Effective Integration of Green Infrastructure into CSO Control Planning
Abstract:In response to a Consent Decree entered into Federal Court in August, 2005, the Louisville & Jefferson County Metropolitan Sewer District (MSD) is developing an Integrated Overflow Abatement Plan to eliminate or control SSOs and CSOs in the MSD system. MSD is implementing a unique approach to incorporating green infrastructure into its CSO abatement plans.
When used in CSO control, green infrastructure programs reduce the quantity of runoff that reaches the combined sewer system by increasing the effective pervious area for storm water to soak into the ground. In highly urbanized areas, restoring the natural environment and introducing green components alone typically will not eliminate combined sewer overflows. While green infrastructure techniques can be important components of CSO control programs, they typically serve as a supplement to the traditional gray systems.
Since green infrastructure relies primarily on infiltration or evapotranspiration of stormwater, it is heavily dependent on available land. In urban environments, these land-based control measures usually rely on willing community partners who install green infrastructure on property not owned or controlled by the wastewater collection agency. It is this lack of ownership and control that have made regulatory approval of green infrastructure solutions difficult in the past.
MSD's approach to green infrastructure integration is based on a front-end consideration of the probable benefits of projects developed through community partners. Closely monitoring the cumulative benefits within individual sewersheds allows an adaptive management approach to sizing gray infrastructure. The approach begins with a careful assessment of green infrastructure opportunities in each CSO sewershed and a further evaluation of probable “market penetration” and effectiveness of green solutions. After each sewershed is evaluated for green potential, a target reduction in impervious surfaces is established for each sewershed.
The targeted reduction in impervious surfaces then becomes the base condition for developing gray control solutions. Each gray solution is sized assuming the estimated green controls will be as effective as predicted. Close monitoring of actual results will allow right-sizing of the gray components after green solutions have demonstrated the actual level of run-off control achievable. This adaptive management step, and an assurance that gray solutions will be rightsized in the future has led to a high level of regulatory agency acceptance, and assurances of support when the plan is submitted.
An essential principal of the MSD approach is that while green best management practices (BMP's) on individual sites are a step in the right direction, a green plan that establishes connectivity between individual sites, neighborhoods, and ultimately the entire MSD CSO service area results in far greater benefits to the community than the sum of the individual components. This multi-scale approach is critical to the overall success of the program. Additionally, when compared to more traditional engineering solutions, the green techniques have a much greater potential for leveraging funding from sources other than sanitary sewer and storm water user fees.
Furthermore, a critical element to MSD's Green Infrastructure Approach is the relationships that form the foundation to the program. As noted previously, partnerships and collaboration with public agencies, private developers, and homeowners throughout the CSO service area are extremely critical to the success of the program. As members of the community, these stakeholders can offer excellent insight and funding to improve the environment in which they live. Partnerships are stronger and longer lasting when both parties receive benefits from the partnership. To encourage community partners to participate in green infrastructure projects, MSD has developed the business case to support a program of subsidies and incentives that represent the value to MSD's customers provided by a green project. For example, a green roof serves many functions, but MSD's primary benefit comes in the form of roof-top storage of rainwater. The volume of water stored by each square foot of green roof has an equivalent cost if this storage is provided by a gray storage tank. The equivalent storage cost is offered to potential partners as a subsidy that represents good business practice for both parties. In addition, greening a roof can result in reduced drainage fees, recognizing that a green roof is essentially pervious, not impervious area, at least up to a given storm level.
It is important to recognize that green infrastructure will be unique to every area and that it will require investigation and understanding of baseline information and pertinent features in the MSD CSO service area. The multi-scale approach not only considers regional concepts, neighborhoods, and individual sites; but it considers analysis of the underlying geological features of the land, soil characteristics, natural systems, and impervious surfaces. MSD has utilized this baseline information for the entire CSO service area to identify areas, neighborhoods, and sites suitable for green infrastructure practices.
This paper will detail the development of MSD's overall green infrastructure program. It will include examples of detailed sewershed green potential evaluations, and show how these evaluations translate into reduced runoff volumes. Programs to achieve the targeted runoff reductions will be described, including the development of the subsidy and incentives for partners.
Finally the paper will describe the process used to engage the entire Louisville Metro community, and the partnerships that have developed.
Document Type: Research Article
Publication date: January 1, 2009
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