5.4. SR 06-05-2006 - Laserfiche WebLink

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Page 3 May 24, 2006 I\tls. Rebecca Haug Minnesota Pollution Control At:encv Comments - Dated April 13.. 2006 Item 10 While Twin Lakes lies at an elevation lower than the proposed project site, there is no stonn water discharges from the proposed proj ect site to Twin Lakes; Viking Boulevard creates a natural banier. All storm water runoff from the proposed project site is conveyed to and treated in ponds prior to discharging to an existing wetland basin and ditch system. Storm water ultimately discharges under Twin Lakes Road in a westerly direction to Trott Brook. Rates of discharge are mitigated to pre-existing flow rates, thereby minimizing bank instability. Changes in aquatic invertebrate are expected to be mInor. Section 1 7 Fully mitigating the volume increase resulting from the conversion of agriculture use of the land to urban residential is likely not to be accomplished for storm events with recurrence intervals greater than the one year event (i.e. 10 year, 100 year). Storm water treatment ponds will be evaluated for incorporating low COSt/19W maintenance infiltration capabilities. The additional maintenance associated with bio- retention cells, swales and rain gardens is an expense the City of Elk River, the ultimate Owner, is not prepared to undertake. The storm \vater model created for this project is releasing flow from the developed site at a rate of less than or equal to'pre-development rates. The receiving streams are two unnamed ditches. Both ditches cross TvvinLakes Road (CH 13) and ultimately discharge to DNR Wetland 71-238W. Neither ditch appears to be experiencing detrimental bank erosion. The extended duration of runoff is not expected to cause detrimental bank erosion in the existing streams. Section 18C The Elk River City Engineer's Office states that sufficient capacity exists for the additional flow from 444 new single family homes and a 900 student school if they [the City of Elk River] are unable to expand. Comments from Alan Hoebelheinrich - Undated Drainage patterns from the north side of Viking Boulevard are preserved with this proposed project (see attached exhibit). Storm \vater runoff enters the project site (Wetland Basin 6) from the north via a 15 inch culvert under Vikillg Boulevard. Tllis culvert will not be disturbed or rendered in-operable by the proposed project. Drainage will continue through Wetland Basin 6 in a southerly direction to a proposed culvert under proposed 197th Avenue. Drainage continues in a southerly direction through Wetland Basin 6 until it is intercepted by a second proposed storm se\\:er pipe, \vhere it \\'ill be conveyed to the southwest and be discharged into the existing ditch, \\Thich lies along the common southerly lot line of the Middlestadt property. At this location, storm \vater runoff drains to the existing culvert under T\\'in Lakes Road.