In the Wash

Las Vegas Wash, the carotiod artery of southern Nevada's water supply system, is undergoing a 12-year, $98 million upgrade.

Essentially, storm runoff flows into the 14-mi.-long wash, which empties into Lake Mead, the Las Vegas Valley's main source for drinking water. The wash can carry more than 150 million gallons of water daily.

Problem is, severe flash floods and neglect have eroded the wash, passing sediment and debris into Lake Mead.

"If we don't do something to stabilize the wash, 2.5 million tons of sentiment could erode over the next 20 years," said Neil Laudati, spokesman for the Las Vegas Valley Water District. "That's enough dirt to fill a football field 90 stories tall."

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To deal with the problem, the Las Vegas Wash Coordination Committee was formed to repair and protect this vital resource, covering a 1,600-sq.-mi. watershed area.
Parsons, a Pasadena, Calif., firm, is the project manager.

A program started in 1998 aims to slow the rate of flow through the wash and minimize its erosion. Presently, water passes through the wash at a rate of 350-cu. ft. per second. The multi-year undertaking hopes to cut that figure in half by building 22 weirs or "mini-dams" that range from rock boulder spillways to a 20-ft.-tall, 200-ft.-long steel-and-concrete weir.

"All of the weirs are designed to be site-specific," said Jerry Hester, the Las Vegas Valley Water District's facilities manager. "They are also natural looking … [and] almost invisible as the vegetation begins to grow."

Seven weir structures have been completed to date: Boise-based Washington Group International completed three; Las Vegas Paving Corporation finished two and the U.S. Bureau of Land Management constructed two.

The project is funded through federal environmental grants and a .25-cent sales tax increase approved by Clark County voters in 1998. Upon completion in 2010, the wash will consist of park-like recreational settings with trails, a nature center and informational kiosks.

Currently, Contri Construction Co. of Las Vegas is working on the $6.9 million Rainbow Garden weir, which is scheduled to finish in June. The 160-ft.-wide, 10-ft.-tall weir is one of the program's largest. Set atop 147 drilled piers, averaging 50-ft. deep and 36-in. diameter, the weir has an 18-in.-thick reinforced concrete mat foundation.

Designed by the Louis Berger Group Inc., of East Orange, N. J., and Pasadena-based Montgomery Watson Harza Inc., the structure will consist of .75-in.-thick sheet metal and a combination of reinforced and roller-compacted concrete.

The Rainbow Garden weir will require 8,000-cu. yds. of concrete to complete. Natural rock will be placed on top of the structure once completed.

"The roller-compacted concrete is a cheaper, quicker placement method, requiring less time to cure," said Eric Gilmore, a project manager with Parsons. "Its application is similar to that of asphalt paving."

The wash preservation program also entails lining 9 mi. of bank walls with rocks in order to prevent further erosion. Where possible, the weirs and rip-rap use recycled concrete from demolished hotel-casinos and other old buildings. The Las Vegas Valley Water District estimates that it saves roughly 100,000 -cu. ft. of material from hitting the landfill each year.

The Bureau of Land Management serves as the contractor for much of the bank stabilization work.

Additionally, the wash committee plants up to 20,000 bushes and trees annually, using mostly willows and cottonwoods with deep -root systems.

"The vegetation acts as a natural cleanser for storm water before it hits the wash," Laudati said. "It also helps to restore the wetlands habitat around the wash."

At the same time, about 10 acres worth of tamarisk trees are uprooted and burned annually. The tree's aggressive growth prevents other types of vegetation from flourishing. The tamarisk tree trunk will begin re-growing if not uprooted within 30 minutes after being chopped down.

Southern Nevada's wetland acreage has dramatically reduced over the last 20 years, dwindling from 2,400 to less than 200 acres today. Wetlands play a critical role for 300 plant and wildlife species.

Since 1999, $98.5 million has been spent on reversing the wash's deterioration.