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<br /> <br /> <br /> <br />Civil Site Group, PC • 4931 West 35th Street • Suite 200 • St. Louis Park • Minnesota • 55416 <br />www.CivilSiteGroup.com • (612) 615-0060 <br /> <br />Water Quality – Stormwater water quality treatment volume must be provided equal to 1.0” over all new <br />impervious surfaces (includes all newly constructed impervious surfaces only, re-constructed impervious <br />surfaces are not included). <br />Volume Control – Must consider volume reduction if feasible and not prohibited on site. The required <br />infiltration volume is equal to the water quality volume described above. <br /> <br />3.0 Stormwater Calculations: <br /> <br />3.1 Proposed Stormwater Management Strategy & Facilities Description <br />This project is greater than one acre. Infiltration is not feasible on this site due to past uses which have <br />resulted in contaminated soils. A lined stormwater filtration system will be used to provide water quality <br />treatment and volume abstraction for 1.1” of runoff from the site’s impervious surface. Live storage within the <br />filtration system will provide rate control. <br /> <br />3.2 Rate Control <br />Rate control for the 1-year, 2-year, 10-year and 100-year storms with the Atlas-14 rainfall distribution is <br />provided by the filtration system. See the table below and the full drainage calculation summary in the <br />appendix. <br /> <br />Overall Stormwater Rate Summary <br />1-Year Event 5.91 1.17 <br />2-Year Event 6.93 1.43 <br />10-Year Event 10.72 2.99 <br />100-Year Event 19.07 8.42 <br />Existing Conditions <br />Rate (cfs) <br />Proposed Conditions <br />Rate (cfs) <br /> <br />3.3 Water Quality <br />Water quality is provided by the sand filter in the filtration system which drains by 6” perforated drain tile at the <br />base of the filter. A lined pretreatment row of the proposed Prinsco HS75 chamber system provides <br />pretreatment. <br /> <br />3.4 Volume Control <br />1.1” of volume abstraction is provided to satisfy the 60% TP and 90% TSS removal requirements. See the <br />table below showing the required and provided volumes. <br />New Impv. Area (sf)Required Volume (cf) <br />PR1 51989 4766 <br />PR2A 2107 193 <br />TOTAL 54096 4959 <br />Inf. Area (sf)Assoc. Inf. Height (ft)Drawdown Time (h) <br />Infiltration Basin 1 5684 7252 0.78 5.88 <br />TOTAL 5684 <br />Proposed BMP Area Provided Vol (cf)Drawdown Time Calculations (1.6"/Hour) <br />Required Infiltration Vol. Summary Infiltration Volume = <br />1.1"*Dist. Impv. Area <br />Drainage Area <br /> <br /> <br />4.0 Conclusions: <br /> <br />To the best of our knowledge, this project meets all State, City and Watershed District stormwater <br />management requirements. <br /> <br />