CCP 09-30-2002 - Laserfiche WebLink

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. . . . 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. The mitigation measures for treating storm water runoff associated with the proposed development include several surface water treatment strategies for each identified drainage area undergoing site plan changes. The strategies proposed are based on the best available information, 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. [The Rice Creek Watershed district received the storm water treatment strategies, outlined in this AUAR, in mid-September for their review and comment.] 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 runoff 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 devices 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 12. As can be seen in Table 6, while three individual sub-watersheds show an increase in total impervious area, the overall amount of impervious acreage will decrease by approximately 2.2 acres. In particular, the amount of pavement (parking lot and roadway surfaces) will decrease by roughly 7,2 acres, which is significant in terms of the types and levels of pollutants associated with pavement versus rooftop runoff. Guidant Campus Master Plan Draft ADAR - 22- September 2002