Ordinance No. 131 Consolidated Land Use Ordinance

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3. Water Quality. Stormwater management must comply with the requirements of the Minnesota Pollution Control Agency's NPDES/SDS Construction Stormwater General Permit MNR100001 (CSW Permit) requirements and guidelines set forth in relevant total maximum daily load studies and other local, state, and/or federal regulations. No direct (untreated) discharges of stormwater to natural or improved waterbodies are allowed. 4. Waste Disposal to Waters. Stormwater management must not result in the discharge of any regulated substance, hazardous or biological waste, or petroleum product, whether treated or untreated, to best management practice devices that may have a deleterious effect upon water of the state (surface and groundwater), unless the discharge is in compliance with Federal, State and local regulations. 3. Criteria. Stormwater management plans shall comply with the following criteria: 1. All hydrologic data shall be completed using NRCS methodology; i.e. HydroCAD or TR20/TR55, XP- SWMM or a comparable, City approved method. Hydraulic calculations will be accepted in the rational method format or in commonly used software packages such as FHWA HY-8, or XP-SWMM or a comparable, City approved method. These computations shall be submitted to the City as outlined in this section with additional detail provided upon request. 2. Water quality calculations will be accepted from commonly used software packages such as MIDS calculator, P8 (with a standard NURP 50th percentile particle size distribution), or a comparable, City approved method. 3. Local storm sewer systems shall be designed for the 10-year storm event. The Rational Method shall be the preferred methodology for the design of local systems. Culvert crossings or storm systems in County or State right-of-way may have a design frequency which differs from the City's 10-year design storm. The designer shall contact each agency/unit of government to determine the appropriate design frequency for hydrologically -connected systems. 4. Energy dissipation shall be provided at culvert and storm sewer outfalls in accordance with the Minnesota Department of Transportation Drainage Manual. If stable vegetation is acceptable, temporary erosion control during and immediately following construction shall be used until vegetation becomes established. 5. High water elevations for landlocked areas (basins where no outlet exists) shall be established by first estimating the normal or initial water surface elevation at the beginning of a rainfall or runoff event using a documented water budget, evidence of mottled soil, and/or an established ordinary high-water level. The high-water level analysis shall be based on runoff volume resulting from a 100-year/10-day runoff (7.2 inches and saturated or frozen soil conditions [CN=100]) and/or the runoff resulting from a 100-year back- to-back event. The high-water elevation shall be the higher of these two conditions. 6. All permanent stormwater management facilities and new discharge points to all wetlands and surface waters shall include pretreatment. 7. Design of all Best Management Practices (BMPs) will be consistent with the Minnesota Stormwater Manual, 2005, and as amended and the MPCA CSW Permit. 8. Stormwater facilities must provide: 1. An identified overflow spillway and downstream route sufficiently stabilized to convey a 100- year critical storm event; 2. Outlets designed to prevent short circuiting of the flow from the facility inputs to the outlet; 3. An outlet skimmer to prevent migration of floatables and oils for the 2-year event; and 4. Access for future maintenance that is free of plantings and impediments. 9. Regional ponds and practices can be used to provide for stormwater management based on the following criteria: 1. Regional ponds are required to be designed based on ultimate conditions for the contributing subwatershed; and Updated January 2025 — Gem Lake Consolidated Land Use Ordinance No. 131 111