Climate conditions 600 feet below Saddle Island on the shores of Lake Mead in Nevada speak nothing of the decade-long drought that's plagued the American Southwest in general and the Colorado River Basin in particular.
As galosh- and rainsuit-clad miners drill and blast an erection chamber for a soon-to-be-assembled tunnel-boring machine, water steadily rains down the access shaft, streams from the walls, and gurgles up from drill casings on the tunnel floor like an artesian well. Thermal in some places and cool in others, the water creeps through fissures in the metamorphic rock — a Precambrian block of amphibolite and gneiss surrounded by youthful tertiary volcanic rock — and drops as it pleases on the workplace below, despite continuing efforts to minimize it.
No shortage of water here.
But aboveground, the drought has prompted the Southern Nevada Water Authority to build a third intake on the lake, which provides 90% of the water supply to the 600-square-mile Las Vegas Valley but is at 43% of its storage capacity — a fact that's painfully evident by the 130-foot-high “bathtub ring” around the 248-square-mile lake. The authority includes seven member Nevada agencies and was formed in 1991 to manage the region's water needs.
Flows on the Colorado River vary widely from year to year, depending on the amount of snowfall and temperature fluctuations across the watershed basin, but they've been well below average in eight of the past 10 years and water levels in the lake have dropped alarmingly — by more than 100 feet, in some cases. Climate uncertainties make it impossible to guess whether levels will recover or continue to decline.
The possibility that they'll keep falling raised concern about the future operability of one of the lake's two intakes, and in 2005 the authority's board of directors approved the design and construction of a third intake. Its primary objective is to protect water supply to the Las Vegas Valley's 2 million residents from significant loss of system capacity should the lake continue to dry up.
Higher-quality water from below the thermocline will be drawn through a 16-foot-diameter intake and conveyed 3 miles through a 20-foot-diameter tunnel to an existing pumping station via a connecting tunnel. The new intake will be deep enough to function at a lake elevation of 1,000 feet, 50 feet deeper than the intake it will replace.
Capital improvements are funded through fees for new water connections, surcharges added to the retail rates for water use, and a ¼ cent sales tax — all of which have fallen along with the economy and affected the project's scope (more about that later). Still, Engineering Director Marcus Jensen says these sources have allowed the authority to build up $400 million in cash reserves while securing long-term financing for nearly $2 billion in capital projects.
At Lake Mead, three intake-related contracts are moving toward completion in 2013.
On board since March 2008, Vegas Tunnel Constructors, a joint venture of construction firms Impregilo SpA and S.A. Healy Co., won the $447 million design-build contract for the tunnel access shaft, tunnel, and intake components. Also part of the design-build team is Arup, which is providing design services, and Brierley Associates LLC, which brings geotechnical expertise. MWH and CH2M Hill provided preliminary design services and are helping with design review during construction.
The authority opted for design-build to give the builder more opportunity to match design features with preferred construction methods and more control in mitigating risks when striving to meet the aggressive deadline. A 24-foot-diameter, mixed-shield boring machine will drill through varied geological formations below the lake bed with a maximum static water pressure of 17 bars. Underwater blasting and tremie-placed concrete work will be used to build the intake in more than 250 feet of water at the bottom of the lake.
Renda Pacific was awarded a $42 million contract to build the underground tunnel that will connect the new intake with Intake 2 and a stub tunnel that will connect the new intake with Intake 1. The contractor will excavate both shaft and tunnel using a drill-blast-muck process.
Barnard of Nevada Inc., which began work on its $30 million contract in May 2008, is digging nearly 400 feet belowground to make the connections and modify Intake 2 to allow water to flow from the new intake to Intake 2 and its pumping facilities.
The firm's tight schedule includes critical work during a six-week complete shutdown of Intake 2 in December and January 2010 to make the final tunnel connections. Workers will first install a large isolation gate to separate Intake 2 from Renda Pacific's work on the connector tunnel. The gate assembly requires precise and complex concrete formwork to ensure the gate can be tightly sealed to protect crews working on the connector tunnel. A temporary cap will be placed on the Intake 2 opening to allow dewatering of the existing intake. During the shutdown, an inspection and modifications will take place inside the intake shaft and tunnel, and the final 50-foot section of rock will be excavated to allow the connection to be made with the new intake once all three components are complete.
An 85% drop in new-connection fees and a 14% drop in sales tax revenues since 2006 forced engineers and project planners to look for ways to reduce overall expenses while maintaining operational objectives. Although cash reserves were available to support financing, drawing down reserves to finance the project's entire scope could have negatively affected bond ratings and future borrowing costs.
As a result, significant engineering adjustments were made to defer building a pumping station and discharge pipeline from the new intake to a nearby treatment facility, a decision that knocks $200 million off the entire project's estimated $817 million price tag.
The authority planned to replace the pumping station at Intake 1, which is in jeopardy of being rendered inoperable by falling lake levels, with a 600-mgd pumping station at the new intake. Instead, larger-capacity pumps will be installed at Intake 2. This increased capacity, together with the sharply curtailed growth in demand due to improved conservation and stalled development, may allow the authority to defer the new pumping station for many years.
These cost savings will be passed along to the seven agencies that govern the Southern Nevada Water Authority in the form of lower construction expenses.
The West is no stranger to scarcity of resources, and the Lake Mead Intake No. 3 project exemplifies Southern Nevada's response to such challenges with planning, engineering, and ingenuity.
— Robin Rockey is the project information manager for the Southern Nevada Water Authority.