10-10-1988 - Agenda Packet City Council - regular meeting

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�L t1 McCombs Frank Roos Associates, Inc. i' tTwin Cities ik 15050 23rd live N Telephone rr'y"" St. Cloud Planr 612/476-6010 Plymouth, MN Sun 55447 August 4, 1988 M. Joe Pallansch 01. Washington Scientific Industries, Inc. 2605 West Wayzata Boulevard r..'.. P.O. Box 340 Long Lake, MN 55356-0340 SUBJECT: Washington Scientific Industries, Inc. Storage Pond 100 Year Storm Run-off MFRA #8749 Dear Mr. Pa_lansch: Encloscd are seven copies wilich show the contours of a storagealsoshondtto store run-off from a 100 year frequency storm. drainage area in blue, from which this run• )ff will occur and include approximately 90% of the proposed new addition to your building. Basically this is the only portion of your property that will have increased run-off due to the proposeu addition iand therefore, wen -off in this have area computed the required storage e volume from tchanges The total area that we are concerned with is 8.3 acres. It is estimated that the coefficient of run-off changes from 0.53 to J.56 due to the addition of the roof area as a result of the new coristructinn. This will increase the run-off from this drainage area from 20.7 cubic feet per second (CFS), to 21.9 CSF. The rate of flow allowed from this area is the original r n—offrate beforoff construction and that was 20.7 CFS. In order to maintain this from the area, it is required to construct a storage pond which will oe constructed aloe,., :, existing swale approximately 200 east of the west property line. The storage pond can be constructed by cleanir,g out and lowering the bottom of the drainage ditch and putting in a small dam in the area arshown 2onithe flow from the drainage h RCP accompanying maps. To regulate the A wil! ;.,+• placed in the dike to drain the water out of the swiillgallow d13.5oCF5e ditcr, uownstream. It is estimated that this 21 inch pipe to flow through it thus backing water into the storage pond as the run-off from a 100 year storm will be 21.9 CFS after construction of the proposed addition. Our computations show that a storage volume of 17,726 cubic fetis required. The ponding area that we show will hold a volume of approximately ,500 cubic feet at the 1,013 contcur elevation. The invert of the upipe rvecshould becioat 1,010.5. We are also including with this 1• computed by the rational method. Copies of this Lrifurmation should be (jeliverad to Mr. Kevin Larson, of Eugene A. Viickock and Associates, b), the e,xi _)f the day on August 8, 1988.