A new 1,100-acre off-channel reservoir planned for construction in southeast Texas by the Lower Colorado River Authority, of Austin, will have a storage volume of 40,000 acre-ft and feature a river intake and pump station that will transfer flows from the Colorado River to an existing canal, which will convey the water via gravity to a re-lift pump station that will discharge the water into the reservoir. From there, flows will discharge by gravity to existing nearby irrigation systems as well as return to the river. Lower Colorado River Authority
The Lower Colorado River Authority of Austin, Texas, has selected CH2M HILL to design a new off-channel reservoir to serve primarily agricultural clients.
July 23, 2013—Seeking to augment its water supplies and improve system reliability, the Lower Colorado River Authority (LCRA), of Austin, Texas, is moving forward with plans to construct a 1,100-acre off-channel reservoir with a storage volume of more than 40,000 acre-ft. Expected to cost $206 million and be completed in four years, the project will enable the LCRA to better meet the water demands of its downstream industrial and agricultural customers while reducing the amount of water that the authority must release from its existing upstream reservoirs. Following its recent selection as the project designer, CH2M HILL, of Englewood, Colorado, has begun work on the preliminary design of the new reservoir and its related facilities, which will be located in Wharton County in southeast Texas.
A conservation and reclamation district created by the State of Texas, the LCRA manages water supplies along the lower 600 mi of the Colorado River, which flows through central and southeast Texas before joining the Gulf of Mexico at Matagorda Bay. The LCRA operates six dams on the Colorado River in central Texas for the purposes of providing water and recreation, controlling flooding, and generating electricity. The dams form six water bodies collectively known as the Highland Lakes, the two largest of which—Lake Travis and Lake Buchanan—hold a combined 2 million acre-ft when full. However, in recent years, drought conditions have reduced the volume of the two reservoirs to less than 40 percent of their capacity.
Expected to increase existing water supplies by 90,000 acre-ft/yr, the new off-channel reservoir will greatly assist the LCRA in meeting its goal of adding 100,000 acre-ft to its water supply. “LCRA is looking to make more water supplies available for a growing service area and to make more efficient use of our existing supply,” says John McLeod, Ph.D., P.E., M.ASCE, a senior project manager for the river authority.
Known as the Lane City Reservoir because of its proximity to the unincorporated community of Lane City, the off-channel reservoir will also enable the LCRA, for the first time, to hold water within the river basin downstream of Lake Travis, which is located northwest of Austin. As a result, the authority will be able to meet some of the water needs of its downstream customers without having to release flows from the upstream reservoirs. Whereas urban water users predominate in the upper reaches of the LCRA’s jurisdiction, industrial and agricultural users create much of the demand for water downstream. Under current operations, some downstream customers, primarily agricultural interests that rely on river water for irrigation, may experience reductions during periods of drought. Along with making more efficient use of river flows below the Highland Lakes, the reservoir will help farmers in particular, McLeod says. “Irrigation customers will benefit from possible reductions in future curtailments,” he says.
In April, the LCRA selected CH2M HILL to provide engineering services for the off-channel reservoir project, including the development of a preliminary engineering report and the design of the reservoir, berm, and associated intake, pump stations, and other conveyance facilities. Having a maximum capacity of 561 cfs, the river intake and pump station will transfer flows from the river to an existing canal, which will convey the water via gravity to a second pump station. Known as the re-lift pump station, the second pump station will discharge the water into the reservoir, which will be designed to convey flows by gravity to existing nearby irrigation systems as well as return water to the river. Ultimately, the toe of the reservoir will be situated approximately 1 mi away from the river. Because the new system is to be located at the site of an existing intake and pump station on the Colorado River, the design team is evaluating whether the structures can be repurposed for the project, says Ken Hall, P.E., M.ASCE, the design project manager and global technology leader for water supply services for CH2M HILL.
Water will be transferred from the river to the reservoir depending on various factors, including time of year, hydrological conditions, and water rights obligations. Among the design challenges to be addressed, Hall says, is the complexity of determining when and how much water may be pumped from the river, so as to optimize the efficiency of the off-channel reservoir. The goal is to “get the most out of the storage,” Hall says. The cost of water from the reservoir is expected to total $212/acre-ft.
The roughly 28,000 ft long earthen berm will enclose a rectangular area of about 1,100 acres and stand between 40 and 50 ft high, making the berm one of the larger such structures to be constructed in the United States of late, Hall says. “You don’t see many reservoirs this size being built recently,” he notes. One advantage offered by the Lane City site is its significant quantity of clay materials, which likely will obviate the need for a synthetic liner within the reservoir. However, an ongoing geotechnical investigation will determine the extent to which the native clay materials will be used for seepage control and embankment construction, Hall says.
The project will be delivered by means of a construction management-at-risk approach. To this end, the LCRA is in the process of evaluating six prequalified contractors and anticipates awarding a contract this September, McLeod says. Major construction is expected to begin in 2015, though certain early work packages could begin in 2014, depending on the timing of permit approvals and input from the constructor, McLeod says. Reservoir construction is expected to be completed in mid-2017.
To attain its goal of adding 100,000 acre-ft of new water supplies, the LCRA also is seeking permits to access up to 10,000 acre-ft/yr of groundwater in Bastrop County. Meanwhile, the LCRA has expressed an interest in potentially constructing two additional off-channel reservoirs in southeast Texas: a $266-million 1,024-acre reservoir in Colorado County and a $200-million 798-acre reservoir in Matagorda County. The authority continues to evaluate the possibility of building the new reservoirs, McLeod says.