Shani Wallis, Editor

Section of the new intake

A new third intake in Lake Mead is to be built by the Southern Nevada Water Authority (SNWA) to protect Southern Nevada’s water supply from significant loss of system capacity resulting from a continuing decline in lake water levels.

Estimated in these early budget stages at some $650 million, the project’s main element will be a large-diameter concrete-lined tunnel of about 3 miles (4.8km) to an intake shaft and structure on the lake bed about 600ft (182m) below water level. It will also include a new pumping station; a connection tunnel from the new intake to the existing No 2 intake about 3,000ft (915m) away; and a connecting pipeline of about 12ft diameter and 3,000ft long (3.6m x 914m) to the existing Alfred Merritt Smith Water Treatment Facility.

White band marks the drop in Lake Mead water levels

The vast reservoir created by the majestic Hoover Dam on the Colorado River is a main source of potable water for the expanding populations of southern California, Arizona, Nevada, Colorado and New Mexico. The 1.8 million residents of the Las Vegas Valley alone rely upon the Colorado River for 90% of their drinking water supply. It is also the headwater of the Hoover Dam hydroelectricity power station, which is a major provider of electricity to the southwest region.

Water levels in Lake Mead are also influenced by an ongoing and severe drought in the Colorado River basin. Recent and projected declines in water levels have raised concerns about the future operability of one of SNWA’s existing intakes. The new intake, approved by the SNWA Board of Directors in May 2005, is designed to draw water from a deeper elevation at a level that will function at the lowest expected lake levels and draw water of a higher quality from below the thermocline.

According to Michael Feroz of Parsons Water Infrastructure Inc and Principal Construction Manager of SNWA, construction is planned to start in November 2007 and the completed facility commissioned by December 2011.

“This No 3 Lake Mead tunnel intake will be a challenging project”, said Feroz. “The new intake lies some 200ft (60m) deeper in the lake than the existing intakes, both of which are also tunnelled structures - the first built in the 1970s and the second in the late 1990s. Where those tunnels were drill+blast excavations, the new tunnel is likely to be a TBM operation starting in the open-mode but having to go into closed slurry or EPB pressurised mode as the tunnel advances into softer formations where the heading might be subjected to as much as 600ft (182m) of hydrostatic head in the lake above.”

Site investigating rig

LakeMead retrieved cores

Geological investigations indicate that Saddle Island, the most likely site of the new intake pumping station and from where the intake tunnel will advance, is a block of Precambrian amphibolites and gneiss with the oldest known rocks in the area dating back 1.7 billion years. The proposed tunnel alignment then heads into softer rocks and the water bearing silts and sands of the Muddy Creek Formation created by ancient rivers 5 to 11 million years ago. Core samples, catalogued and stored for further analysis, also indicate ground water temperatures as high as 100°F at tunnel elevation.

“Design-build is being considered for procurement of the tunnel works”, said Feroz, “with the pump station and ancillary works let on a design-bid-build basis.”

Reconnaissance and water level studies for the new project progressed from April to November 2005; geotechnical investigations are almost complete; environmental and permitting procedures are progressing; and preliminary design of the facilities started in August 2005.

Two configurations are currently under consideration for design of the new intake pumping station; either twenty to thirty turbine pumps suspended in 5ft diameter vertical well shafts of about 500ft deep with motors and electrical gear housed in a building at ground surface; or a dozen or so centrifugal pumps installed in a deep underground chamber with ancillary chambers for valves and piping. Forebays and access shafts of various sizes and configurations will be required depending on the final design of the facility. The conveyance pipeline from the new pumping station to the Alfred Merritt Smith treatment plant is anticipated as a cut-and-cover construction, as long as lake levels remain low, and a tunnel heading will connect intake No 3 with intake No 2. Intakes 1 and 2 are not interconnected and connection between Intakes 2 and 3 will allow pump station 2 to pump water from intake tunnel 2 or 3 with better quality of water coming from the deeper Intake 3.

The No 2 intake tunnel and shaft project, completed in the late 1990s by Kiewit (operating as Lake Mead Construction), was built under 225ft (68m) of water head, some 34m lower than the No 1 intake, and was the topic of several conference papers including three in the RETC proceedings of 1999: Risk management strategies (p103); Shaft construction (p761); and Drill+blast operations and water control measures (p1047).