04-13-2005 Council Agenda - Laserfiche WebLink

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9€%'% (-60; 2C/da 515 Little Canada Road, Little Canada, MN 55117-1600 (651) 766 -4029 / FAX: (651) 766 -4048 www.ci.little-canada,mn.us MEMORANDUM To: Mayor Blesener and Members of City Council From: Bill Dircks, Public Works Date: March 24, 2005 Re: Manhole Cover Replacement MAYOR Bill Blesener COUNCIL Jim LaValle Rick Montour Barbara Allan John Keis ADMINISTRATOR Joel R. Hanson City staff has been working on the problem of inflow and infiltration into the city's sanitary sewer system. Inflow is clear water discharging into the city sewer system through such things as sump pumps, roof drains, cross connections with the storm sewer system, and manhole covers. Infiltration is clear water that enters the sanitary sewer system from the ground through deteriorated pipes, pipe joints, cracks in manhole walls, service line deficiencies, and other similar means. The public works department has investigated the videos of the sewer system, checked lift station pumping records to observe spikes in flows, checked manhole chimneys for deterioration, and investigated every sanitary sewer manhole cover in the city to try and get a grasp on what is causing the city's major UI problems. The more 1/I we can eliminate, the smaller the bill we will receive from Metropolitan Council Environmental Services. The public works department believes that a significant amount of inflow into the sanitary sewer system is occurring through holes in our manhole covers. The holes are designed to accommodate a pick or hook to ensure removal of the covers. They are typically called "pick holes ". Technology in manhole covers has advanced to the point where covers are now designed with concealed pick holes that do not allow any water through the cover. We did an inspection of all of our manhole covers and found that over 450 of our manhole covers have at least one open pick hole, with many containing two. A study was performed at the Neenah Foundry in 1983 on the amount of inflow that can occur through open pick holes. The relevant data from that study; including the test conditions, the graph showing relationship of pick hole size to gallons per minute of inflow, and the table of data collected from each test condition has been included with this memo.