Home to the largest contiguous cotton-growing region in the world and encompassing more than 115 square miles, Lubbock is the economic, education and health care hub of the South Plains of Texas. With a population of approximately 212,000 in 2006-estimated to exceed 250,000 by 2060 according to the U.S. Census-Lubbock is also one of the fastest growing cities in west Texas.
A decade ago, forward thinking local officials realized Lubbock was expanding beyond the capabilities of its municipal water supply system. Lubbock's water resources included surface water from Lake Meredith and ground water from a regional well field. Two pressure planes, an east pressure plane and a west pressure plane, served the city, with the division generally occurring near the city center. (A pressure plane is a designated area for water distribution defined by water pressure in feet above sea level.) Eight pump stations and three elevated storage tanks supplied water to the distribution system in the two pressure planes.
The residential and commercial development in the city's southwest portion placed a tremendous burden on Lubbock's water storage and distribution system. To sustain reasonable water pressures throughout Lubbock and particularly into the expanding areas, the city decided to expand its water service capabilities in the southwest.
In 2001, Lubbock initiated a comprehensive Water Distribution System Study to determine the projected water demands through the year 2050 and the improvements necessary to meet those demands. Based on the results of a steady-state hydraulic model that projected the required water demands at peak conditions, the following recommendations were made:
- Create a new southwest pressure plane via the addition of a 5 mgd pump station and a 0.5 million gallon elevated storage tank. This pressure plane would allow the anticipated growth in southwest Lubbock to be served at pressures above minimum requirements.
Build a new 14 mgd pump station and a 5 million gallon ground storage tank to serve the existing west pressure plane.
Finding a Cost Effective Solution
In 2004, in response to the city's solicitation, Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering and program management firm headquartered in Houston, and Enprotec/ Hibbs & Todd (eHT), a local civil, environmental and geotechnical engineering firm, proposed a number of solutions to implement the creation of the new southwest pressure plane and the pump station improvements necessary to meet the demands in the west and southwest pressure planes.
Convinced that the proposed solutions offered the advantages the city wanted, Lubbock officials awarded the contract to the team. LAN was chosen to provide design and construction management services, which included modeling and sizing of the new pump station, selection and sizing of pressure sustaining valves and design of the pump station and ground storage tank. eHT's services included forecasting future growth in the area, environmental studies on the pump station site and pipeline route, and design of the offsite pipelines and pressure sustaining valves.
The team thoroughly reviewed the city's existing distribution system. Through extensive data conversions, the H20Net water model was updated to a WaterCAD model and multiple scenarios were evaluated. The steady state hydraulic model used in the 2001 study was transformed into an extended period simulation and updated to include future growth. Steady state analysis takes a snapshot in time, at peak water usage and minimum water usage periods, whereas an extended period study analyzes the system hour by hour for a more thorough review. The model also analyzed the existing system, determined fill and draw capabilities using the existing system components, and the pump station site serving the west and southwest distribution areas.
"We not only looked at the near term conditions and growth projections, but also the improvements that should be made to the pump station in the next 15 to 20 years," said Graham Moore, LAN's project manager.
Based on this hydraulic modeling analysis, the team recommended the following solutions:
- Combining the facilities suggested in the 2001 Water Distribution System Study into a single facility to reduce construction costs. The team suggested designing and constructing a 14 mgd dual pump station that would take suction from a new 5 million gallon ground storage tank. This would serve the new southwest pressure plane and the existing west pressure plane.
Deferring the construction of the proposed elevated storage tank in the southwest pressure plane and instead installing two pressure sustaining valves coupled to the existing west pressure plane. In the event of a pump station failure, the pressure sustaining valves open, allowing water from the elevated storage tanks in the existing west pressure plane to back feed into the southwest pressure plane.
Driving the project team's design of the new southwest pressure plane and associated pump station improvements were two major considerations: operational flexibility and ease of access. A number of features that would maximize the distribution system's usage and service capabilities, minimize operational impacts during maintenance periods and accommodate the future expansion of the site were incorporated.