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Strategic Asset Management Planning for Separate Storm Sewers

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Gwinnett County, GA, located in metro Atlanta, with a population of approximately 750,000 owns and operates a relatively young storm sewer system consisting of about 1,300 miles of storm sewer pipe. Growth has slowed and the county is maturing from its development phase and is moving swiftly towards a redevelopment and maintenance footing.

Following some preliminary analyses, conducted several years ago, the county understood that it was facing a significant short to mid term financial threat in the management of much of this donated storm sewer system. Fully 80% of the County owned storm sewer in Gwinnett was constructed with various types of corrugated metal pipe (CMP) which is estimated locally to have a median useful life of approximately 30 years. With replacement costs averaging $480/lf, and several hundred miles of galvanized and bituminous coated CMP already older than 20 years, the county considered and adopted a Stormwater Utility in 2006 with the primary purpose of funding the rehabilitation or replacement of the aging storm sewer system. In 2009 Gwinnett's Stormwater Utility fee will provide dedicated stormwater management funding in the order of 31M.

In 2007 the GA Environmental Protection Division, which is delegated with authority to issue MS4 NPDES permits in GA, advised that they would be requiring that 20% of all storm sewer infrastructure be inspected annually. This in conjunction with the financial threat described above, and the fact that the County's Water Resources epartment had recently created an Asset Management Work Group, provided the necessary impetus for the Stormwater Utility to commence the development of a Strategic Asset Management Plan (SAMP) for the storm sewer system.

Initially, a SAMP Project Work Group with broad representation from front line construction and maintenance staff to engineers, Asset Management staff and consultants was formed. A SAMP plan framework was completed over the course of several months and facilitated by the consultants. At the end of this initial stage utility staff took over the development of the plan almost exclusively and further refined the document through many additional meetings and workshops.

A major data gap was quickly identified. The condition of the storm sewer system in Gwinnett had never been adequately assessed. Some minor level of condition assessment, consisting exclusively of top of manhole inspections, had been completed during the county's storm sewer infrastructure inventory between 2000 and 2007. With the knowledge that resources were limited, the SAMP work grouprealized that prioritization of infrastructure inspections, and thoughtful allocation of rehabilitation and repair (R&R) dollars, would be of utmost importance. In response, and within the larger context of SAMP development, the utility identified available data sources that could be used to support a consequence and likelihood risk analysis of the storm sewer system.

Each of the pipes within the county system was assessed using consequence and likelihood criteria developed by the SAMP Work Group. This resulted in the 83,000 storm sewer pipes in Gwinnett being ranked from highest to lowest risk of failure. A critical subset of these pipes was identified and the utility's condition assessment program is now being directed based on these results.

Much was learned about the system through the process of developing the SAMP, and with scheduled annual reviews and updates to the plan, it is believed that the utility will be better able to manage the financial risk associated with maintaining, rehabilitating and repairing the county's storm sewer system.
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Keywords: MS4; Municipal Separate Storm Sewer System; SAMP; Strategic Asset Management Planning

Document Type: Research Article

Publication date: 2010-01-01

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