Hawthorne Bioswale, Millvale Borough, Pennsylvania

Project Completed:  September 2013

Project Sponsors: Western Pennsylvania Conservancy, Treevitalize Pittsburgh, Sisters of Saint Francis & The Borough of Millvale

The Hawthorne Bioswale is designed to collect runoff from both the Mount Alvernia Campus and the 11.1 acre area tributary to the upstream curb cut along Hawthorne Road.

Much of the storm water from this 11.1 area is collected above the site in the existing storm sewer system that runs down Hawthorne Road to Girty's Run.  The area along Hawthorne Road has a separate sanitary and storm sewer system until Evergreen Road, where the system becomes a combined sewer owned by the Girty's Run Joint Sewer Authority.

Prior to proceeding with the design of the project, the existing storm sewer was televised to determine its condition and to locate the source of significant dry weather flows that were observed in the line. It was determined that the water was entering from a Pittsburgh Water and Sewer Authority (PWSA) water line that was leaking.  PWSA was informed of the leak and repairs were made to the line.  It was also noted that the existing clay storm sewer, owned by the Borough of Millvale, was cracked and broken in many locations. The Borough was contacted and the Millvale Public Works Department replaced over 400 feet of the 18" storm sewer with new high density polyethylene (HDPE) pipe. This unforeseen issue was handled without any additional cost burden.

The constructed bioswale is 390 feet in length and has an 8% average slope.  The bioswale is broken up into several pool and riffle sections and  contains approximately 15,000 gallons of storage. The system is designed to delay flows for a significant period. The system is being monitored, using a grant obtained by the North Area Environmental Council from the Allegheny County Conservation District, to determine the amount of attenuation and flow reduction that occurs during various rainfall events.  A report on the follow-up flow study will be prepared by Groundwork Civil in 2014.  In addition to flow attenuation, some of the runoff that enters the system also infiltrates into the ground and is taken up by the plants located throughout the bioswale.