In May 2010, Nashville experienced a flood of epic proportions that inundated the city with nearly 14 inches of rain in just 36 hours. The event resulted in significant damage and destruction of private and public facilities, including the K.R. Harrington Water Treatment Plant—one of two 90-MGD water treatment plants that provides potable drinking water to the Nashville metropolitan area and its surrounding communities.
Located at the confluence of the Cumberland River and Stones River, the 35-year-old water treatment facility was completely engulfed by the floodwaters, shutting the plant down and exposing weaknesses in the overall water treatment and water distribution system. GS&P engineers worked with Metro Water Services’ (MWS) engineering and operational staff to assess the condition of the facility and helped expedite the recovery of its operations within 30 days of the superflood. Joe Whitson, executive vice president of GS&P’s Water Resources market, explains how a planned electrical system upgrade for the facility turned into a comprehensive flood-mitigation effort to protect the plant from future flood events:
“GS&P has worked at K.R. Harrington since 1988 when the firm was hired to design a plant expansion, so we had an in-depth knowledge and understanding of the system’s operations from our past experience. Just prior to the flood, we had been selected to design the replacement of the plant’s existing switchgear, which had outlived its anticipated life cycle. Before we could get started on the project, the flood event dramatically changed the project’s original purpose and scope.
“Getting the plant back in operation and online as quickly as possible was an enormous effort unto itself. During that process, we determined there were several things we had to do to make sure the facility would not fail under another flood, and were tasked by Metro Water Services with fully protecting the entire plant’s electrical supply and distribution system from a 500-year flood event in addition to replacing the original switchgear. Previously, the majority of the facility was only protected to the 100-year flood elevation.”
A Phased Approach
To replace the plant’s existing 4,160-volt switchgear and at the same time keep the facility operational required the installation of a new electrical building on the site. This presented the design team with one of the project’s biggest challenges.
“Site space was limited due to several factors, including an existing electrical generator building and substation as well as raw water lines,” says Dale Mosley, senior vice president of GS&P’s Water Resources market. “The flood plain along the Stones River also presented limitations. Because we lacked the area needed for the building footprint, we had to make the structure two stories.”
Designed to stand 2 feet above the 500-year flood elevation, the first floor of the new building would house four 2,500 kW diesel generators relocated from the existing electrical building and allow space for two future generators. The upgraded switchgear and variable frequency drives (VFDs) would be situated on the second floor.
“The plant’s four existing generators had been partially submerged in the flood. A big part of the project was relocating those generators to the new electrical building,” says Mosley. “Each generator weighed several tons, so extensive coordination was required to move them to the first floor where they’d be safe above the flood level. To keep the plant operating with emergency backup, we kept two of the existing generators running while we moved the other two generators to the new building. Once the relocated generators were operational, we then transferred the remaining generators. This phased approach was absolutely critical because it allowed the plant to remain online throughout the extensive construction, and supplementing Nashville’s drinking water, which was being produced at the Omohundro Water Treatment Plant.”
“It was a remarkable achievement to have a project of this scope and size and be able to replace the plant’s entire electrical system—the cables, the switchgear, and the instrumentation and controls—without shutting it down,” adds Whitson. “We essentially replaced the facility’s brain and nervous system while it kept on working.”
In addition to being flood-resistant, to meet stringent International Building Code requirements, the new electrical building was also designed to be earthquake-resilient.
“The building and the equipment inside were specifically designed to withstand natural disasters such as an earthquake and still remain in service,” says Mosley. “The structural steel superstructure and metal insulated panels allow movement during a seismic event. So, if there’s a major earthquake in Middle Tennessee, the facility will remain intact and be fully operational.
“From an aesthetics standpoint, GS&P selected a metal building type that complements the color of the plant’s mostly brick structures. From a distance, you can’t tell it’s a metal building on the far end of the facility. It simply blends in with the rest of the plant.”
Flexible, Efficient and Resilient
Early in the design, GS&P and MWS established four key goals for success: reliability, energy efficiency, maintainability and operational efficiency. Mosley discusses a few of the ways these drivers were incorporated into the new facility:
“To increase reliability, we designed redundancy—or backups—into the various electrical systems so if there’s an issue with one piece of equipment it doesn’t shut half the plant down. For example, the upgraded diesel generators transferred from the existing building can produce enough electrical power to operate the entire water treatment plant at the rated 90-MGD capacity. This provides a standby electrical power source for the plant. We increased energy efficiency by installing 4160-volt VFDs on the high service pumps to vary treatment system flow rates to match the water distribution system demands. This allows the plant to alter flows and fill the system during lower-pressure periods, which requires less energy.
“The use of VFDs on the high service pumps for flow control add to operational flexibility because of their ability to function by just turning on a switch at the control station. In terms of maintainability, we looked closely at the serviceability and reparability of equipment. We spoke with maintenance personnel about what equipment they liked, what had worked well for them in the past, and what equipment hadn’t worked for them. Using that input and our own electrical system experience and knowledge of plant operations both pre- and post-flood, we helped MWS select equipment that could not only be easily maintained but also quickly restored to service in the event of a failure.”
Ready for the Future
Changing course from a standard switchgear upgrade to a major flood-protection effort, GS&P’s design solution provided Metro Water Services and K.R. Harrington staff with a flood-protected, earthquake-fortified, energy-efficient facility that provides the city of Nashville and Davidson County with a reliable source of water.
“The key to the way this project is designed is that in the event of a flood, when the water hits a certain level, plant personnel can turn off the entire electrical system and leave the facility. When the water level goes down, staff can return to the plant, flip a switch, and restart the facility without worrying about a system failure because all of its processes are above water and protected,” says Mosley.
“This project defines what we do for our clients,” adds Whitson. “As trusted advisors, we form a bond with our clients and function as part of their staff as they become part of our team. I’m proud of our successful delivery of a complicated project, and that Nashville and Davidson County now has a 90-MGD water treatment facility that is essentially weatherproof.”