Drought of 2007: Drastic times cause for drastic measures
Gresham, Smith and Partners
As was the case in most of the southeastern United States throughout the summer of 2007, Elmore County, Alabama experienced record lows when it came to the amount of rainfall it received. This lack of rainfall severely affected water levels in a number of lakes in Alabama, including Lake Martin, which is the main water supply for the Central Elmore Water and Sewer Authority (CEWSA). After several meetings with the Alabama Power Company, who controls the release of water from Lake Martin for power generation, CEWSA realized that if this situation was ignored and the drought continued, they were going to run out of water.
"To ensure a continuous and uninterrupted supply of water to more than 66,000 people, we worked in partnership with water services engineers from Gresham, Smith and Partners (GS&P) to develop a quick, strategic and effective way to solve this problem," commented Robert L. Prince, Jr., general manager of CEWSA.
CEWSA and GS&P started by assessing the situation and looking at the raw water pump station to determine at what water level elevation in Lake Martin, the area's main water supply, the raw water pumps would begin to cavitate. The pump's raw water intake is fed by two intake lines in Lake Martin. The "upper intake" was dry and the lake level was continuing to fall to record lows for this time of year, meaning the same fate was possible for the "lower" intake. After much consideration, it was agreed that CEWSA must go forward with the Emergency Drought Intake Plan.
|The emergency pumps are designed to deliver raw water at a rate of 6,000 gallons per minute. This floating barge will rise and fall with the lake level, ensuring reliable operation.|
Emergency Drought Intake Plan
GS&P considered various pump types, layouts, piping schemes and pump feasibilities due to existing site conditions. CEWSA elected to work with a local supplier, Hydra-Service out of Warrior, Ala., who would provide all equipment and some labor to install the system. After a meeting with GS&P, CEWSA and Hydra-Service, a floating barge system was conceptually laid out along with required controls and connections. Understanding the urgency of the problem, Hydra-Service began fabricating barges the next day.
The system was designed to be able to deliver 6,000 gallons per minute (gpm) to the raw water pump station. The unconventional part was that the static head condition, typically a fixed quantity in hydraulics, was going to vary from the initial condition to an assumed "doomsday" lake level when the drought had hit its peaka level taken to be approximately 18 feet lower than current levels. This was achieved by sizing the pumps for the worst case and utilizing variable frequency drives to reduce pump speed and flow as required.
|Approximately 550 feet of 18-inch high-density polyethylene pipe were fused together to provide a discharge line for the floating pump station. A track hoe is moving the line into place for connection to the existing 24-inch raw water intake line.|
The Floating Barge Concept
The barge consists of four separate floats, three of which each house a 95 HP submersible pump and the fourth supporting the necessary pipe header. Each pump can handle approximately 4600 gpm. This means two pumps together achieve the flow required, leaving the third pump as a backup. The associated control panels are set up on shore. Power is supplied from the existing motor control center with the existing 1000 kW genset providing backup power. Eighteen-inch HDPE pipe is suspended just below the water surface by floats and serves as the raw water line between the barge and the connection point.
The modifications to existing infrastructure were twofold. The first required an approximately 10-foot-deep excavation to get to the upper intake line. It was cut and a 24" tee with a 24" plug valve was installed. The valve was closed and the pump discharge was hooked up to the tee. This valve allowed the intake screen to be closed off and isolated from the emergency pumping system. All joints on the upper intake line were dug up and restrained to ensure the pipeline could handle the operating pressures it would see.
The second modification was necessary because of the need to hydraulically connect the existing wet well, which was split into two separate wet wells by an internal partition wall, while isolating the lower intake screen from the system. Differing water levels in this wet well were not desirable and would also make the control system very complex. The solution was to install a 24" plug valve in the lower intake line. This modification required an excavation approximately 30' x 40' and 25' deep. The excavation required one six-inch pump to provide the necessary dewatering during the installation of the plug valve. Nylon covers were custom-made and placed on the lower intake screen by divers. The line was cut and the valve was, after much effort, installed and closed. Mission accomplished.
"The success of this project boiled down to the cooperation of environmental professionals all working together to develop an innovative and non-traditional solution to a unique problem," stated Bo Linder, project manager from GS&P.
After five weeks of efficient teamwork, the conceptual design, fabrication, delivery and construction were complete and the system was up and running. CEWSA did not experience any interruption in their ability to provide water service to their customers. Furthermore, this emergency pump concept, as well as the modifications to the existing infrastructure, allows CEWSA to operate their existing raw water pump station in the same manner as prior to the drought. It also provides easy connections in the future should extreme drought conditions strike again.
Andy Yarbrough, project engineer from GS&P added, "This project posed a unique challenge in that we were operating against a very serious deadline being imposed by nature. Ultimately, success was a direct result of the collaboration among all parties involved. Equal input from CEWSA, GS&P and Hydra-Service led to a reliable solution that maintained the quality of life for the citizens of Elmore County."
Layton Meng can be reached at (615) 770-8463 or Layton_meng@gspnet.com.