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Rskspvlffi� REQUEST FOR COUNCIL ACTION Department Approval Date: 5119108 Item No.: 13.a City Manager Approval Item Description: Rainbow Foods Combined storm sewer system update BACKGROUND With the redevelopment of Rainbow Foods at Larpenteur and Fernwood, City Staff has been working on developing a partnership with the developer and property owner to construct additional storm water storage for the Gottfried Pit subwatershed underneath the newly reconstructed parking lot. This would be accomplished through increasing the size of Rainbow's proposed underground chamber system. At this time, the system is proposed to be twice the size of the system required for Rainbow Foods redevelopment project. As a result, we would anticipate that half of the system cost would be City responsibility. POLICY OBJECTIVE One of the goals from the City's CSWMP is to provide flood protection for all residents and structures as well as protect the integrity of conveyance channels and stormwater detention areas. In 2003, the City of Roseville cooperated on a study of downstream flooding and water quality issues with Capitol Region Watershed District, Falcon Heights, the City of St. Paul, and Ramsey County for the northwestern watershed area to Como Lake. Gottfried Pit is located within this area. The results of the study demonstrate that to reduce the flooding issues in this subwatershed, improvements need to be accomplished upstream of Gottfried Pit. It also shows that flooding in Larpenteur Avenue occurs with the 4.2 inch (I 0 -yr) rain event. As a part of their building permit, Rainbow Foods is reducing the volume of water coming off their 7.7 acre site and going to the Gottfried Pit. The system that they are building significantly reduces the rate of runoff from their site. For the 4.2 inch event (10 -yr) it is reduced by almost 90 %. The underground storage volume of their system is 1.1 acre -feet. Roseville drains 315 acres to this system via Gottfried Pit near Fernwood Street and Larpenteur Avenue. 00 of these acres are to the northeast of the Rainbow site and are conveyed through a 30 inch pipe that runs through an easement between the redevelopment site and the shopping center to the east. We have worked with Rainbow on designing a system that would maximize the amount of water from this larger drainage area, without detracting from potential future redevelopment of the site. The design that is being proposed at this time is to double the size of the Rainbow infiltration system, creating a 2.2 acre -foot underground system. The additional 1.1 acre -foot of storage would reduce the volume of runoff from the City's system that goes to Gottfried Pit by 13 %. While it would not completely eliminate the frequent flooding of Page 1 of 2