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O& MLENK Real People. Real Solutions. January 9, 2018 City of Orono Attn: Jeremy Barnhart 2750 Kelley Parkway Orono, MN 55356 RE: Preliminary Plat Application 18-3999 Southways Addition Engineering Review # 1 Dear Jeremy: Resolution Exhibit C 6835 2638 Shadow Lane Suite 200 Chaska, MN 55318-1172 Ph: (9521 448-8838 Fax: 19521 448-8805 Balton-Menk.com As requested, I have completed an engineering review of the documents submitted for the above referenced project. In addition, Adam Edwards has provided continents for inclusion with this review. We offer the following observations, comments, and recommendations for your consideration: 1.0 GENERAL: 1.1 Stormwater management for the site to meet regulatory requirements is proposed through use of wet retention basins. The following should be considered regarding the proposed system: a. Volume control and water quality measures must be provided as well to meet NPDES requirements. Plans should be revised to include volume reduction equal to 1" of runoff from the new impervious surface. If infiltration is considered infeasible, other methods of volume reduction must be considered. It appears that there are sufficient areas high enough above the water table to provide localized filtration practices or amended soils. Also, the overall estimate of Total Suspended Solids (TSS) and Total Phosphorus (TP) from proposed improvements must be reduced from pre -project conditions. Calculations detailing volume control and water quality compliance must be submitted for review and approval. If the applicant pursues a design with less than the normally required infiltration volume, sequencing of design to provide treatment to maximum extent practical must be provided. b. An Outlet Control Structure (OCS) that provides skimming must be installed on the wet retention ponds in order to keep floatables in ponds. The following should be considered regarding OCS design: i. The inlet pipe from the pond to the OCS should be submerged with back slope from the OCS to the pond to facilitate maintenance. The normal water level should be controlled by a weir in the structure or the outlet pipe. The inlet pipe should be one nominal size larger than the outlet pipe to limit flow control and turbulence at the pond interface. ii. A sloped rim should be utilized, per City Standard Details, with the bottom of the rim placed at the predicted 100 year high water level. iii. A trashguard should be installed on the outlet pipe. c. Wet retention basins must provide a minimum 1800 cubic feet per acre of drainage area. The stage -storage curves for permanent pools must be submitted for review to verify A:iiORNO\C13115486�x-18.3999 Brackets Falnt\5nuttmys-PrePl R"iew-i 2019A1-09.docx Bolton S Menk is an