Physical Condition Evaluation Pipelines

The City’s collection system condition ranges from Good to Poor, reflecting considerable variation in the structural integrity and capacity of the pipelines in the network. The condition of the pipelines is a function of a variety of factors including age, pipe material, soil conditions, past maintenance, initial construction, activities in the vicinity of the pipeline and the amount of growth which has occurred since the pipeline was initially designed and constructed.

As part of the Clean Water Atlanta program, the City has implemented a systematic approach for evaluating, rehabilitating and upgrading its collection system network. This approach was initiated in 1999 and is anticipated to be complete for the entire system by

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2014. The process the City is using to evaluate its collection system network includes both modeling and field investigation, and is generally as follows:

x A system-wide hydraulic modeling plan was developed that identified separate trunk and outfall sewers and their associated sewersheds.³ Using available information supplemented with field information, the sewersheds were evaluated with respect to a variety of criteria (i.e., frequency of overflows, risk associated with failure, capacity, etc.).

The sewersheds were then grouped into six sewer groups where the groupings reflected common level of priority. Those lines with the most critical need for evaluation, repair and replacement were placed in Sewer Group 1, with subsequent groups populated by lower priority lines. Sewer Group 6 includes those lines with the least need of evaluation, repair, and replacement. A map showing the sewersheds and the six sewer groups is shown in Figure 3-4.

x Beginning with Sewer Group 1 and proceeding progressively through Sewer Group 6, a sewer system evaluation survey (SSES) is being conducted. Under the SSES contracts, the sewer lines are located, physically inventoried, and the condition of the line is determined using a variety of field investigation techniques (e.g., physical verification, surveying, closed circuit TV, smoke testing, robotics, etc.) As part of this work, a detailed and standardized process for identifying and recording defects is used to facilitate subsequent evaluation. These evaluations employ a set of established service level criteria to determine which defects are to be repaired. These evaluations are consistent with the condition scoring defined in Table 1-1 and used in this report; however, since individual pipelines were not assigned a condition rating in this report, the SSES field defect analysis was not reviewed in detail. The rehabilitation work defined as part of the SSES evaluation is performed sewer group by sewer group in order from Sewer Group 1 to 6.

x Concurrent with the development and implementation of the SSES program, the City created a macro model of its collection system. This model included information for all lines within the City’s collection system that were greater than or equal to 24-inches in diameter. Upon completion of the macro model, micro-modeling of the City’s collection system was initiated. The micro model incorporates the additional data collected from the SSES program and adds any additional pipelines within the City’s collection system that are 8-inches or greater in diameter. Micro-modeling activities are currently being performed and will continue to be performed throughout all six stages of the sewer group renewal and rehabilitation. The micro models are designed to allow the City to more efficiently and effectively manage the life-cycle cost of the pipeline network by evaluating system conditions and performance on an on-going basis.

x The Department has an adopted rehabilitation manual that specifies the appropriate rehabilitation methodology based on specific type(s) of defects and levels of severity as noted in the SSES evaluation. EPA and EPD concurrence with the design criteria specified in the rehabilitation manual was obtained during the initiation of the Clean Water Atlanta program. In general, pipeline defects with a condition score of “Poor” and

“Very Poor” (or “4” and “5” scores) are proposed for rehabilitation. Rehabilitation consists of either pipeline lining, pipe bursting, point repair, replacement, service line reconnection and, when needed, service line “lower lateral” repair or replacement.

3Sewersheds are a subdivision of a sewer basin that typically consists of 10,000 to 50,000 linear feet of hydraulically linked sewers that are tributary to a point in the sewer system which has permanent or temporary flow meters.

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Sewersheds and Sewer Groups (SSES & Rehab Area)

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x In Fall 2008, the Department reviewed the design criteria to ensure that appropriate rehabilitation decisions were being made based on the more extensive knowledge of pipeline condition that has been obtained from the completed SSES work. Based on this review, the Department modified its approach to management of the sewer system asset so that capital improvement funding would be applied to correct “very poor” conditions and some “poor” conditions with a potential to adversely impact operations, and operations funding would be applied to long term preventative and corrective maintenance. The Department’s intent was to identify changes in the pipeline rehabilitation design process which could lead to near term cost savings with only limited increase in operational risk. A pilot study on portions of Sewer Group 2 Rehabilitation contracts was conducted to verify, validate and quantify the reductions in cost and scope.

The pilot study was completed in December 2008 and found that approximately 23 percent of the proposed rehabilitation CIP budget for the remaining work in Sewer Groups 2 through 6 would be deferred until after 2014. In general, pipeline defects with a condition score of "Very Poor" and some of the potentially high impact "Poor" scoring pipelines will be recommended for rehabilitation. In January 2009, the EPA and EPD were informed of the proposed revisions in the design criteria to ensure there are no regulatory objections prior to full implementation.

x Using the hydraulic model in conjunction with flow data collected from numerous permanent and temporary flow meters, the City has identified those areas of the collection system that are capacity limited and require capacity relief projects. These projects will be constructed as part of the Sewer Group 1 through 6 Capacity Relief Projects. Note that the sewer groups for the capacity relief projects were initially identical to the sewer groups for the SSES and rehabilitation work as shown in Figure 3-4. In 2005, the Georgia EPD approved the City’s request to change the geographical boundaries of the sewer groups with respect to the construction of sewer relief projects.

This change allows the sewer relief projects to be constructed for an entire sewer basin at one time, thereby allowing the improvement to become effective as early as possible while improving the efficiency of both design and construction. This approach requires the SSES work to be completed earlier than initially scheduled in order to have the necessary information available for design of the capacity relief projects. The locations of Sewer Groups 1 through 6 for the sewer relief projects are shown in Figure 3-5.

x The City has also developed a capacity certification program whereby site-specific sections of a wastewater pipeline can be evaluated to determine if a cost-effective improvement is available which will sufficiently improve the capacity of the pipeline section to enable new connections. To the extent possible, the Department requires benefiting developers to share capacity expansion costs. This capacity certification program has allowed the City to continue development and economic growth in capacity-limited areas of the City through the use of infiltration/inflow (I/I) credits that offset the flow added by new development.

x In addition to the SSES, rehabilitation and relief projects, the Department maintains numerous annual contracts for pipeline investigation, repair, rehabilitation and construction. These contracts are used on an as-needed basis to address immediate problems and concerns that cannot reasonably be postponed until the scheduled time for the sewer group work to be performed in that area. The Department is also making every attempt to coordinate the replacement of sewer pipes in areas where extensive water main work is occurring and vice versa.

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Sewer Groups (Relief Projects)

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Pump Stations

Each year the Department evaluates the capacity of its remote pump stations. As part of the capacity analysis conducted in 2008, along with routine operational evaluations, a variety of maintenance and capital needs were identified. In spite of these issues, all of the pump stations are considered to be in Good condition, consistent with their age and use.

Future capital improvements to the pump stations to accommodate growth in projected flows, enhance operational reliability and efficiency, and provide for appropriate renewal and rehabilitation include:

x The Flint River Pump Station, a 15 mgd, two-stage pump station, will be replaced with a single stage submersible pump station (up to 25 mgd) and will include up to 5 mg of storage. The existing two-stage pumping system is not energy efficient. The current system allows for flow equalization to the plant through on site storage.

x Niskey Lake No. 1 and No. 2 Pump Stations are 25 years old and have single phase power, which is inefficient and results in power outages. The pump stations will be replaced with a submersible pump station. This will also eliminate confined space issues, alleviate flooding concerns and provide additional capacity for future growth.

x A third pump will be installed at the Rebel Forest Pump Station to increase its capacity.

This is being done in conjunction with upgrades to the force main and addition of a relief line to the Forest Park receiving sewer to enable it to accept increased flow.

Replacement of the adjacent east influent pipe to the wetwell is also planned to increase future pumping capacity and eliminate surging in the Forest Park receiving sewer.

None of the above listed improvements are required to be completed in order to maintain a Good condition rating for the facilities, but are associated with future capacity or improved operating efficiency needs.

Force Mains

The Flint River force main has experienced significant deterioration in various locations due to hydrogen sulfide corrosion. In 2003 a gravity section of the pipeline collapsed when repairs were being conducted to address a small sinkhole in the vicinity. This section of the pipeline was immediately replaced as part of Phase 1A of the Flint River Transmission Main Replacement Project. The remainder of Phase 1, which replaced gravity segments of the Flint River Transmission Main where the potential effects of corrosion were of significant concern, was completed in 2007. Phase 2 replaces the remainder of the gravity section of the Flint River Transmission Main and was completed in March 2009. Phase 3 is a future phase that addresses future capacity requirements in the force main portion of the transmission main. The gravity section capacity was increased during Phase 1 by a bigger pipe size since the pipe was being replaced. An evaluation will be completed in 2009 to determine when Phase 3 may be required. With the improvements, the Flint River Transmission Main is considered to be in Good condition.

All of the force mains are operating well within the expected operational lifetime for pipelines of this nature. As such, all of the force mains are considered to be in Good condition.

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Tunnels

The Three Rivers and Nancy Creek Tunnels were bored through rock and lined with concrete in select locations and have no known problems. Each is considered to be in Very Good condition.

Proctor Creek Diversion Structure

The Proctor Creek Diversion Structure is located on the Proctor Creek Trunk. The Proctor Creek Diversion Structure was upgraded in 2007 to replace the stop logs with gates and motorized gate operators, and install telemetry back to the Utoy Creek and R.M. Clayton WRCs. This improved the City’s ability to use the diversion structure to control flow and eliminated the need for Department employees to enter the diversion structure. The Proctor Creek Diversion Structure is considered to be in Very Good condition.

The FY 2009 to 2014 Capital Improvement Program contains projects to upgrade pipelines and complete force mains to address the assets assigned poor or very poor condition ratings.