Agenda - Council Work Session - 02/23/2021

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the Rum River are made by the Minnesota Pollution Control Agency, and shall be addressed through MPCA regulatory processes. Water ual tv Treatment of storm water to MPCA Stormwater guidelines is required prior to stormwater discharge to a lake, stream, or wetland and prior to discharge from the site as part of development. The stormwater treatment facilities shall provide at least an annual removal efficiency of sixty (60%) phosphorus and at least an annual removal efficiency of ninety percent (90%) total suspended solids. The onsite abstraction of runoff may be included in demonstrating compliance with the total suspended solids and total phosphorus removal requirements. Peak Flow Rate Co Post -development peak runoff rates shall not exceed existing rates for the 2, 10, and 100 year storm peak discharges. The Rational Method shall be the preferred methodology to calculate peak flow rates for the design of minor systems that do not require hydrograph routings. If a minor system requires the use of a hydrograph method for routing purposes, only methods pre -approved by the LRRWMO will be accepted. If the method is not currently approved by the City, documentation of the methodology used shall be submitted with the calculations. Hydrograph methods currently approved by the LRRWMO include: USDA SCS, TR-55 and TR-20, HydroCAD, and EPA SWMM. Volume Control A volume equal to one inch of runoff from all impervious surfaces on the site shall be infiltrated on - site. In cases of redevelopment, this volume control requirement applies only if >50% of the project area is disturbed. Project area is defined as the parcel(s) to which the permit would apply or the area encompassed by the common plan of development, whichever is greater. At a minimum, abstraction from 1-inch of runoff from the on -site impervious surfaces shall be provided. Abstraction, through infiltration, shall not be allowed within a 1-year travel zone of a public well as determined by the municipal well -head protection plan. Unless determined by the LRRWMO to be exempt or granted a waiver, the following shall be addressed for stormwater management of all sites: 1. Stormwater volume management practices shall attempt the equivalent of retaining 1-inch of runoff, while recognizing that meeting a full 1-inch may not be practical. 2. These practices should utilize pervious area for the retention of runoff from impermeable surfaces to the maximum extent practical to provide treatment for both water quantity and quality. Use of an existing regional treatment facility with available treatment capacity for the required volume will be allowed as an exception to the infiltration requirements if the LRRWMO and the city where the project is occurring agree the facility has the capacity. For all ponds, infiltration through the pond bottom, draining dry within 48 hours, does count toward the required volume to infiltrate. Dead storage alone, without infiltration of that stored volume, does not count toward the required infiltration volume. Evaporation or transpiration from pond surfaces Comprehensive Watershed Management Plan Lower Rum River Watershed Management Organization October 2011 (Re Ihir S p r 00 1 8 ,'"1V E-5