08-24-11 Council Agenda - Laserfiche WebLink

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• Water quality goals — Non -point source pollution reduction BMPs shall achieve 90% total suspended solids removal from the runoff generated by a NURP water quality storm (2.5" rainfall). IV. Proposed Development The proposed drainage system will capture all stormwater runoff on -site for water quality, rate, and volume measures. To provide treatment for the project, a filtration basin will be constructed in the north center of the site capturing the western half of the project. The remaining eastern half of the site, including the bus loading zone, will be directed to an underground storm sewer system (see Appendix A) and provide additional rate control and water quality treatment. The drainage pattern will stay roughly the same with the exception of 1.3 acres diverted east to the unnamed wetland per discussions with RWMWD and Capital Region Watershed Districts. Effects due to the increased drainage are anticipated to be minimal; this is under review by Capital Region Watershed District. The project's primary outfali will be the unnamed wetland to the east. A smaller area currently draining to the T.H. 36 will be incorporated into the site stormwater system. Site runoff will not discharge to the T.H. 36 ditch system. To meet the Ramsey - Washington Metro Watershed District ( RWMWD) requirements for stormwater management, the site will have pretreatment areas for snow removal These include a 15 feet bench /swale between the north retaining wall and parking area, east basin adjacent to the bus turnaround, and a 20 feet wide pretreatment area adjacent to the central filtration basin. A variance from the 50' average wetland buffer is required. Because of constraints from internal bus movements, ramp clear zones, access points, and grades, some elements of the project construction will be within the 50' buffer zone. Based on Watershed District comments from the 50% plans dated July 20, 2011, additional measures have been undertaken to further reduce buffer impacts, these include; increase slopes to 2.5:1, additional retaining wall, relocated stormsewer outfalls, and modified pond storage area. The soils and contaminants found on the site limit infiltration practices. Therefore the project follows the alternative compliance sequencing approach per RMMMD rules. Based on the watershed district volume reduction spreadsheet, the required infiltration for the project is 7,840 CF (based on 104,500 SF of proposed impervious area). Hwy 36 and Rice Street Park and Ride Storm Water Narrative — 9096 Review Set 8/5/2011 32 Page 1 4