Agenda - Council Work Session - 02/23/2021

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For landlocked areas - Available storage volume shall be established by estimating the water surface elevation resulting from back-to-back 100-year frequency rainfall events, or the 100-year, 10-day snowmelt, whichever provides the higher water surface elevation. In the snowmelt scenario, frozen soils shall be assumed (CN=100). Flood levels in landlocked basins are difficult to predict, as they depend upon the starting water elevation at the beginning of a storm event. The starting elevation of the waterbody prior to the runoff event shall be established by one of the following: Existing Ordinary High Water level established by the Minnesota Department of Natural Resources, Annual water balance calculation approved by the LRRWMO, • Local observation well records, as approved by the LRRWMO, or • The elevation of hydric soils, or the highest anticipated ground water table or elevation shown by a geotechnical study accepted by the LRRWMO. Infiltration within the basin during frozen conditions will be allowed in the calculation of basin sizing. The infiltration rate used must be approved by the LRRWMO. Ultimate development, based on the approved common plan of development and the city's future land use plan within the area tributary to the pond/waterbody, shall be assumed. Modeling methodology acceptable to the LRRWMO. Methods currently approved by the LRRWMO include: USDA SCS, TR-55 and TR-20, HydroCAD, and EPA SWMM methodology. Precinitation Data Sources Rainfall amounts for hydrologic analyses should be based on: National Oceanic and Atmospheric Administration's updated Precipitation Frequency Atlas for the Midwestern States (Atlas 14, Volume 8). (Reference: w More recent updates of these documents should be used, if available. Low Floor Elevation The lowest floor elevation of all development, including basements, must be at least 3 feet above the highest anticipated ground water table, 2 feet above the designated or designed 100-year flood elevation, or 1 foot above the emergency overflow, whichever is higher. This requirement may be waived if evidence that a lesser separation can be achieved is: • submitted and certified by a geotechnical engineer, • reviewed and approved by the city engineer or other party designated by the city engineer, and • approved by the LRRWMO. If this process is pursued, the developer should consult with the LRRWMO Engineer before beginning study to discuss acceptable methodologies. Discharaes to the Rum River The Rum River is classified as an Outstanding Resource Value Water. State Rules 7050.0180 prohibit new or expanded discharges. Determinations about discharges that may or may not impact Comprehensive Watershed Management Plan Lower Rum River Watershed Management Organization October 2011 111 lu) L Li A 131 Li E-4