Flood Control in the Desert? For Vegas Residents, it’s a Wash

Amidst all the glitz and glamor that is Las Vegas, a small, innocuous tributary carries stormwater, urban runoff, shallow groundwater and highly-treated wastewater into Las Vegas Bay at Lake Mead. Called the Las Vegas Wash, it is an integral part of the city’s ecosystem. However, time and the elements have been wearing away at many parts of the system, particularly its Main Branch which runs from Las Vegas Boulevard to Lake Mead Boulevard in North Las Vegas. With the overall effectiveness of the Wash reduced, the Southern Nevada Water Authority (SNWA) and the Clark County Regional Flood Control District have contracted with Meadow Valley Contractors to stabilize the channel, replace several flow-inhibiting structures and tie the newly-upgraded segment in to other existing parts of the system. It’s a project which seemed ideally suited to the benefits GNSS technology offers, so Meadow Valley’s use of machine control from Topcon Positioning Systems was a sure bet—something you do not see a lot of in Las Vegas.

Time Takes a Toll
The Las Vegas Wash runs easterly for 12 miles from just past the Las Vegas Valley’s main wastewater treatment plants, and through a series of weirs until it eventually empties into Lake Las Vegas. Built in 1975, the system has had to deal with a number of issues including: erosion, quality of groundwater and urban runoff, as well as degradation of both the wetland and the habitats located along the Wash. According to Scott Allen, Meadow Valley’s general superintendent, the project to improve the system’s performance, started in July, 2012.

“Though some areas of the Wash have already been upgraded, this roughly 2.5 mile section hasn’t,” he said. “This was originally a trapezoidal earth channel which had just been continually eroding over the years. It’s gotten to the point now, where it is creating a potential risk for homes located alongside it. One of the most significant fixes we are addressing is converting this to a stabilized channel with a concrete floor and walls. But we are also addressing replacing a culvert where the channel runs under Pecos Blvd.; an area that is prone to regular flooding. Both of those efforts will improve the flow through this section in a huge way.”

Because roughly 400-500 gallons of water is always present in the Wash (runoff from sprinklers, irrigation, and so on), Allen says Meadow Valley first had to reroute that “nuisance flow” before they could begin work on the channel itself. To do that, they placed a temporary dam upstream at Cheyenne Blvd., in a section in which the channel has already been improved. Pumps with automatic floats were then used to pump the nuisance flow to an area behind the active work site. “That’s enabled us to have a nice dry channel in which to work,” he says.

Ditching Some Manpower
The initial stages of the project entailed a good deal of excavation, roughly 220,000 cubic yards in Phase 1 and another 60,000 cy. in phase 2. Allen says some of the material removed has been exported to various other sites, some has gone to a crusher where it will become Type 2 road base to be used at the Wash site, and some is going directly to an area pit.

“The excavation portion of the job was significant—the existing grade sat two-foot or so higher than what we needed and the channel had no uniformity to it,” he said. Essentially, we had a deep V-ditch in the middle where all the nuisance flow ran, and the rushing water over the years just wore away at the original trapezoidal shape. We did the mass excavation with an excavator and then set to work setting the subgrade.”

In the past, establishing that subgrade would have been a time-consuming, challenging, multi-person effort in which a surveyor would first come onsite to establish points and offsets. A Meadow Valley grade checker would then pull those points and offsets, paint lines to indicate where to cut, and cuts would be made—oftentimes removing the painted indicators. That would mean calling back the surveyor, re-checking the grades, and so on. Today, the process has been streamlined significantly through the use of GNSS and machine control equipment.

“We got into GPS several years back and a couple years ago added Topcon machine control (3D-MC) to our first machine, a Cat D5 dozer,” said Allen. “But though we’ve been using GNSS-based equipment for a while now, this project really underscored how valuable it can be for us. We are doing the bulk of the dirt with a Cat excavator, then following right behind it with our dozer cutting down to subgrade. No staking, no need to flatten then have the blade come back, reset the grade, check it, set hubs and so on.  You really begin to see how quickly the final grade of each lift can get done. We’ve since also added 3D-MC to our Cat 14H motor grader.”

At the heart of Meadow Valley’s GNSS effort is a Topcon HiPer II which the company has mounted to one of the bridges crossing over the jobsite. In addition, the company utilizes a pair of Topcon FC 250 handheld units which it uses for onsite data collection and periodic grade checks.

Three Against One
The speed with which the floor of the Main Branch structure is being laid is particularly impressive, given all that’s going into it. After the subgrade is established, Allen’s crews come in and cover it with a synthetic black fabric and 18 inches of rock—again placed using the machine-controlled dozer. Fabric then also covers that layer, and Type 2 rock, placed to grade, tops it off in advance of the concrete pour.

“We have three side-dump trucks pulling alongside the channel and steadily dumping to the area below. A loader continually feeds the GPS-equipped dozer and my operator, by having all the grade info on the screen in front of him, has been easily able to keep up with the pace of those three trucks.”

As mentioned, additional work was needed in some areas to maintain a good flow within the Wash and alleviate recurrent flooding problems. With that in mind, reducing surface tension—which can impede the flow of water within the channel—was the goal of a key culvert replacement.

“Previously there was a seven cell culvert system under the Pecos/Lake Meade overpass—with each cell essentially being an 8 X 8 square concrete box,” said Allen. “With so many walls in such a system, there was substantial surface tension, resulting in poor flow and repeated flooding during peak rainfall times. We replaced those seven cells with three 24-foot wide by 7-foot tall reinforced concrete box (RCB) culvert sections. Now there are only two internal walls which lessens surface tension and improves flow.”

Support System
To bring all the GNSS benefits to bear on the Las Vegas Wash project, Allen and Meadow Valley have been working through Nevada Laser, the area Topcon dealer. Allen said that the level of support—key when transitioning to new technology like this—was outstanding.
 

“Eddie Brown and everyone at Nevada Laser—as well as the folks at Topcon itself—have been great to deal with,” he said. “There’ve been a number of occasions on this job where we’ve had to rely upon them because we don’t yet have the expertise with the equipment that they do. They’ve always been there for us, which not only gives us peace of mind, but has also helped keep things on track.”

He added that the impact—including increased production—adding a GNSS capability has had on the Wash job can’t be overstated

“But the labor savings is where we are seeing the biggest benefit,” he said. “When compared to other projects we’ve done in the past, there is hardly any manpower on the ground. This new approach has allowed us to put one operator on a machine and let him tackle everything from mass excavation to subgrade excavation to aggregate for concrete placement. There is no waiting for a grade checker, no overcutting or undercutting. Just real onsite efficiency.”

Meadow Valley has also won the bid for Phase 3 of the Wash project, which includes construction of pedestrian bridges, a 12-foot wide paved walking trail, and more. Work on all three phases is expected to wrap up around July, 2013.

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