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H:\ORNO\_Surface Water Management\MS4 Administration\MCM-5_Post-Construction Stormwater Management\2_Site Plan Reviews\Long Bridge\Long <br />Bridge_PrePlatReview_2021-03-01.docx <br />March 1, 2021 <br />City of Orono <br />Attn: Jeremy Barnhart <br />2750 Kelley Parkway <br />Orono, MN 55356 <br />RE: Preliminary Plat Application <br />Long Bridge <br />Engineering Review #1 <br />Dear Jeremy: <br />As requested, I have completed an engineering review of the revised documents submitted for the above <br />referenced project. We offer the following observations, comments, and recommendations for your <br />consideration: <br />1. Four dry retention basins 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. Per <br />NPDES requirements, the system must also treat the water quality volume, calculated as 1” <br />times the sum of new and fully reconstructed impervious surface. If infiltration is <br />prohibited as outlined by NPDES requirements, a wet sedimentation basin or filtration <br />basin may be utilized. <br />b. Based on suspected soil conditions and the submitted calculations, site soils do not seem <br />conducive for infiltration. If infiltration is not possible due to site soils, volume control will <br />not be required per NPDES requirements, but the water quality volume must still be <br />treated. A permanent volume must be provided below the outlets for each basin to treat this <br />volume. If using filtration, an underdrain must be installed with the basins to ensure <br />complete drawdown between rainfall events. In addition, filtration basins should be located <br />a minimum 3’ above the water table to ensure proper functionality. <br />c. Per City requirements to track TMDLs and reduce loads to meet required Waste Load <br />Allocations, the calculations for Total Suspended Solids (TSS) and Total Phosphorus (TP) <br />loading must be provided. <br />d. Facilities necessary for the ultimate development (street construction, sewer, basins, etc.) <br />must be installed and the site mass-graded at the same time as street construction to ensure <br />all drainage ways, watersheds, and runoff management systems are constructed per the <br />approved Stormwater Management Plan and are sized adequately for proposed <br />improvements. <br />e. An 8’ wide bench must be provided around each basin to provide access to the outlet and <br />for basin maintenance. <br />f. Basins must be permanently vegetated prior to issuance of any building permits. <br />g. Native vegetation should be utilized in basins and wetland buffers. <br />LA21-21 <br />Council Exhibit F