6.2. SR 11-19-2007 - Laserfiche WebLink

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TABLE 2 SUMMARY OF MODELED LOADINGS GENERATED IN EROS TP Load' (Ibs/yr) TSS Load i (Ibs/ r} ": Runoff Volume :! (ac-ft/ r} Baseline Condition 1,230 440,600 1,050 Present Condition 1,920 678,600 1,740 2020 Condition w/o infiltration 2,230 772,700 2,000 2020 Condition w/infiltration 450 54,100 400 The results presented in the table above are based on the assumption that 80% volume reduction implies 80% phosphorus reduction (though greater than 80% removal likely occurs) and 93% 755 reduction. Application of the 0.5-inch infiltration requirement is adequate to meet Baseline conditions far all three parameters based on this example. It is also possible to consider the decrease in load by a runoff depth comparison between agricultural land use and urban residential land use. 0.14 Rv Agriculture * 31.5 inches average annual precipitation = 4.4 inches/year 0.21 Rv Urban residential * 31.5 inches average annual precipitation = 6.6 inches/year 6.6 inches/year' 20%= 1.3 inches/year This simple calculation shows that the runoff depth for urban residential with implementation of the infiltration requirement is considerably less than the runoff depth generated with agricultural land use. City staff and the City Council will consider whether an adjustment is needed to the infiltration requirement to meet nondegradation requirements. Structural and non-structural BMPs developed as a part of BMP summary sheet 5a-1 should be designed to meet the infiltration requirement. To address the need for a citywide infiltration requirement, the City of Elk River proposes BMP 5d-1, implementation of the citywide infiltration ordinance. IMPLEMENTATION OF BEST MANAGEMENT PRACTICES WITH PUBLIC PROJECTS Another means of attaining nondegradation compliance is for the City to begin retrofitting BMPs to areas through routine and periodic public improvement projects such as street reconstruction, park and other public facilities projects. In the case of street reconstruction projects very often these are mill and overlay and not full reconstruction projects. Nonetheless, some opportunities might present themselves for retrofitting different BMPs depending on the specifics of each project. A potential annual load reduction would be speculative if offered now. Rather, the BMP summary sheet for this SWPPP modification emphasizes adaptive management through experience and reporting through the annual reporting required by the permit. To address the need for retrofitting BMPs, the City of Elk River proposes BMP Sf-1, implementation of BMPs with public projects. Elk River 1482-07001 Nondegradatlon Report Page 5