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Stormwater Narrative/Summary December 5, 2021 <br />Tanager Creek <br />Orono, MN <br />3 | P a g e <br />Methodology <br />HydroCAD <br />The hydrologic characteristics of the site were modeled using HydroCAD software. TR55/TR20 methods <br />were utilized. Existing and proposed drainage areas were determined based on existing survey and <br />proposed grading. <br />The 2, 10, & 100-year frequency events where analyzed for peak runoff rate control in the existing and <br />proposed conditions. The MSE Type III- 24 hr distribution was used in analysis. Runoff from pervious and <br />impervious surfaces were calculated separately to more accurately model the runoff volume from the site <br />surfaces. For our purposes, only the portion of the property that will be modified as a result of this project <br />has been modeled. Results of this analysis are summarized below, and a report can be seen in Exhibit <br />D: HydroCAD Report. <br />Filtration Basin <br />The project proposes to use a shallow filtration basin to control and treat stormwater runoff. The proposed <br />basin is situated to most efficiently gather a majority of runoff from impervious surfaces prior to discharging <br />to wetland area. <br />The proposed filtration basin will provide 12” of water retention prior to outletting via earthen weir. The <br />bottom of the basin is proposed to be lined with MPCA Filtration Mix C and includes a 6” perforated <br />underdrain. <br />Phosphorus Control <br />Minnehaha Creek Watershed District requires control phosphorus within stormwater runoff. For <br />redevelopment projects, the requirement is considered met if the proposed conditions meet the <br />requirements of the Volume Control regulations. See below for more information. <br />Rate Control <br />Minnehaha Creek Watershed District requires post-development conditions to maintain pre-development <br />peak runoff rates. The table below summarizes the pre- and post-development peak rates for the <br />property: <br />Storm Event Pre Development <br />Rate (CFS) <br />Post Development <br />Rate (CFS) <br />2yr – 2.86 2.39 1.31 <br />10yr – 4.25 5.91 4.90 <br />100yr – 7.28 15.20 13.21 <br />Volume Control <br />Minnehaha Creek Watershed District requires the retention and infiltration of 1.0” of stormwater runoff <br />from new impervious surfaces, where feasible. Due to restrictive on-site soils, the project has proposed a