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H:\ORNO\C13120357\z_Shoreline Estates\ShorelineEstates_PrePlatReview_2021-1-19.docx January 19, 2021 City of Orono Attn: Jeremy Barnhart 2750 Kelley Parkway Orono, MN 55356 RE: Preliminary Plat Application Shoreline Estates Engineering Review #3 Dear Jeremy: As requested, I have completed an engineering review of the revised documents submitted for the above referenced project. We offer the following observations, comments, and recommendations for your consideration: 1. One rain gardens and soil amendments are proposed for stormwater runoff management to meet regulatory requirements. The following must be considered regarding stormwater management: a. Per City requirements, the proposed stormwater management system must limit proposed peak runoff rates to that of existing for the 1, 10, and 100 year – 24h hour events. Per NPDES MS4 requirements, the system must treat the water quality volume, calculated as 1” times the sum of new and fully reconstructed impervious surface. b. Per the proposed hydrographs submitted, 2.55 acres impervious surface is proposed resulting in a water quality volume of 9,257 c.f. Therefore, if infiltration is utilized, the basin must be expanded to provide this volume below any surface outlet. c. Based on field inspection and a shovel test, site soils did not seem conducive for infiltration. If infiltration is not possible due to site soils, volume control will not be required per NPDES requirements, but a rain garden is still necessary to treat the water quality volume. Also, if tight soils are present, an underdrain must be installed with the basins to ensure complete drawdown between rainfall events. d. Per City requirements to track TMDLs and reduce loads to meet required Waste Load Allocations, the calculations for Total Suspended Solids (TSS) and Total Phosphorus (TP) loading must be provided. e. Facilities necessary for the ultimate development (street construction, sewer, rain garden, etc.) must be installed and the site mass-graded at the same time as street construction to ensure all drainage ways, watersheds, and runoff management systems are constructed per the approved Stormwater Management Plan and are sized adequately for proposed improvements. f. Rain garden must be permanently vegetated prior to issuance of any building permits. g. An impervious surface exhibit must be provided to verify areas match the calculations. The exhibit must indicate existing and proposed impervious surfaces. h. Insufficient space is provided for the swale proposed south of the house pad on Lot 1. Grading must be revised to provide adequate space for a swale or the Proposed Drainage Council Exhibit A Resolution Ex B