3 PCSR 10-19-1993 - Laserfiche WebLink

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• Steve Ach October 15, 1993 Page Four It will be necessary to coordinate the construction of this outlet with the proposed development of the commercial areas such that there is not a negative impact from the rise of the water elevation in this ponding area. 2. A much smaller portion of the development site is proposed to flow southerly into the existing storm sewer system. The system ultimately outlets to the Mississippi River near the old City Hall site at TH 10 and Main Street. This is the same system that in recent large storms has caused isolated flooding at the intersection of Main Street and Evans Avenue. The increase in flow to this system from the proposed development is relatively small at 14 cfs in a 100-year rainfall event. However, unless provisions are made to provide an alternate outlet for the Main and Evans intersection, this additional flow will only aggravate the existing problem and large frequency rainfall events. Also, this storm drainage flowing south from the proposed development opens up the portion of the School District property which may ultimately develop as commercial property to providing additional flow into this system. As part of the Draft Comprehensive Storm Drainage Plan, a solution to the Main and Evans isolated flooding problem has recommended. The City has • authorized additional work on preliminary design of this including cost estimates and funding scenarios. Coordination of the Main and Evans improvements will be necessary to preclude any increase in problem in that area caused by additional flow from this proposed development. 3. The storm drainage design for the Elk Park Center Development has been revised such that the sedimentation ponds now meet the design criteria laid out in the Best Management Practices guidelines. There are a total of six sedimentation ponds proposed on the site. These will all have existing standing water ranging from 3 to 6 feet. During a 100-year rainfall event, the maximum depth of water in these ponds will increase to a minimum of 7 feet and a maximum of 10.9 feet. These existing standing water depths and maximum depths raised the issue of whether or not the liability associated with these types of ponds would require that some type of fencing be provided. 4. The design of the ponds calls for a 4:1 side slope above the static water level with a benched area approximately 10:1 at the water level. The pond slopes beneath the standing water level are shown as 3:1. These slopes are in accordance with the Best Management Practices guidelines. However, the 3:1 slope below the water level may also be a concern given the fact that these ponds will have a standing water depth of approximately 3 to 5 feet. Should • a small child wander into one of these ponds, it would be difficult to exit given a 3:1 side slope. If the ponds are not to be fenced, consideration should be given to lessening the slope below the water level to something that individuals could easily get out of if they should fall in. 152-1501.00T