08-11-11 Planning Comm. Agenda - Laserfiche WebLink

Home Browse Search

e 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 portion of the site. The remaining eastern portion 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; the primary outfall 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 slope from the parking lot to the central basin. The soils and contaminants found on the site limit infiltration practices on the site. Therefore the project follows the alternative compliance sequencing approach per RMWMD rules. Based on the watershed district volume reduction spreadsheet, the required infiltration for the project is 8,175 CF (based on 109,000 SF of proposed impervious area). Using filtration BMPs, and the 70% filtration credit, the volume of runoff requiring filtration increases to 11,808 CF. The underground filtration system consists of chambers and angular stone. In addition, to meet the District requirements, filtration is provided by adding 18 inches of clean washed concrete sand, 6 inches of stone, and an under drain (see Appendix A). The concrete sand filtration layer design is consistent with the Minnesota Stormwater Manual. The Central Pond provides NURP stormwater treatment for 1.2 acres of parking lot runoff. Additional BMPs above and beyond the RWMWD permit program requirements are a consideration that Metro Transit would like to pursue under the RWMWD Cost Share Program. This includes porous concrete sidewalk and tree trenches at the bus stop area. Hwy .36 and Rice Street Park and Ride Storm Water Narrative — 50% Review Set 7/20/2011 Page 1 4