4 PCSR 06-10-1993 - Laserfiche WebLink

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Mr. Steve Rohlf • June 4, 1993 Page 3 9. There is an existing 100-foot wide U.P.A. easement through Block 4 which is proposed to be realigned. This realignment along the west line of Block 4 is shown to be through the middle of one of the proposed storm drainage ponds. We would recommend that it is not appropriate to have these types of utilities under a storm drainage pond because of the potential maintenance problems associated between the two. C. STORM DRAINAGE PLAN 1. The majority of the storm drainage from the proposed development is shown to flow easterly under Trunk Highway 169 to an existing ponding area on the east side of the highway. This amount of storm drainage into this ponding area will create a rise in the water level of several feet. Negative comments were received relative to this increase in water level as part of the EAW. The City is currently in the process of developing a comprehensive storm drainage plan. One of the recommendations of this plan will be to provide an outlet to this • large ponding area on the east side of Trunk Highway 169, the outlet ultimately flowing to the Mississippi River. It will be necessary to coordinate the construction of this outlet with the proposed development of this commercial area such that there is not a negative impact from a rise in the water elevation of this ponding area. 2. As part of the draft comprehensive storm drainage plan, it has been recommended that the City of Elk River follow the Best Management Practices guideline set out by the Metropolitan Council. Sedimentation ponds designed following these policies would typically have a static water level of approximately four feet. Two feet of this would be for sediment buildup and two feet for actual sedimentation of runoff. As the bottom two feet fills with sediment and the total depth of the pond is reduced, this leads up to the time that the pond needs to be cleaned out. The design of the sedimentation ponds for this development have assumed that, rather than a four foot static water level at the time of a rainfall event, the ponds will actually be dry. Assuming that the ponds will be dry, the storm drainage calculations indicate that the ponds are sized to hold the entire runoff from a two- inch rainfall event, thus allowing sediment to settle to the bottom of the pond. This assumption allows the size of the ponds to be smaller than they normally would be following the Best Management Practices. We would recommend that this assumption not be allowed in the storm drainage design, but rather the • policies and design criteria as laid out in the Best Management Practices be followed. The reason for this is over time sediment will wash into the ponds, thus reducing the ability of the ponds to percolate water, resulting in standing water in the ponds at the time of a rainfall event. Thus, the assumption that the ponds will 152-0401.jun