04-15-2019 Planning Commission Packet

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BO LTO N 2638 Shadow Lane Suite 200 f , A & M E N K Chaska, MN 55318-1172 Real People. Real Solutions. Ph: (952)448-8838 Fax: (952)448-8805 Bolton-Menk.com April 8, 2019 City of Orono Attn:Jeremy Barnhart 2750 Kelley Parkway Orono, MN 55356 RE: Preliminary Plat Application LA19-000022 Willow Bay Sanctuary Engineering Review#1 Dear Jeremy: As requested, I have completed an engineering review of the documents submitted for the above referenced project. We offer the following observations, comments, and recommendations for your consideration: 1. It is unclear what is proposed for stormwater runoff management to meet regulatory requirements. The Preliminary Grading Plan depicts a dry retention pond on the north side of the proposed street. The Preliminary Erosion Control and SWPP Plan depicts a wet retention pond on the north side of the proposed street and rain gardens on Lots 2, 4, 5, and 6. The hydraulic calculations indicate a wet retention pond with a filter bench and the rain gardens will be used to meet requirements. The following should be considered regarding stormwater management: a. The Stormwater Management Report and Preliminary Plans should be coordinated to indicate the same stormwater management system. The system must limit proposed peak runoff rates to that of existing for the 1, 10, and 100 year—24h hour events. The system must limit proposed discharges of runoff volume, Total Suspended Solids, and Phosphorus to that of existing conditions as well. Since more than one acre of new impervious surface is proposed, the system must include volume reduction equal to 1" of runoff from the new impervious surface per NPDES requirements. The stormwater management plan should be revised as necessary to meet all requirements and resubmitted for review prior to Preliminary Plat approval to ensure adequate siting is available for required facilities. b. Storm sewer and inlets are located on Brown Road near the proposed intersection, but these facilities are not indicated on the plans. The Storm Water Management Plan should be revised to include these accordingly. c. All proposed culverts should be included in the hydraulic calculations to verify adequate sizing, except the driveway culverts for Lots 1,4, 5, and 7. 2. Facilities necessary to meet regulatory requirements should be installed and the site mass- graded at the same time as street construction to ensure all drainage ways and watersheds are constructed per the approved Stormwater Management Plan. If done piece-meal, coordination of required stormwater management facilities can be difficult, and facilities required may be forgotten during future construction. 3. Long Lake is impaired for nutrients, and the City of Orono has been assigned a load reduction of 125 lbs/yr. The City should consider reviewing potential of maximizing stormwater runoff H:\ORNO\C13117856\z_LA19-000022 Willow Bay Sanctuary\WillowBaySanctuary_PrePlatReview_2019-04-08.docx Bolton&Menk is an equal opportunity employer.