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<br />H:\ORNO\C13120357\z_Orchard Hills\OrchardHills_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 /> Orchard Hill <br /> Engineering Review #2 <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. An infiltration basin is 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. Proposed plans only include stormwater management facilities necessary to meet <br />governmental requirements for street improvements. No facilities are indicated for <br />potential lot improvements, so it is assumed the determination of location, type, and design <br />of management facilities for each lot would be the responsibility of the lot owner. When <br />done piece-meal, coordination of required stormwater management facilities is difficult, <br />and facilities required may be forgotten during future construction. Therefore, facilities <br />necessary for the ultimate development (street construction, driveways, buildings, etc.) <br />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 />c. Based on field inspection and a shovel test in location of proposed basin, site soils are <br />likely not conducive for infiltration. A soil boring or infiltration test in the location of the <br />proposed basin must be provided to confirm infiltration is possible. If infiltration is not <br />possible due to site soils, volume control will not be required per NPDES requirements, but <br />a retention pond or filtration system will be necessary to treat the water quality volume and <br />provide pollutant control. <br />d. The outlet pipe from the basin must be directed at a 45 degree angle to the creek to limit <br />erosion potential in the opposite bank. <br />e. 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.