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Water special: Las Vegas − Putting it back

The State of Nevada’s already scarce water supply is under significant threat from population growth and climate change. Jo Stimpson finds out how Royal Haskoning has introduced innovative design to form an integral part of the solution.

Nevada’s Mojave Desert is a dry place − and it’s getting drier. Its primary source of potable water − Lake Mead, the reservoir created by the Hoover Dam − was once able to supply not just Nevada, but also Arizona and California with ease. But while the population grows, the lake is shrinking.

Lake Mead relies on the melted snows of the Rocky Mountains being channelled into it via the Colorado River. But climate change, coupled with several years of drought, means that supply is drying up, and the lake levels are dropping accordingly. From its original level of 381m, the water level had dropped to just 339m by January this year.

Compounding the problem, the area’s population has exploded in the last 20 years − but authorities are still only able to take the same amount of water from the reservoir as they did almost 90 years ago.

“Las Vegas hardly existed in the 1930s. But since then Las Vegas and Nevada have grown immeasurably, so the water they’re allowed to take out is not enough now.”

David Watson, Royal Haskoning

The Colorado River’s neighbouring states signed an agreement in the late 1920s allocating each a proportion of the water. “California, which was well developed, and Arizona, which had some big cities, took the lion’s share, whereas Nevada − even though half the lake is in Nevada − took a very small proportion,” explains Royal Haskoning sector director for marine pipelines David Watson.

“Las Vegas hardly existed in the 1930s. But since then Las Vegas and Nevada have grown immeasurably, so the water they’re allowed to take out is not enough now.”

The solution is what is known as return flow credits. Because the water apportionments are net allocations, Nevada is able to take an extra gallon of water out for every gallon that it puts back − in form of treated effluent.

The current system sees treated wastewater flow into the 19.2km long Las Vegas Wash channel, which conveys it to the lake. This discharge programme has been in place for around 40 years.

An alternative way

However, as more has become understood about water quality, it has become clear that this is not the ideal way to return water to the reservoir. As effluent travels through the Las Vegas Wash, it picks up untreated contaminants such as groundwater and urban runoff, which are then carried into Lake Mead, affecting the lake’s water quality and its aesthetic appeal.

Water quality suffers further from climate change − Lake Mead’s falling water level means its dilution capacity has decreased significantly. Both effluent and nutrients such as chlorophyll become concentrated in certain areas rather than being diffused throughout the lake.

Lake Mead

Lake Mead’s water levels are dropping dramatically − the difference is visible on the banks

For these reasons, the Clean Water Coalition (CWC) − a body whose members include the Clark County Water Reclamation District, the City of Las Vegas, the City of Henderson and the City of North Las Vegas − initiated the Systems Conveyance and Operations Program (SCOP) at a cost of $800M (£502M). The programme was set up to explore and provide an alternative way of discharging effluent into the lake that would allow the state to maximise its return flow credits and result in higher water quality and better dilution.

The chosen solution was to build one large pipeline to collect the majority of the tertiary-treated wastewater discharged by the CWC members’ water treatment plants and convey it through the River Mountains via a 14.4km tunnel. The effluent will pass through a hydropower station where the tunnel emerges from the mountains − a 30m descent will provide the hydraulic head necessary to produce electricity − before entering a distribution system that will take it deep into the lake via five pipelines.

“If you have five pipes you can direct it to whichever pipe is best suited at the time depending on lake level, water temperature and so on.”

David Watson, Royal Haskoning

The discharge points of the five pipes are staggered. “This gives us control,” says Watson. “Because if you have five pipes you can discharge at different water depths and in different locations. In low flow conditions you may only need one pipe, and then you can direct it to whichever one is best suited at the time depending on lake level, water temperature and so on.”

The discharge points will be near Boulder Island − a location that won out over four others following dye studies to test the dilution in that part of the lake. This distribution system is the part of SCOP with which Royal Haskoning is involved.

MWH is the lead design engineer for the pipeline from the hydropower station to the discharge point − Royal Haskoning was employed to collaborate on the pipelines because of its reputation with similar projects. “They’ve employed Haskoning because of Haskoning’s extensive experience in marine pipelines,” says Watson, who is project director.

Royal Haskoning shares the design with MWH where the pipelines lie in a trench on the lake bed after they first hit the water, but Royal Haskoning takes sole responsibility after the point at which the pipelines emerge from the trench and lie on the lake bed itself. The pipes will be made of 1,600mm High Density Polyethylene (HDPE). In the trench, they will be 2,320m long. On the lake bed, their lengths will vary from 900m to 2,530m.

Environmental benefits

Crucially, the pipes will be produced on site, at a temporary manufacturing facility. Watson says this was a major driver in the choice of HDPE as a material.

“Obviously when you transport pipes to site you’re transporting a lot of air,” he says. “If you can manufacture it on site you’re just bringing in the solid raw material, so you’re cutting down the amount of vehicles to bring in the pipe. That was one of the environmental benefits.” Further environmental benefits will come from the continuous concrete collar that Royal Haskoning has designed for the pipes.

Watson explains that the original plan was to take the dredged material from the lake bed to an onshore storage area, to be brought back once the pipes had been installed.

“With plastic you normally need special backfill around the pipes,” he says. “But with the Haskoning system of weighting, the pipes are protected with a continuous concrete collar and therefore any material will do. So we can dredge and just put the dredged material to the side of the trench underwater, lay the pipe, and then pick it up and put it back.”

Las Vegas

Las Vegas sends treated effluent back to Lake Mead

Another innovative solution is the air capture chambers that were designed to replace the original control structure for managing effluent flow and air release.

“These were going to be in a huge chamber, and it would be totally underwater. Health and safety was an issue to operate this and maintain it, so we looked at alternatives,” says Watson. “MWH and ourselves designed a better system using air capture chambers, which slow the flow down and allow the air to come out, and then a network of pipes take the air away.”

“When you’re dealing with lakes, you can’t just phone up a marine contractor and get a big dredger or a big pipeline barge. Equipment has to be transported by land and put into the water.”

David Watson, Royal Haskoning

The SCOP project has presented Royal Haskoning with its fair share of design challenges − not least those that come with planning for working on a landlocked body of water. “When you’re dealing with lakes, you can’t just phone up a marine contractor and get a big dredger or a big pipeline barge. You can’t get them there,” says Watson.

“So we’ve had to design bearing in mind that equipment has to be transported by land and put into the water, which has been quite a challenge. You don’t think about it when you’re working on the coast.”

Environmental issues have been challenging too. “The lake is a national park, very highly used by tourists, so one of the philosophies that we had from the start was to minimise any environmental impact,” says Watson. In practice, this means the work must be carried out in seasonal windows over a period of years.

“Because of the tourist aspect of the lake they don’t want any disturbance during the boating season in the summer,” he says, “so the dredging and any works on the lake have to take place in the six months over winter − October to March.”

Construction is expected to start in the second half of 2010, and is expected to be completed in 2013. The diffusion pipeline will have an estimated contract value of £50M.

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