CCP 01-21-2003 - Laserfiche WebLink

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I I. I I I I I I I Ie I I I I I I I. I I . The permanent pool for wet ponds should have a volume equal to or greater than the runoff resulting from a 2.5-inch rainstorm under complete watershed development. (RCWD Rules, adopted August 12, 1998, and LSMP) . Storm water runoff rates for the proposed project must not exceed existing runoff rates for the critical one-year or two-year and 100-year frequency events. (RCWD Rules) . While not a requirement, the RCWD promotes the use of infiltration devices capable of infiltrating the impervious surface runoff from the Minneapolis-St. Paul median storm in 72 hours. (RCWD Rules) The drainage patterns proposed for the expanded campus are similar to the eXlstmg condition, but provide for storm water treatment prior to discharging to any wetland, wherever feasible. Small diversions of water from an existing drainage area may result in order to provide treatment for changes in other parts of the campus. The quantity of storm water runoff will likely increase in some drainage areas due to the increased impervious surfaces proposed for these areas, however, there will be an overall decrease in impervious surface area within the AUAR study area compared to existing conditions. Guidant continues to coordinate with the Rice Creek Watershed District (RCWD) to develop mitigation measures for treating stonn water runoff associated with the proposed development. These measures will include several surface water treatment strategies for each identificd drainage area undergoing site plan changes. The strategies proposed are based on the best available infonnation, but will require further study and discussion with the Watershed District prior to final design. During the design phase of this project, soil and groundwater data will be required to ensure that appropriate treatment areas and Best Management Practice (BMP) designs are selected. To the extent possible, storm water runoff from new impervious areas will be directed into infiltration basins and wet ponds prior to leaving the site. Infiltration basins provide mitigation for the increase in runotT volume by infiltrating storm water and provide water quality treatment by filtering particulate matter from the runoff. Wet ponds provide water quality treatment by settling out sediment and attached nutrients and by skimming floating debris and provide rate control for the water leaving the site. Design of the treatment deviccs will be in accordance with the City and Watershed District water quality guidelines. Design of the outlets for the treatment devices will be in accordance with the Watershed District rate control guidelines. Table 6 summarizes the existing and proposed impervious surface and proposed treatment strategies for each drainage area shown in Figure 10. As can be scen in Table 6, three of the six individual sub-watersheds show an incrcasc in total impervious area, however; overall there is approximately a 2.2 acre decrease in impervious area within the six sub- watersheds. Also by comparison to the existing condition, the proposed condition will reduce pavement surfaces (parking lot and roadway surfaces) by 7.2 acres which is substantial in terms and levels of pollutants associated with pavement versus rooftop runoff. GuiJant Campus Master Plan Final AUAR and Mitigation Plan January 2003 - 22-