Editor's note: Here, we highlight just a portion of the Evanston, Ill., collections system. For more information, check out the December cover story, "Innovation And Proactivity Put The City Of Evanston Ahead Of The Storm." 

To deal with stormwater overflows, cities across the country are separating combined sewers and digging new storm sewer lines — at a considerable expense.

Not so in Evanston, Ill. The city has taken a different approach, installing Reg-U-Flo flow restrictors from Hydro International in alley and street drainage inlets to its combined sewer system, which prevents the combined system from overloading during heavy rains.

The stormwater that stays on the street due to the inlet restrictors can flow down the street for up to two blocks where it is intercepted by high-capacity inlets that flow directly into the relief sewer system.

The approach is working and saving money over installing separate storm sewers throughout the city.

“The flow restrictors have really protected our basements from backups,” says Dave Stoneback, Evanston utilities supervisor. “They cause stormwater to stay on the street and move about two blocks to drains to our relief storm sewer system.”

Restricting the flow
The flow restrictors are self-activating valves that use vortex principles to control and slow the runoff rate of stormwater without the need for moving parts or external power requirements. The units are usually applied to manhole sidewalls or drainage structures.

Under low or normal flow conditions, the Reg-U-Flo valve unit provides a relatively large orifice that lets stormwater pass directly through the valve and discharge to the combined sewer system.

As flow increases during a period of heavy runoff, however, the water enters the valve and is spun — in a vortex swirling motion — by the valve’s interior design. As flow increases, the spinning action speeds up and the air core inside the valve expands, throttling the flow of water. As a result, discharge from the valve is restricted or reduced.

“The valve slows the flow while maintaining a larger opening compared to traditional flow restrictors,” says Phillip Taylor, stormwater solutions specialist with Hydro International. “The water is restricted but debris such as leaves passes through. It doesn’t end up plastered to the side of the valve.”

The restrictors are fairly common for stormwater applications in the U.K., where they are written into stormwater management software. But they are relatively new in the United States, Taylor says. They have been used previously in wastewater, but new U.S. stormwater discharge regulations are creating new applications, such as the one in Evanston.

The Reg-U-Flo valves have traditionally been custom-made using 304 stainless steel, but Taylor points out that a newer model using high-density polyethylene is now available, reducing cost.

The valves are not cheap, Taylor says. However, they have multiple applications and can offer considerable advantages when compared to orifice plates in a cost-benefit analysis.

Evanston application
Evanston is one of the leaders in applying the Reg-U-Flo restrictors. According to city information, the flow restrictors are installed in hundreds of alley and street drainage inlets and are connected to the combined sewer system to prevent overload during moderate and extreme rainfall events including the 100-year design storm.

This provides basement backup protection under nearly all situations, according to the city. The water can stay on the street for about two hours until it drains away into the relief sewer inlets.

“What Evanston is doing,” says Taylor, “is reducing or delaying the peak flow that hits the [sewer] pipe, and temporarily holding water upstream [on the street]."

Stoneback credits the flow restrictors with protecting the city’s basements. “They keep the stormwater on the street and divert the water to our relief sewers,” he says. “They’ve worked well.”

Maintenance has been normal, according to Stoneback. Crews use a two-part device that fits into the outfall pipe and pulls the vortex head up to the street for cleaning.