Agenda - Public Works Committee - 06/17/2014

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Sec 117.384-410 Revised June 5, 2014 to include 2013 Construction Stormwater Permit, 2013 MS4 Permit requirements and 2012 LRRWMO Third Generation Plan [deletion;,, additions] (Code 1978, § 9.27.10; Ord. No. 06-32, § 1, 11-28-2006) Sec. 117-392. - Permanent stormwater pollution controls. (a) The applicant shall install, construct, or pay the city fees for all stormwater management facilities necessary to manage increased runoff from development.se that the discharge rates from stormwater troatmont !xtc nc for the predevelopment ten_„ear and 100 year peak storm discharge rates are not increased. Hydrologic and hydraulic design calculations must be submitted for the pre -development and post - development conditions for the 2, 10, and 100 year events as well as the 10 day snow melt event. Such calculations shall include: (i) description of the design storm freauencv, intensity and duration, (ii) time of concentration, (iii) Soil Curve Numbers or runoff coefficients, (iv) peak runoff rates and total runoff volumes for each watershed area, (v) infiltration rates, where applicable, (vi) culvert capacities, (vii) flow velocities, (viii) data on the increase in rate and volume of runoff for the design storms used, and (ix) documentation of sources for all computation methods and field test results. These predevelopment rates shall be based on the last ten years of how that land was used. Accelerated channel erosion must not occur as a result of the proposed land disturbing or development activity. An applicant may also make an in -kind or a monetary contribution to the development and maintenance of community stormwater management facilities designed to serve multiple land disturbing and development activities undertaken by one or more persons, including the applicant. (b) Sizing of stormwater basins shall be based upon: (1) For basins that are not land locked — Available storage volume shall be based on 100-vear return period (7.1 inch Atlas 14, Anoka County). 24hr duration, SCS Type II storm with average soil moisture conditions (AMC 2) (2) For land locked areas — Available storage volume shall be established by estimating the surface elevation resulting from a 100 vear/ 10-dav runoff or 100 vear/10-dav snowmelt, whichever is more 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) 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-vear flood elevation or 1 foot above the emeraencv overflow, whichever is higher. The LRRWMO rules designate a procedure for modifying these requirements. (d) 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 95th 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. Ramsey, Minnesota, Code of Ordinances Page 7