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~L~~RIN~ ~- ~SSO(I~AT~S iRUt HORi!! Memo TO: Mayor & City Council Members FROM: Lee Elfering, City Engineer DATE: July 8, 2009 RE: Culvers Infiltration Rate This memo is to provide details on the remaining issue to be resolved for the Culvers site, detail background information, and provide a recommendation prior to the Council taking action. The remaining issue involves the amount of time allowed for infiltration of storm water on the site. Culvers is requesting that a 72-hour draw down time be allowed for the subsurface infiltration practice instead of the 48-hour time required by the City's Storm Water Ordinance. Prior to 2009 a 72-hour draw down time was allowed for subsurface infiltration practices within the City. However, the Watershed revised this time to 48 hours in their most recent standards adopted this year and these new standards were utilized in the City's Storm Water Ordinance that was adopted this spring. Therefore, under the current ordinance the maximum draw down time allowed for an infiltration practice would be 48 hours. The implication of the change is that a subsurface infiltration practice would be able to fully infiltrate and treat the first 1-inch of rainfall from a site every 2 days instead of every 3 days by the old standard. While the current Watershed rules require a 48-hour draw down time, they have granted approval of the development with a 72-hour draw down time. It is also important to point out that the Market Place Pond serving this area was designed to provide the required treatment for the Culvers site based on the standards in place at the time it was constructed. However, this treatment is not considered as part of the sites storm water treatment for review purposes due to the infiltration standards that are now in-place. In reality, the storm water that comes off the Culvers site will still go through the Market Place Pond and receive treatment. Summary and Recommendations Due to the fact that the Watershed has approved the development with a 72-hour draw down time and that treatment of storm water leaving the site is achieved by the Market Place Pond, we would recommend the City grant Culvers requests fora 72-hour draw down time. If you have any questions please call me at (763) 780-0450 extension 13. 10062 Flanders Court NE -Blaine, MN 55449 - (763) 780-0450 fax (763) 205-2341 Z:\PROJ\120118E - Culver's\Memo-070809-CC.doc