2016-04-26_PWETC_AgendaPacket - Laserfiche WebLink

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e) Volume Reduction Calculation: The volume reduction (in cubic feet) provided by surface infiltration practices shall be computed using the following: 1) For sites required to obtain a watershed permit within CRWD, RWMWD, or RCWD jurisdiction, follow Watershed District Rule C. 2) For sites that do not require a watershed district permit due to project size, provide volume calculations based on the following formula: V=Area x 1.1 inch/12 Where V=Required Volume Reduction in cubic feet(cf) A =New or Reconstructed Impervious Area in square feet(sf) f) Storm Sewer Conveyance Design: Local storm sewer systems shall be designed for the 10-year storm event within the crown of pipe. 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. g) Outfall Energy Dissipation: For culvert outlet velocities less than or equal to four (4) feet per second (fps), check shear stress to determine if vegetation or riprap will be adequate. If vegetation is used, temporary erosion control during and immediately following construction shall be used until vegetation becomes established. For velocities greater than four (4) fps, energy dissipaters shall be designed in accordance with Mn/DOT Design Criteria. h) Landlocked Basin HWL Determination: 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 (10.0 inches and saturated or frozen soil conditions [CN=100]) or the runoff resulting from a 100-year back-to-back event (7.4 inches followed by 7.4 inches). The high water elevation shall be the higher of these two conditions. i) Building Low Opening: The lowest floor openings of all buildings shall be set: 1) At least two (2) feet above the 100-year high water elevation and at least one (1) foot above a designated emergency overflow. 2) For landlocked basins, at least two (2) feet above the higher of the elevations determined in Part 4h. j) No Net Loss of Storage Capacity: If encroachments within storm water retention basins are approved by the City Engineer, then calculations indicating the volume of encroachment and plans for volume mitigation must be submitted. 5) Volume Control/Infiltration Practices Design Criteria. a) Infiltration systems are prohibited: 1) Where the bottom of the infiltration basin is less than three (3) feet to bedrock or the seasonally high water table; Page 4 of 8 DRAFT