Agenda - Planning Commission - 08/07/2014

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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 AV consider reducing the need for z%r ivrata nian cment facilities by in3orponating the use of natant Ipobtap4iy and land cover suer oforaaturul vnalcs and depressiorra az they exist before development to the degree that they can accommodate the additional flow of treated (e.g., settled) water without eoinproni�zirr the integrity or quality of the wetland or pond. (Commentary: The arzzitivity-g a wetland to degradation warier vrith the type of veg tatian. Ledge ma dov rz, oven bogs and vnaiirs coniferous bogs, calcareous fens, low p'&ries, lowland hardwood m7ae pz, and seasonally flooded b,2irrz-are highly sensitive to degradation. Floodplain forests, reed canary g z. owz, obaRow (reed canary grass, era/ tail, g ait Y loosestrife) marshes arc only slightly sensitive to degradation. See the current version of the Minnesota Pollution Control Agency's publication "Storm Water and Wetland Ilkxxing and Eialuation Guidelines for Addressing PotontkLl Impacts of Urban Storm Water ird grrwr MJt Runaff 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 reauirements. (d) The hlbwrirg ziormwata nian emcnt practicer,. iazi he investigated in developing the z%rnivrataf managemAt part of the stormwatx pokkition eont1 i11 Arm in the following descending order of preference: (1) Protect and preserve as muA natant or vegetated arca on the site az-eta✓.Je, niininiiirr i xvioav-Arfaces, and diroo irr 1un3ff to vegetated areas rather than adk3inirg Ara -ski, z173 PM sewers and ditches; (2) Flow att3 Cation of treated stormwater by use of open vegetated vnah and natara11 depressions; (3 f%tiwater wet detention facilities (in„±id rig pafetation facilities); and (1) A Son%✓.nation of successive practices may be used to achieve the applicable minimum control requirements specified in vaheection (a) of this section. The applicant taR 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 aualitv. (2) Retain the post construction runoff volume on the site for the 95"' percentile storm. These practices shall prevent the off -site discharge of the precipitation from all rainfall events less than or equal to the 95th 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-proiect rates and the pre-proiect hvdrograph 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.