Stormwater | Project Profile

Public works officials in Clark County, NV, had a situation: Adverse gravity conditions were causing an urban stormwater detention pond to drain improperly, leaving standing water over a 10-acre area.

A Clark County, NV, employee examines the control panel.

Their solution? To install a pre-engineered package lift station at the Desert Inn Detention Basin that pumps the last of the water in the basin to the elevation of the existing storm sewer main lines. "Clark County Public Works requested that the adverse gravity flow problem be corrected so the detainment basin could be fully drained," says project engineer Jeff Herrick of VTN Nevada, "and the lift station solved the nuisance water drainage problem that was causing problems."

The efficient pumping of stormwater is becoming more important to municipalities across the country as evidence mounts of the environmental and health hazards associated with urban runoff. As development increases, so do the risk factors—more oil, antifreeze, and other urban detritus collecting in the runoff, more surface area impermeable to water, and more water that is no longer soaking into the bare ground.

A crane places the top slab to finish off the wet well installation. The base, barrel, and top slab are pre-aligned for ease of installation.

The wet well fills with water after startup; the storm pipe is visible in the wall to the left.

In Las Vegas, officials were concerned with odor and health concerns associated with the standing water at the Desert Inn site, particularly with mosquito abatement. Warm, standing water is an ideal breeding ground for the insects, some species of which can carry the West Nile virus. The virus, which has been reported in nearly every state, made its first appearance in Nevada last year, with Clark County logging two confirmed human cases.

The Desert Inn Detention Basin is part of Clark County Regional Flood Control's drainage system, which consists of 350 miles of channels that bring water into 62 detention basins and, ultimately, to Lake Mead. The area is subject to potentially devastating flash floods, caused by intense rainfall that runs rapidly off steep mountain slopes and armored desert surfaces, concentrating in the urbanized areas at lower elevations. Clark County officials say the detention basins have helped reduce flood damage by up to a factor of three. In one 90-minute storm in 2003, for example, the detention basins captured 7,600 cubic feet of water per second.

After collecting the stormwater and urban runoff, the basins are designed to drain in a metered flow through a pipe system. However, an adverse-to-grade condition at the Desert Inn pond, coupled with a flat outlet pipe resulting in frequent backups, consistently left 4 to 5 inches of standing water over the surface of the 10-floor-acre pond.

The change in elevation from the floor of the Desert Inn pond to the discharge point was just 4 feet. One option considered was to "chase the grade," which would have entailed digging up half a mile of roadway to lay pipe that would correct the adverse-to-grade situation and move the water into the storm drains.

"But that's a busy intersection," points out Blair Lecker, project engineer for Clark County Public Works' Construction Management Division. "To put in a low-flow storm drain that would accommodate the difference in elevations would have meant excavating down a heavily trafficked arterial. We opted for the lift station."

VTN Nevada and county officials worked with Romtec Utilities of Roseburg, OR, on the design and submittal process. Romtec Utilities' Pre-engineered Package Lift Station fit well with the design standards used by Clark County Public Works, Herrick says. "That was the big advantage with the Romtec system —the concrete housing, pipe, and the pumps fit in well with the county's existing inventory.

"I was impressed with Romtec's customer service, especially through the shop drawing/design and submittal phase," Herrick continues. "They were really Johnny-on-the-spot getting any customization done that needed to be done." One of the custom features was the elimination of the pre-assembled concrete valve vault, which was unnecessary because the lift station is pumping water only 15 feet through the dual forcemains.

All of the other components of the lift station, including the pre-cast, self-cleaning wet well, the factory-assembled control panel, and pre-tested pumps and controls, arrived at the Desert Inn site last August in one shipment, ready to be installed.

The general contractor on the project was Diamond Construction of Las Vegas. Public works employees watched as a crane lowered the precast barrel sections into the hole excavated by contractor Underground Inc. and then added the precast top slab containing the Flygt access cover. Installation of the lift station took about six hours, with an additional day for finishing the pumps, piping, and electrical work.

Flygt representatives were on hand to answer questions about the submersible duplex pumps and the MultiTrode monitoring and control system. The Desert Inn lift station controller responds to a sophisticated rain monitoring station located about 1,000 feet from the far side of the detention pond. When it's raining, the pumps start; when it's not, they stop. The controller technology, coupled with the lift station's MultiTrode liquid level sensing probe, gives the county the ability to read the pump activity remotely.

The lift station itself is located at the intersection of two six-lane roads on the west side of Las Vegas. The only evidence of its existence is the pre-cast top slab that sits just above ground level, the pump disconnect stand, and the control panel enclosure.

Herrick visited the site recently after a rainfall, and reports the lift station works as advertised. "Everything seems to be working well, and we look forward to having the lift station in place when our winter rains start."

SW March/April 2005

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