05-04-2020 Council Work Session Packet

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Ms. Katie Larsen 4/13/2020 Page 2 Nadeau Acres Engineering Review Grading revised. See comment 10 under the Stormwater Management section. 6. 10 foot side yard drainage and utility easement required on Lot 101. Revised 7. 10 foot side yard drainage and utility easement required on Lots158 and 159. Revised. Now lots 157 & 158, side yard drainage and utility easement added. 8. Drainage swales do not meet minimum 2% requirement at the following locations: rear lot line of Lots 116, 117,119, 121. Slopes revised Lot numbers have changed, but grading in the areas mentioned has been updated. 9. Grade break elevations needed on Lots 124, 139, 165. Spot elevations added at right-of-way for rear yard swale slope verification. Lot numbers have changed, but grading has been updated. 10. The development is a two-phase development. It is anticipated that grading for the entire site and for the stormwater ponds will be completed at once and not in two phases. Noted. 11. Provide construction details for the turf maintenance accesses. Individual lot surveys will be required to include the turf maintenance access locations (Lots 2 & 3 of Block 1, Lots 6 & 7 of Block 2, and Lots 7 & 8 of Block 6). Maintenance access detail added to detail sheet 12 • Stormwater Management The stormwater on the site flows to three different locations from the site. The majority of the site drains to an existing private ditch/wetland area to the north of the site. A small portion at the south of the site drains through an existing culvert along Birch St (CSAH 34). The western portion of the site drains to the northwest to an existing private ditch. The surface water will flow through three new ponds. Pond 200 located in the middle of the site will pick up surface water from the south portion of the site and direct it towards Pond 100. Pond 100 will pick up surface water from the northwest portion of the site and discharge to the west to an existing private ditch. Pond 400 will receive the remaining surface water and discharge to an existing ditch to the north. After the project is completed, the site will include approximately 8.8 acres of new impervious surfaces including the streets, sidewalks, paths, parking lots, and rooftops. Due to poor soils in the south portion of the site and a high groundwater table in the north portion of the site, infiltration is not feasible. Groundwater was encountered at depths ranging from 4 to 19 feet. Topsoil was encountered at ½ to 1 foot. Below the topsoil, soils consist of glacial outwash sands and native clayey glacial till and are classified as hydrologic groups HSG D in the southern portion and HSG C in the north. The applicant used a HydroCAD model based on the Atlas 14, 24-hour storms to design the storm sewer conveyance system and ponds. The site is designed to reduce site runoff rates for the 2-, 10-, and 100-year rainfall events. Pre- and Post- Development Discharge Rates (cfs) Condition 2-Year 10-Year 100-Year Pre-Development 18.9 19.2 46.4 46.7 103.4 103.3 Post-Development 13.1 12.5 41.7 35.6 91.9 87.8 Post-development stormwater discharge rates are less than the pre-development rates.