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Page 11 of 26 Permit No: IVIN R 100001 d. Basin outlets must be designed to prevent short-circuiting and the discharge of floating debris. Basin outlets must have energy dissipation. e. The basin must provide a stabilized emergency overflow to accommodate storm events in excess of the basin's hydraulic design. f. Adequate maintenance access must be provided (typically 8 ft. wide) for future maintenance of the basin. 2. Infiltration/Filtration Infiltration/Filtration options include but are not limited to: infiltration basins, infiltration trenches, rainwater gardens, sand filters, organic filters, bioretention areas, enhanced swales, dry storage ponds with underdrain discharge, off-line retention areas and natural depressions. Infiltration must be used only as appropriate to the site and land uses. Settleable solids, floating materials, oils and grease should be removed from the runoff to the maximum extent practicable before runoff enters the infiltration/filtration system. Filtration systems must have a reasonable chance of achieving approximately 80% removal of total suspended solids. The Permittee(s) must evaluate the impact of constructing an infiltration practice on existing hydrologic features (e.g., existing wetlands) and try to maintain pre-existing conditions (e.g., do not breach a perched water table which is supporting a wetland). For a discussion of ground water warnings, design measures, maintenance considerations or other retention, detention, and treatment devices, see the MPCA's Protecting Water Quality in Urban Areas found on the MPCA's web -site. a. Infiltration systems should not be excavated to final grade until the contributing drainage area has been constructed and fully stabilized. b. During construction of an infiltration system, rigorous sediment and erosion controls (e.g., diversion berms) should be used to keep sediment and runoff completely away from the infiltration area. The area must be staked off and marked so that heavy construction equipment will not compact the soil in the proposed infiltration area. c. To prevent clogging of the infiltration or filtration system, a pretreatment device such as a vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) must be used to settle particulates before the storm water discharges into the infiltration or filtration system. d. Infiltration or filtration systems shall be sufficient to infiltrate or filter a water quality volume of '/2 inch of runoff from the new impervious surfaces created by the project. e. The water quality volume shall discharge through the soil or filter media in 48 hours or less. Additional flows that cannot be infiltrated or filtered in 48 hours should be routed to bypass the system through a stabilized discharge point. A way to visually verify that the system is operating as designed must be provided. f. Appropriate on-site testing shall be conducted to ensure a minimum of 3 feet of separation from the seasonally saturated soils (or from bedrock) and the bottom of the proposed infiltration system. Calculations and computer model results that demonstrate the design adequacy of the infiltration system must be included as part of the SWPPP. Wq-strrn2-5 I