The American Public Works Association (APWA) – Minnesota Chapter recognized the City of Bloomington at the annual conference on November 22. The Penn American Stormwater Storage project received the Project of the Year award for their work to develop a linear storage system that combined retention and conveyance of the runoff while mitigating the impact to Upper and Lower Penn Lake.
During even moderate rainfall events, runoff overwhelmed the existing storm sewer system and was stored above ground, within the streets and adjacent properties, until the existing pipe system could reduce the flow. The highly impervious, fully developed nature of the contributing watershed combined with significant overland flows to the low storage areas only worsened the flooding. Because the area is fully developed, there is limited safe storage above ground before flood damage can occur. The city's plan for high-density, pedestrian friendly, and transit-oriented redevelopment of the area limited the open space for excavating a stormwater detention basin as a potential solution. The most viable solution was determined to be an increase in the outfall storm sewer capacity.
Bolton & Menk was hired to engineer the outfall design of this critical system and, in support of this work, also evaluated and refined the city’s preliminary XPSWMM modeling for the area to confirm the watershed’s hydrology and the influencing design parameters. The final project included an underground conveyance and storage system including 700 feet of dual 16-foot by 9-foot box culverts, 2,600 feet of 144-inch diameter corrugated metal pipe, 900 feet of 72-inch RC pipe, and more than 12 acre-feet of underground storage.