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Anderson Daycare October 19, 2016 professional wetland delineators. MSA set a "worst-case scenario"setback line by assuming that the wetland boundary is one foot above the toe of slope on the western edge of the site. This line also approximates the 10-yr flood elevation of the adjacent Yahara River. This line is depicted on Sheet ST.2 of the attached construction plans. MSA is currently working with WDNR staff to either gain concurrence with this worst-case assumption or perform a wetland delineation (potentially in Spring of 2017). As depicted on Sheet ST.2, the redeveloped impervious areas will all lie within the footprint of existing impervious surfaces. Further, there will be a significant reduction in impervious surface area lying within the 75 ft. setback from 12,550 sq. ft. in the existing condition to 6,930 sq. ft. in the redeveloped condition. The redevelopment project will reduce the square footage of impervious area lying within the 75 ft. wetland setback regardless of where the wetland boundary is set. Post-Construction Stormwater Management Analysis Water Quality Treatment Grassed swales and a biofilter will be constructed to provide water quality treatment. Because no soil borings have been advanced for the project, infiltration rates are not depended on to provide water quality treatment. The biofilter will have approximately 350 sq. ft. of floor area and a 9-inch depth of live storage. Runoff directed into the biofilter will pass through a two ft. depth of engineered fill before reaching a tile underdrain. MSA constructed a WinSLAMM model to assess the level of water quality treatment. The vegetated swales and biofilter treat the runoff from the western watershed to a 69.8% performance level. Including the untreated eastern watershed, the modeling results indicate a TSS capture rate of 48.9% for the redeveloped site in aggregate. Because infiltration rates for the vegetated swales were modeled as zero inches per hour, the anticipated water quality treatment performance level will be greater. The WinSLAMM modeling data is attached. Peak Discharge Peak discharge rates will be less than in the existing condition for any storm event due to the significant reduction in impervious area. MSA prepared a HydroCAD model of the site to analyze shear stress within the vegetated swales and the grassed overflow weir of the biofilter, the output of which is attached to this memorandum. Infiltration Although the site is exempt from infiltration requirements, the redeveloped site will passively infiltrate more runoff than in the existing condition due to the significant reduction in impervious area. Infiltration will be further enhanced by the installation of the stormwater management features. Page 3 of 6 18226000_memo_101916