Trihydro was selected by the City of Cheyenne to prepare design plans and specifications for construction of 850 linear feet of stormwater line in Snyder Avenue in Cheyenne, Wyoming. The purpose of the new stormwater line was to connect an upgraded stormwater collection system in Pershing Boulevard to an existing stormwater line south of the Snyder Avenue and Randall Avenue intersection. The existing stormwater system and outfall line were undersized, resulting in frequent flooding of Pershing Boulevard corridor and the adjoining intersections. Adding the additional stormwater outfall line in Snyder Avenue was a critical component to improving the drainage of the historic neighborhood along the Pershing Boulevard corridor.
A unique design component of the project was dealing with the connection of the new 48-inch pipeline to an undersized existing 36-inch outfall pipeline to Crow Creek. The City had identified the existing 36-inch pipeline in Snyder Avenue to be upsized as part of future drainage project, but that project could not be completed before the start of the Pershing Boulevard project. To prevent the increased flow from the upgraded stormwater system in Pershing Boulevard from flooding the smaller existing downstream pipeline, a series of restrictor plates were designed and installed in the new upstream manholes in Snyder Avenue. The restrictor plates were designed to limit the flow by using the upstream manholes and pipes as small detention cells. After downstream outfall line was upsized the restrictor plates could then be removed to allow full flow in the pipeline.
An additional challenge included routing the 48-inch stormwater pipeline down a narrow two-lane urban corridor, which involved moving existing parallel utilities, as well as threading the pipeline under a 30-inch water main and 12-inch sanitary sewer main. Crossing under the water and sanitary sewer mains required the design of two transition vaults to change from a 48-inch pipe to four 24-inch pipes back to a 48-inch pipe. There were also several areas where the parallel utilities could not be moved to provide the required separation distance between a potable water line and stormwater pipeline. To receive approval from the Cheyenne Board of Public Utilities and the Wyoming Department of Environmental Quality to place the pipes closer, Trihydro negotiated the use of Steel Reinforce Polyethylene (SRPE) pipe with pressure-rated joints as an alternative to Reinforce Concrete Pipe. This was the first use of SRPE in the City, and because of this project's success, the City opted to use the SRPE pipe product on several subsequent projects.