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<br />H:\ORNO\C13120357\z_Shoreline Estates\ShorelineEstates_PrePlatReview_2020-08-03.docx <br /> <br />August 3, 2020 <br /> <br />City of Orono <br />Attn: Jeremy Barnhart <br />2750 Kelley Parkway <br />Orono, MN 55356 <br /> <br />RE: Preliminary Plat Application <br /> Shoreline Estates <br /> Engineering Review #1 <br /> <br />Dear Jeremy: <br /> <br />As requested, I have completed an engineering review of the documents submitted for the above <br />referenced project. We offer the following observations, comments, and recommendations for your <br />consideration: <br /> <br /> <br />1. Five rain gardens are proposed for stormwater runoff management to meet regulatory <br />requirements. The following must be considered regarding stormwater management: <br />a. Per City requirements, the proposed stormwater management system must limit proposed <br />peak runoff rates to that of existing for the 1, 10, and 100 year – 24h hour events. In <br />addition, per NPDES MS4 requirements, the system must limit proposed discharges of <br />runoff volume, Total Suspended Solids, and Phosphorus on an annual average basis to that <br />of existing conditions as well. <br />b. Based on field inspection and a shovel test, site soils are likely not conducive for <br />infiltration. If infiltration is not possible due to site soils, volume control will not be <br />required per NPDES requirements, but rain gardens are still necessary to treat the water <br />quality volume and provide pollutant control. Also, due to the tight soils, an underdrain <br />must be installed with the basins to ensure complete drawdown between rainfall events. <br />c. The number of rain gardens should be reduced to consolidate treatment locations and future <br />maintenance. The basin proposed near the entrance does not have adequate vertical <br />separation from the anticipated water table, so it should be eliminated. In addition, the two <br />basins proposed in the rear yards pose access and maintenance issues. <br />d. Facilities necessary for the ultimate development (street construction, driveways, buildings, <br />etc.) should be installed and the site mass-graded at the same time as street construction to <br />ensure all drainage ways, watersheds, and runoff management systems are constructed per <br />the approved Stormwater Management Plan. <br />e. Rain gardens should be permanently vegetated prior to issuance of any building permits. <br />f. Calculations indicate all driveways will be pervious. Since construction and maintenance <br />of pervious pavement by private property owners often results in inadequate treatment and <br />management of stormwater runoff, all driveways must be calculated as impervious surfaces <br />for the purpose of rate control verification. <br />g. A stormwater management plan outlining how the proposed improvements will meet all <br />requirements must be submitted for review prior to Preliminary Plat approval to ensure <br />adequate siting is available for required facilities.