Vortechs Hydrodynamic Separator
In the late 1950s the area around End Street in Chicopee, Massachusetts, was developed for residential use. The developer installed a stormwater drainage system with a 24-inch outfall that discharged into a ravine leading to a nearby stream. Over time, the outfall site eroded from small ditch to small channel to larger gully. By the late 1990s, water had eroded the gully site into a huge scour hole with banks of unstable sandy soil that were 30 feet high, 150 feet long and 70 feet apart.
“It was like a mini Grand Canyon,” said an engineer who worked on the project, noting that the change in elevation from the ravine’s top to its outlet was more than 80 feet. The scour hole was undercutting the drain pipe itself.
“We got to the point where we were just calling it “the hole,” said Chicopee Department of Public Works Supervisor Stanley Kulig. He and other city officials were concerned about the public safety hazard that the hole posed. “It was an attractive nuisance for kids in the area, who rode bikes and four-wheelers in it. We found forts and other evidence that kids were playing in it.” Increasingly, the city became more concerned about liability. The hole was also beginning to encroach on nearby roadways. It also threatened a power line in a utility easement that ran through the area, forcing the local power company to move one of its poles.
The City of Chicopee retained Green International Affiliates, Inc. (GIA), an engineering firm based in Medford, Massachusetts, and embarked on a collaborative planning process to design a permanent solution that would be approved by environmental regulatory agencies. The design team included representatives from GIA, the City’s Public Works Department, the Conservation Commission, a wetland scientist from Valley Environmental Services, and a bioengineering consultant from The Bioengineering Group.
The group set out to address three goals. First, they needed to design a system that would stabilize the existing stormwater outfall and stop severe erosion at the site. They also needed to improve water quality through Contech practices specific to the site. Finally, they hoped to restore the property and the environmental resources in context with their ecological surroundings, including a 450-foot-long stream.
The steep grade at the site complicated planning. To eliminate the potential for erosion at the sandy site, stormwater runoff would have to be piped down the embankment to the stream. However, the steep angle of the pipes meant that in larger storms, high outlet velocities could potentially cause erosion and threaten wildlife habitat in the stream below.
The City wanted to improve water quality using best management practices (BMPs) even though the site, which had qualified as a restoration project, did not have to meet the state’s most stringent stormwater regulations. “We ended up using a unique combination of pipes and stormwater treatment technologies to come up with the solution,” said Michael Pelletier, the lead engineer with GIA.
GIA specified a stormwater treatment system sized to treat a one-year or smaller storm event with a high flow bypass for larger storms. A Vortechs® System, a hydrodynamic separator manufactured by Contech, was installed at the top of the ravine near End Street to treat runoff from up to a one-year storm prior to entering the stream. Installing the Vortechs near the road provided the city with easy access for routine maintenance and cleanout.
A diversion structure at the top of the slope controls discharges so that low flow is maintained in the natural stream and high peak flows from storm events are diverted to the larger bypass pipe. Because of the steep angle of the slope, GIA specified HydroBrake vortex valves to slow the velocity of the water and minimize its impact on the stream. A 24-inch two-year storm pipe has one HydroBrake valve, and a 36-inch bypass pipe has two HydroBrake valves in series where the major changes in slope occur. The HydroBrake valves function using vortex principles to control the velocity of the water as it circulates around the inside of the housing prior to discharging through an orifice in the bottom. By using the HydroBrake valves, the system could be constructed on the steeper part of the slope without threatening the stream bed below. The project was funded as a demonstration project through EPA’s 319 Grant Program.
“The HydroBrake valves allowed us to effectively slow down the water without using large energy dissipating structures at the outlets, or as many as 25 manholes that would have been required to traverse down the slope to maintain velocities of less than 12 feet per second,” said Pelletier. “The vegetated stream was designed for a maximum water velocity of five feet per second and the HydroBrake valves have successfully maintained the outlet velocities below this limit.”
The results have proven to be a resounding success. The public safety hazard has been eliminated and the use of BMPs means that cleaner stormwater is going into the waterways. The ecology of the area is rebounding with nearly all of the planted vegetation thriving. Additionally, the solution demonstrated that restoration projects requiring the rehabilitation of existing infrastructure can meet the stormwater BMP requirements under the Massachusetts Contech Policy and NPDES regulations.
In recognition of GIA’s work on this project, the American Consulting Engineers Council (ACEC) of Massachusetts presented them with the 2001 ACEC/MA Engineering Excellence Award and the 2001 ACEC/MA Small Firm Award.
Vortechs Hydrodynamic Separator