controlling CSOs and improving water quality. In addition to green infrastructure designs such as porous pavements, green streets, green and blue roofs, swales, rain gardens, street trees, constructed wetlands, and other strategies, the Green Infrastructure Plan will also use targeted, cost-effective grey infrastructure investments and measures to optimizing the existing system to control CSOs and reduce stormwater runoff volumes.

According to analyses by the New York City Department of Environmental Protection (DEP), which examined areas of the New York Harbor where water-quality standards have not been met, the biggest remaining challenge is to further reduce CSOs. Since 2005, the city has spent more than $1.5 billion on CSO reduction, including infrastructure improvements and CSO storage facility upgrades. A conventional approach for CSO reduction would include the construction of large piping networks to store or separate stormwater and wastewater. However, according to the September 2010 NYC Green Infrastructure Plan report, these types of CSO reduction projects are very expensive and do not provide the sustainability benefits that New Yorkers have come to expect from multibillion dollar public fund investments. Furthermore, officials feel that while meeting water-quality goals is the primary consideration for future DEP investments, the long-range alternatives it considers should also be consistent with the city’s sustainability goals. CSO reduction strategies, according to the report, would be more valuable if they incorporated a sustainable approach, managing stormwater at its source through the creation of vegetated filtration (i.e., rain gardens, street trees, and constructed wetlands) and green infrastructure.

Conclusions in the city’s Sustainable Stormwater Management Plan found that green infrastructure could be more cost-effective than certain large infrastructure projects such as CSO storage tunnels. DEP modeling efforts demonstrated that the use of green infrastructure in combination with other strategies would be more effective at controlling CSOs as compared to grey strategies alone, but would also provide the additional benefits of cooling the city, reducing energy costs, and increasing property values. Moreover, green-based strategies would provide further economic benefits in terms of lower O&M costs; a greater distribution of O&M costs toward jobs, potentially resulting in job creation; improved air quality; and reduced carbon dioxide emissions.
Performance Comparisons Between Green and Grey Strategies. DEP evaluated and compared two different infrastructure investment plans for long-term CSO management and reduction: a green strategy and a grey strategy. The main components of each strategy are shown below.

Green strategy:

• green infrastructure
• cost-effective grey infrastructure investments
• system optimization and reduced flow

Grey strategy:

• cost-effective grey infrastructure investments
• potential tanks, tunnels, and expansions

Using an InfoWorks computer model to estimate future city CSO flows, DEP modeled CSO volume projections under both strategies to access and compare future CSO control performances for each alternative.

One of the assumptions made by DEP in reference to modeling of green infrastructure—which would be implemented as a combination of infiltration and detention technologies—included the capture and infiltration of the first inch of rainfall on 10% of existing impervious surfaces in each combined sewer watershed in the city.

According to predictions by DEP, implementation of the green strategy over 20 years will reduce CSO volumes from approximately 30 billion gallons per year to approximately 17.9 billion gallons per year. This is nearly 2 billion gallons per year more CSO reduction than the grey strategy, which was estimated to reduce CSO volumes down to 19.8 billion gallons per year.

Economic Benefit
In addition to significant citywide CSO reductions every year, DEP also predicted considerable economic benefits in several areas that would result from implementation of a green strategy as compared to a grey strategy.

Total Citywide Costs. According to DEP estimates compiled in the Green Infrastructure Plan report, costs associated with full implementation of the green strategy are anticipated to be considerably less than those for the grey strategy.

As shown, the total cost of the grey strategy is approximately $6.8 billion (2010 dollars), which includes $3.9 billion for the potential tanks, tunnels, and expansions component of the plan. The cost for the citywide green strategy, however, is estimated at approximately $5.3 billion, of which $2.4 billion would be allocated toward green infrastructure programs for capturing 10% of the combined sewer watersheds’ impervious areas. In total, the green strategy is forecasted by DEP to save the city $1.5 billion over the next 20 years.

The costs for each strategy were also broken down for comparison on a unit cost basis. Based on the cost per gallon of CSO reduction for each respective alternative, the grey strategy is estimated to be the more expensive option ($0.62 per gallon for the grey strategy versus $0.45 per gallon for the green strategy).

The cost per gallon of CSO reduction for each component of both strategies include:

• green infrastructure (green strategy): $1.60 per gallon of CSO reduced
• optimizing the existing system (green strategy): $0.03 per gallon of CSO reduced
• potential tanks, tunnels, and expansions (grey strategy): $1.75 per gallon of CSO reduced
• cost-effective grey infrastructure (both strategies): $0.36 per gallon of CSO reduced

As shown, the cost per gallon of CSO reduced for the green infrastructure component is estimated to be considerably less than the cost per gallon of CSO reduced for the potential tanks, tunnels, and expansions of the grey strategy. Also, as discussed in the report, the overall green strategy is a more affordable alternative than the grey strategy in part because optimizing the existing system—a part of the green strategy—is the most cost-effective component-strategy.

O&M Cost Estimates. DEP also estimated and compared long-term O&M costs to the city under both green and grey strategy scenarios. O&M expenses evaluated included salaries, electricity and natural gas, contracts, supplies and equipment, and fringe costs. From the Green Infrastructure Plan report, O&M costs for the green strategy would be higher in the initial years as green infrastructure controls are implemented relatively quickly. However, according to the estimates, O&M costs for the grey strategy would eventually outrun those of the green strategy as tanks, tunnels, and expansions are completed and come online. Another factor contributing to this cost difference is energy costs, including electricity and natural gas expenses, which are not needed for green infrastructure but would weigh in much heavier under a grey strategy scenario.

Economic Sustainability Benefits. Further value-added advantages predicted by DEP as a result of implementation of the Green Infrastructure Plan include benefits related to a reduced urban heat island effect, greater recreational opportunities, energy savings, improved air quality, and higher property values. In addition, the Green Infrastructure Plan shows a greater distribution of funds to support maintenance-related activities in the form of salaries and benefits. For every year scenario, there is a greater distribution of monies to support jobs rather than to pay for utilities (electric and gas). This is an important finding, as job creation is one element of sustainability that is often overlooked.

To estimate these dollar-based benefits, DEP first generated a working model to anticipate the amount of land that would be converted from impervious surfaces to planted areas. DEP’s modeling efforts forecasted that the amount of total citywide vegetated surface area by 2030 would range from 1,085 acres to 3,255 acres. Of this range, DEP assumed that half of all planted green infrastructure would be fully vegetated (such as green roofs), with the other half partially vegetated (to account for a lower ratio of surface area to drain impervious surfaces in the right of way).
Next, DEP estimated a dollar-per-acre benefit for both fully and partially vegetated infrastructure controls. For this process, DEP used the economic values for street trees from the New York Municipal Forest Resource Analysis as well as the energy benefit assumptions for green roofs in Green Roofs in the New York Metropolitan Region, as cited in the Green Infrastructure Plan. Using these data, DEP estimated the annual economic benefits resulting from fully and partially vegetated infrastructure controls on a dollar-per-acre basis in the year 2030.

The results of DEP’s analysis are displayed in Table 5, taken from the Green Infrastructure Plan report. As shown in Table 5, DEP estimates that in the year 2030, every fully vegetated acre will result in a total annual benefit of $14,457, and each partially vegetated acre $7,771 per year. This includes annual economic benefits from reduced energy demand, reduced carbon dioxide emissions, improved air quality, and increased property values.

DEP also estimated a range of accumulated economic benefits from new green infrastructure controls over a 20-year implementation time frame. According to DEP’s modeling efforts, the total accumulated sustainability benefits (through lower energy costs, reduced carbon dioxide, better air quality, and increased property values) will range from $139 to $418 million, depending on the amount of vegetation used in the source controls (New York City 2010).

Conclusions

The previous examples show how incorporating a green infrastructure strategy with LID can help cities and municipalities reduce stormwater runoff volumes entering combined systems, lowering treatment costs. Also, as shown, using a combination of grey and green infrastructure strategies for CSO management can be considerably more economically viable than using grey infrastructure alone.

This was clearly demonstrated in the Portland’s Tabor to the River plan, which showed a cost benefit of $63 million to the city through the inclusion of green strategies in combination with a grey infrastructure approach for upgrading an undersized sewer pipe system in order to help control CSOs and improve sewer system reliability. An economic benefit potentially as much as $19 million was also estimated by Kansas City for incorporating green infrastructure strategies along with a traditional grey infrastructure approach for the Middle Blue River Basin Plan, a part of Kansas City’s citywide overflow control program.

An economic context for the use of LID was also established for the city of Portland’s overall approach for CSO management. The city determined that watershed health initiatives, which included LID and green infrastructure strategies, were cost-effective project alternatives for the city to implement as part of its approach for long-term CSO management.

Chicago’s initiatives demonstrate the city’s commitment to using green infrastructure for the purpose of CSO control. Although economically based information depicting the future cost of construction for CSO separation was not available, the city of Chicago has shown a major reduction of stormwater volume to its combined system as a result of LID.

Additionally, New York City forecasted long-term performance and economic benefits by incorporating a CSO reduction plan that includes green infrastructure in combination with cost-effective grey infrastructure investments. New York City’s estimates also included future economic sustainability benefits in the form of lower energy costs, reduced carbon dioxide emissions, improved air quality, increased property values, and a greater distribution of operations and maintenance costs, which could potentially lead to more employment opportunities.

The projects and plans presented in this article establish an economical and performance-based benefit for LID and green infrastructure. Shown in the context of actual project designs, incorporating these strategies alongside grey infrastructure improvements can result in significant cost savings for cities pursuing and implementing CSO management. This article demonstrates the beneficial economic context for the implementation of green infrastructure and LID design for future CSO compliance projects.

The information provided here is part of a project titled Forging the Link: Linking the Economic Benefits of Low Impact Development and Community Decisions.

References
City of Chicago. 2009. Draft Stormwater Carbon Calculator.

Dobson, L. 2008. Program Manager For Sustainable Stormwater Management. Portland Bureau of Environmental Services, in discussion with the author, 2008.

Kansas City. 2008. “Kansas City’s Overflow Control Program Overview.” Kansas City Water Services Department, Kansas City, MO.

Kansas City. 2009. “Kansas City’s Overflow Control Program Overview.” Kansas City Water Services Department, Kansas City, MO.

Leeds, Terry. 2009. Presentation at the Midwest AWMA Annual Technical Conference (January 2009) by Terry Leeds, Overflow Control Program Manager, Kansas City Water Services Department.

MacMullan, E. 2007. “Economics of LID.” EcoNorthwest, Eugene, OR.

Middle Blue River Basin Green Solutions Pilot Project. 2009. “Meeting the O&M Challenges of Sustaining Green Infrastructure.” Kansas City Water Services Department, Kansas City, MO.

New York City. 2010. “NYC Green Infrastructure Plan.” City of New York, NY.

Owen, B. 2009. Engineering Services, City of Portland, Bureau of Environmental Services, in discussion with the author, 2009.

Portland. 2005. “CSO Sizing and Flow Management Final Predesign Report.” Portland Bureau of Environmental Services, Portland, OR.

Portland. 2007. “Green Street Policy.” Portland Bureau of Environmental Services, Portland, OR.

Portland. 2009. “Brooklyn Creek Basin: Project Objects.” Portland Bureau of Environmental Services, Portland, OR.