Agenda - Environmental Policy Board - 08/04/2014

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restrictive. In the snowmelt scenario, frozen soils shall be assumed (CN =100). The methodology used must conform to current Lower Rum River Watershed Management Organization (LRRWMO) rules. (c) The applicant shall consider reducing the need for stormwatcr management facilities by incorporating the use of natural topography and land cover such as natural swales and depressions as they exist before without compromising the integrity or quality of the wetland or pond. (Commentary: The sensitivity of a wetland to degradation varies with the type of vegetation. Sedge meadows, open bogs and swamps, coniferous bogs, calcareous fens, low prairies, lowland hardwood swamps, and seasonally flooded basins are highly sensitive to degradation. Floodplain forests, reed canary grass meadows, shallow (reed canary grass, cattail, giant reed or purple loosestrifc) marshes arc only slightly sensitive to degradation. See the current version of the Minnesota Pollution Control Agency's publication "Storm Water and Wetlands: Planning and Evaluation Guidelines for Addressing Potential Impacts of Urban Storm Water and Snow Melt Runoff on Wetlands" for details.) Lowest Floor Elevation -The Lowest floor elevation of all development, including basements, must be at least 3 feet above the highest anticipated groundwater table, 2 feet above the designated or designed 100 -year flood elevation or 1 foot above the emergency overflow, whichever is higher. The LRRWMO rules designate a procedure for modifying these requirements. (d) The following stormwater management practices must be investigated in developing the stormwatcr management part of the stormwater pollution control plan in the following descending order of preference: (1) Protect and preserve as much natural or vegetated arca on the site as possible, minimizing impervious surfaces, and directing runoff to vegetated areas rather than to adjoining streets, storm cowers and ditches; (2) Flow attenuation of treated stormwater by use of open vegetated swales and natural depressions; (3) Stormwatcr wet detention facilities (including percolation facilities); and (1) A combination of successive practices may be used to achieve the applicable minimum control requirements specified in subsection (a) of this section. The applicant shall provide justification for the method selected. Storm water volume management practices must be utilized on all new development sites. The options are: (1) Infiltrating or retaining the first one (1) inch of precipitation over the impervious surface of the site. These practices should seek to utilize pervious areas for stormwater treatment and to infiltrate stormwater runoff from driveways, sidewalks, rooftops, parking lots and landscaped areas to the maximum extent practical to provide treatment for both water quantity and quality. (2) Retain the post construction runoff volume on the site for the 95t percentile storm. These practices shall prevent the off -site discharge of the precipitation from all rainfall events less than or equal to the 95t percentile rainfall event. Discharge volume reduction can be achieved by engineered infiltration, canopy interception, soil amendments, evaporation, rainfall harvesting, and /or evapotranspiration and any combination of the aforementioned practices. (3) Match the pre - development runoff conditions. Design and construct stormwater management practices that preserve the pre - development runoff conditions following construction. The post - construction rate, volume, and duration of discharges must not exceed the pre - project rates and the pre - project hydrograph for the 1 and 2 year storms must be replicated through the site design and other appropriate processes. The practices listed above can be used to achieve this goal. Defensible and consistent hydrological assessments and modeling methods must be used and documented.