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Mr. Michael Grochala <br />April 8, 2020 <br />Page 4 <br /> <br /> <br />K:\015144-000\Admin\Docs\AUAR - Update 2020\Stormwater\MEMO - 040920 - Stormwater AP.docx <br />The NPDES MS4 permit requires permittees to provide post-construction water quality <br />standards adopted at the local level. The NPDES MS4 Permit is currently drafted and out <br />for public review (available for comment through January 11, 2020). The MS4 permit will <br />require permittees to meet the requirements of future TMDLs. Currently there is a TMDL <br />Implementation Plan proposed for discharges to Peltier Lake and Hardwood Creek, they <br />are identified as the Peltier Lake and Centerville Lake TMDL and the Hardwood Creek <br />Impaired Biota (Fish) and Dissolved Oxygen TMDL. Depending on the location and <br />proximity to impaired waters, development within the AUAR area may need to complete <br />an anti-degradation analysis for the impaired water. <br /> <br />Water Quantity and Quality Analysis <br />A water quantity and quality analysis was completed for the existing and proposed conditions <br />within the study area. This quantitative analysis uses the NRCS runoff curve number method to <br />calculate runoff and the results are summarized in Table 1 and Table 2. <br /> <br />Table 1 summarizes the total runoff volumes for each development scenario compared to the <br />existing condition. <br /> <br />Table 1. Existing and Proposed Annual Runoff Volumes1 <br /> <br />Existing <br />conditions <br />(AC-FT) <br />2020 <br />Scenario <br />1 w/o <br />Volume <br />Reduction <br />(AC-FT) <br />2020 <br />Scenario 1 <br />with <br />Volume <br />Reduction2 <br />(AC-FT) <br />2020 <br />Scenario <br />2 w/o <br />Volume <br />Reduction <br />(AC-FT) <br />2020 <br />Scenario 2 <br />with <br />Volume <br />Reduction2 <br />(AC-FT) <br />2020 <br />Comp <br />Plan <br />(2030) w/o <br />Volume <br />Reduction <br />(AC-FT) <br />2020 <br />Comp Plan <br />(2030) with <br />Volume <br />Reduction2 <br />(AC-FT) <br />AUAR <br />Area 11,201 11,363 3,326 11,381 3,328 11,383 4,678 <br />1Annual runoff volumes are based on an average of 32 inches of rainfall for the state of <br />Minnesota. <br />21.1 inches represents approximately 90% of all rain events in Minnesota (Minnesota Minimal <br />Impact Design Standards, MPCA), therefore volume reduction of 90% was assumed in all <br />land use areas for proposed scenarios, except Permanent Rural, Urban Reserve and Right-of <br />Way, which are assumed to not require any future stormwater management. <br /> <br />Table 2 summarizes the total pollutant loads for each development scenario compared to the <br />existing condition. <br /> <br />Table 2. Total Suspended Solids and Total Phosphorus Annual Loads <br /> <br />Pollutant1 Existing <br />conditions <br />2020 <br />Scenario <br />1 w/o <br />Volume <br />Reduction <br />2020 <br />Scenario <br />1 with <br />Volume <br />Reduction <br />2020 <br />Scenario <br />2 w/o <br />Volume <br />Reduction <br />2020 <br />Scenario <br />2 with <br />Volume <br />Reduction <br />2020 <br />Scenario <br />2 w/o <br />Volume <br />Reduction <br />2020 <br />Scenario <br />2 with <br />Volume <br />Reduction <br />AUAR <br />Area <br /> <br />TSS <br />(tons/yr) 906 946 299 957 301 925 372 <br />TP <br />(lbs/yr) 8,522 8,407 3,059 8,573 3,076 8,344 4,679 <br />1Pollutant loading was determined using Event Mean Concentration values from the MPCA Stormwater <br />Manual, based on Land Use classification for the AUAR area. <br />