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<br />Page 11 of26 <br />PerInit No: MN RIOOOO 1 <br /> <br />d. Basin outlets must be designed to prevent short-circuiting and the discharge of floating <br />debris. Basin outlets must have energy dissipation. <br /> <br />e. The basin must provide a stabilized emergency overflow to accommodate stonn events in <br />excess of the basin's hydraulic design. <br /> <br />f. Adequate maintenance access must be provided (typically 8 ft. wide) for futnre maintenance <br />of the basin. <br /> <br />2. Infiltration/Filtration <br /> <br />Infiltration/Filtration options include but are not limited to: infiltration basins, infiltration <br />trenches, rainwater gardens, sand filters, organic filters, bioretention areas, enhanced swales, dry <br />storage ponds with underdrain discharge, off-line retention areas and natnral depressions. <br />Infiltration must be used only as appropriate to the site and land uses. Settleable solids, floating <br />materials, oils and grease should be removed from the runoff to the maximum extent practicable <br />before runoff enters the infiltration/filtration system. Filtration systems must have a reasonable <br />chance of achieving approximately 80% removal of total suspended solids. The Permittee(s) <br />must evaluate the impact of constructing an infiltration practice on existing hydrologic featnres <br />(e.g., existing wetlands) and try to maintain pre-existing conditions (e.g., do not breach a <br />perched water table which is supporting a wetland). For a discussion of ground water warnings, <br />design measures, maintenance considerations or other retention, detention, and treatment devices, <br />see the MPCA's Protecting Water Quality in Urban Areas found on the MPCA's web-site. <br /> <br />a. Infiltration systems should not be excavated to final grade until the contributing drainage <br />area has been constructed and fully stabilized. <br /> <br />b. During construction of an infiltration system, rigorous sediment and erosion controls (e.g., <br />diversion benns) should be used to keep sediment and runoff completely away from the <br />infiltration area. The area must be staked off and marked so that heavy construction <br />equipment will not compact the soil in the proposed infiltration area. <br /> <br />c. To prevent clogging of the infiltration or filtration system, a pretreatment device such as a <br />vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) <br />must be used to settle particulates before the storm water discharges into the infiltration or <br />filtration system. <br /> <br />d. Infiltration or filtration systems shall be sufficient to infiltrate or filter a water quality <br />volume of 1/, inch of runoff from the new impervious surfaces created by the project. <br /> <br />e. The water quality volume shall discharge through the soil or filter media in 48 hours or <br />less. Additional flows that cannot be infiltrated or filtered in 48 hours should be routed to <br />bypass the system through a stabilized discharge point. A way to visually verify that the <br />system is operating as designed must be provided. <br /> <br />f. Appropriate on-site testing shall be conducted to ensure a minimum of 3 feet of separation <br />from the seasonally saturated soils (or from bedrock) and the bottom of the proposed <br />infiltration system. Calculations and computer model results that demonstrate the design <br />adequacy of the infiltration system must be included as part ofthe SWPPP. <br /> <br />wq*stn112*51 <br />