The second straw

Las Vegas has an unquenchable thirst for water; the Southern Nevada Water Authority’s Capital Improvements Program will double water treatment capacity to almost one billion gallons a day.

by Clair D. Urbain

There is much more at stake in Las Vegas than bets in the casinos.

It’s water. The Southern Nevada Water Authority (SNWA), made up of seven water and wastewater agencies in the Las Vegas metropolitan area, have combined forces to meet the burgeoning demands of the growing population.

“More than 5,000 people move to the Las Vegas area every month and we have 39 million visitors annually. It’s the fastest growing area in the United States and to obtain water in a desert region takes planning, engineering and construction to keep up with the demand,” says Robin Rockey, SNWA project information manager.

SNWA and Parsons, the company overseeing design and construction of the project, helped the authority develop the $2.1 billion capital improvements program, says Bill Buchholz, Parsons’ project officer.

“The goal is to create a system that will reliably provide the burgeoning SNWA area with water. One strategy was to establish uniformity in the system – treatment plants, pumps and system designs – for efficiency and reliability,” Buchholz says,

The area’s fast growth meant the well-thought out plan had to be expedited. “Because of explosive growth, the plan for 2014 had to be completed in 2006. This has been a strain on everyone involved with construction,” he says.

The project began in 1991 and is approximately 85 percent complete. Lake Mead is the source of more than 90 percent of the area’s water and a second intake is in place and connected to the water treatment and pipeline system. The new intake was put into service in 2002. It’s dubbed the “second straw” Las Vegas has into Lake Mead, the man-made lake behind Hoover Dam.

The second intake draws water from as deep as 230' below the lake’s surface. From there, water flows to a forebay through a 1,700' long x 14' wide x 16' high tunnel. In the forebay, 3,000-hp turbine pumps fit in 22 72" diameter well shafts with 50" casings. At capacity, the pumps can draw up to 600 million gallons a day (mgd) and pump it through a 12'diameter aqueduct to the Alfred Merritt Smith Water Treatment Facility adjacent to Lake Mead, or through the mountains to the newly constructed River Mountains Water Treatment Facility near Henderson. After treatment, the water is pumped to municipal reservoirs. When complete, the system will be capable of delivering 1.2 billion gallons of treated water every day.

Back to Lake Mead
With that much water coming into a desert area, where does it go once it’s used? Treated wastewater flows through the Las Vegas Wash, which meanders across the desert and delivers it back to Lake Mead.

“Las Vegas is one of a very few cities that lives downstream of its own wastewater,” says Buchholz. The SNWA gets credit for water it returns to Lake Mead, further upping its ability to draw fresh water from the lake.

Two water treatment plants process water for the valley. The Alfred Merritt Smith plant, on the shores of Lake Mead that processes up to 600 mgd, was completed in 1997. To transport raw water to the River Mountain plant, the East Valley Lateral was built in 2000. It includes a 78" diameter pipeline that crosses beneath the Las Vegas Wash. “The pipeline required extensive tunnel-boring under the wash to assure a rainfall event would not erode it,” says Joe Pescio, Contri Construction project manager.

The tunnel under the wash is 1,500' long and is just upstream from the Rainbow Gardens weir project. “It’s 60' below the base of the wash, so it is protected from any type of rainfall event,” says Pescio. “It is hard rock, so it was a tough drilling job. The pipeline was a rather straightforward job. It runs close to existing utilities and another water line. To assure there would be no damage to existing infrastructure, the pipeline path was closely marked and potholes were dug ahead of crew to every 200' and at every bend to assure no utilities would be hit,” he says.

Due to high flow rates from Las Vegas area wastewater treatment facilities and high rainfall events, erosion takes its toll on the wash, damaging the wetlands along the wash and carrying unwanted sediment flows into Lake Mead. To counteract the erosive effects of high water flow and give sediment a chance to settle before reaching Lake Mead, several weirs (dam-like structures) and other water control systems are being installed all along the wash.

“The Rainbow Gardens weir work was bid as part of the East Valley Lateral project, which is the last full weir before the water enters Lake Mead. It also serves as the final water flow measurement point used to calculate the amount of water returned to Lake Mead for credit to the SNWA,” he says.

“It made sense for us to work on the weir because we were in the area, which increased efficiencies. The jobs all along the water project are fairly spread out. It can take up to 20 miles of travel between jobs, so you really need to plan what tools you need before heading to the job,” Pescio says.

Tying the East Valley Lateral into the River Mountains Water Treatment Facility created challenges. “The existing pipeline had non-standard connections. We had to connect to a tapered pipe at an angle. We surveyed both ends of the pipe very accurately, then had the connection built.

“When it came time to connect it, we put together a plan. We had a list of all the equipment needed plus backup equipment. We had to shut down and drain part of the system, so we made sure all of the reservoirs were full and that contingency plans were in place if something didn’t go right,” Pescio says. “We planned to finish the connection in 24 hours, but we were able to do it in 12 hours. The amount of planning was three or four times greater than for a normal connection.”

At the weir, construction was more difficult. “The Rainbow Gardens weir was tricky to build because it is an active wash. We had to build a dam and divert the flow around the work area,” says Pescio.

Pescio and his crews paid close attention to rainfall forecasts and water flow measurements upstream. “We had significant rainfall events during construction. Luckily, none were 100-year events. The diversion could handle 5,000 cfs, and during construction we had only one event that approached 3,000 cfs. We recently had a 10,000 cfs event and we were lucky to be finished and out of there,” he says.

The weir is built with roller-compacted concrete. “The mix is very dry and put in place with a telescoping conveyor, then smaller-sized construction equipment formed it into place and compacted it. It was a tight working area, so we used a mini-excavator for prep work, then used aluminum beams and plywood for forms,” he says.

Fast-track water treatment facility
Of the more than 100 individual projects included in the SNWA’s construction program, perhaps the fastest moving project is the River Mountains Water Treatment Facility. The facility is designed to be built in 150 mgd sections. The first two sections will be finished late in 2005 for an operating capacity of 300 mgd, nearly a decade earlier than planned.

The nearly $250 million project is slated for completion in the Fall of 2005. Phase 2 construction has been underway since July 2003.

“While the buildings and infrastructure were built in Phase 1, the Phase 2 project takes present plant capacity of 150 mgd to 300,000 mgd. It’s mostly pipe-fitting and controls work. Tying the new system into the Phase 1 system is challenging,” says Blaine Stewart, job superintendent for MMC Inc., which is in charge of Phase 2 construction at the water treatment facility.

Crews have already tied the second 150 mgd section into the first section put in operation during Phase 1. It required tight planning and clockwork-like activity.

“We only had 13 days to tie the second section into the first one,” Stewart says. “This was an almost completely confined-space job. It took five days to drain the tanks and have the ozone levels drop to a point where the tanks could be entered safely.

“Workers began on the new side of the facility by coring holes to within 2" of breaking through the walls. Once ozone levels were low enough and the tanks were drained, we cored the remaining thickness and then brought in a wall saw to cut out the 12' x 6' holes in the 16"-thick concrete walls. The pieces were cut again into 3' x 3' sections and pulled out of the holes by crane. We continuously monitored the confined areas for ozone and high oxygen levels with badges and monitoring equipment. If ozone levels get too high, it can burn workers’ lungs,” he says. “We tried to do as much work as possible on the new side.”

The Water Treatment Facility is made up of a flow split structure that takes water rushing in from the River Mountains Tunnel, slows its velocity, then transfers the raw water into water processing areas.

The first stop for the raw water is the ozone contactor area, where ozone bubbles through the water and disinfects it. From there, it is pumped to the flash mixing and flocculation basins where materials are added to coagulate microscopic particles which separate out in the filter galleries. At this point the water is treated with fluoride, chlorine and other chemicals to make it safe for drinking.

“We only had 1/2" of clearance to move pipes and valves into the filter gallery,” says Stewart. “We had to be careful of the equipment weight to make sure it wasn’t too much for the weight-bearing capacity of the gullets. We were right at crane capacity when we flew in the 48"-diameter x 38'-long top pipe in the filtration facility. We also had to cut the 48"-diameter pipe already in place to install a two-sleeve valve so a bypass valve could be installed to allow back-flushing of the filter beds,” Stewart says.

Jobsite safety
To date, there have been 196 recordable incidents since construction began and only 55 were lost-time incidents on the entire project since 1995. The overall safety record is excellent, report SNWA officials. The recordable incident rate for the program is less than two-thirds the national average and the lost time incident rate is less than half of the national average.

“Our contractors have safety meetings every Monday to discuss the work and point out safety concerns for the coming week. There are some language barriers. We now do some safety meetings in Spanish. We have some workers who cannot read, so they are more challenging to train. For others, safety is not a big part of their work ethic, and we work with them to change that,” says Buchholz.

Every worker gets nearly a half-day of general training before commencing work on any SNWA site. Every worker must go through desert tortoise training, which helps protect this endangered species found on SNWA’s properties. Workers are instructed on the importance of the 18"-high tortoise fences that line the roads and access points to keep the tortoises out of danger. U-troughs planted across entrances keep the tortoises corralled in areas where fencing is impractical.

Published in the May 2005 issue of Contractor Tools and Supplies magazine.

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