Agenda - Council - 02/26/2002 - Laserfiche WebLink

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Hakanson Anderson Assoc., Inc. 3601 Thurston Avenue, Suite 101, Anoka, MN 55303 Phone: 763/427.5860 Fax: 7631427-0520 MEMORANDUM TO: Brian Olson, City Engineer FROM: Cralig Jochum DATE: February 22, 2002 RE: Storm Water Computations Cc: Dwight Jelle, P.E. STORM SEWER / HYDROLOGY All 100-year flood elevations must be labeled on the grading plan for all ponding areas including back-'yard areas. The hydraulic gradellne for the 100 year storm event must be considered for all backyard catchbasins, The gradeline must start at the discharge point and continue upstream. This case appears to be more critical for some of the stormsewer in future phases, Set 100-yr flood elevations for all backyard Iow areas. Label the emergency overflows for structures 75 and 76. 3. No computations have been provided for the storm sewer system in future phases. Plans must be approved by Lower Rum River Watershed Management Organization. Water quality ponds should have a 10:1 bench at the NWL. Show the storm sewer on the grading plan. 10, Check the runoff volume for water quality for streets only. Using the weighted curve number for the runoff is not accurate. Existing CN should be 39, there is greater than 7'5% grass Cover on the site. Show the outlet for the existing pond in the park. Label the 100-year flood elevation and show the proposed grading. A 21" pipe flows to a 15" pipe in CBMH 104. this needs to be corrected, 11, Will the existing pond in the park act as a water quality pond? If not, SRR6, DA-2 and DA-3 will have to be treated. ~I2, 13, Is ar~a SRR3 treated prior to entering SRR?'? Tailwater needs to be considered when setting the flood elevation of the pond. Civil & M,,m'cipal Engineering 25 Zand Surveyingj3r 'r~ ,.7,,'""~ Page 1 of 2-103-