Engineers plan weir to slow water eroding Las Vegas Wash

The Las Vegas Wash has gone wild in recent years, so the National Park Service wants to calm things down a bit.

Officials at Lake Mead National Recreation Area are working on plans to build a multimillion-dollar weir - also known as a calming structure - to slow the flow of the wash and protect a major bridge in the park.

It's yet another consequence of a shrinking Lake Mead.

As the surface of the reservoir drops, the wash has farther to fall to meet it. The result is a faster current and more erosion.

If left unchecked, the rushing wash could scour the supports out from under the Northshore Road bridge east of Henderson and chew through the dam that forms Lake Las Vegas.

But park engineer Travis Anderson said there is no immediate danger.

"The bridge is good right now," he said. "The bridge is safe."

To keep it that way, the park service wants to construct the new weir - and eventually five more, each costing between $3 million and $5 million - in the wash below the bridge.

Three such water-calming structures were built there in 2002 after an inspection by the Federal Highway Administration determined that without action the bridge could fail.

Anderson said the existing weirs have largely controlled erosion around the bridge, but the force of the water has taken a toll on the one closest to the lake.

"Every time the lake gets low, this becomes an issue," said Gerry Hester, engineering project manager for the Southern Nevada Water Authority.

Hester ought to know. He has been working in the Las Vegas Wash for decades.

The wash used to be an ephemeral stream that filled during downpours and carried the valley's storm runoff to Lake Mead. Its flow became permanent in the late 1960s, as the Las Vegas Valley developed and the wash became an outlet for treated wastewater.

Hester said erosion once cut channels as much as 40 feet deep in the banks of the wash, leaving vegetation high and dry.

He said the community finally got serious about erosion control in 1984, when a flood in the wash exposed what was then the valley's only distribution line for treated water from Lake Mead.

Since the late 1990s, Hester has overseen the construction of 13 flow-control weirs in the wash above Lake Las Vegas.

Three more of the structures are now under construction and set for completion by the end of the year.

In all, nine more weirs are slated to be built by 2015 under a $165 million program funded by sales tax revenue and proceeds from the sale of federal land in Southern Nevada.

The program also has paid for extensive work on the banks of the wash, where "armor" has been laid in the form of concrete chunks from demolished casinos and highway interchanges.

"We have a whole bunch of casinos out there," Hester said.

The National Park Service is gathering public comment on its plans for the wash below Lake Las Vegas. An environmental review of the weir work should be wrapped up by this time next year, with construction to follow in summer 2013.

Michael Boyles, environmental compliance specialist for Lake Mead, isn't expecting much opposition to the project. Aside from protecting the bridge, slowing down the wash should improve water quality and clarity and help beef up riparian habitat along the lower wash.

"I think the long-term effects are all beneficial," Boyles said.

The Northshore Road bridge was built in 1978 to help provide access across the wash to the northern parts of the almost 1.5 million-acre recreation area.

The park service doesn't have much traffic data for the bridge, but Boyles said the 420-foot span is used by roughly 2,000 vehicles a day in the summer.

He didn't know the exact height of the bridge.

"It gets taller all the time with the erosion," Boyles said. "That's kind of a joke, but it's also true."

Contact reporter Henry Brean at hbrean@reviewjournal.com or 702-383-0350.