3535 RIVERFRONT BLVD 2023-02-21 - Laserfiche WebLink

I 5. WATER QUALITY TREATMENT ' Water quality treatment facilities were designed by taking into account the possible future expansion of the proposed parking area. The proposed driveway and the ultimate parking lot configuration constitute to about 36,539.77 square feet(0.84 acres)of pollution-generating hard surfaces within the site. Other hard surfaces Isuch as the trail and the concrete pad that will include the restroom structure and picnic canopy are not considered pollution-generating. I The project area discharges to the Snohomish River,which is a Basic Treatment receiving water body. Bioretention, which is approved for enhanced treatment by the Department of Ecology,was chosen as the preferred water quality treatment option for this project. Therefore,the type of treatment provided for runoff from the pollution-generating surfaces on this project is more than what is required. Approximately 0.80 acres of surface runoff from the driveway and parking lot sheet flows into four bioretention cells. Refer to Appendix B for the Bioretention Tributary Areas exhibit. Since the soils are unstable and do not have good infiltrating I capacity,the bioretention cells have been designed with underdrains to collect and convey the treated runoff. The runoff from the remaining pollution-generating surfaces will be collected via catch basins located at low points of the surfaces and treated by catch basin filter cartridges. Refer to Appendix B for the north and south road Preliminary Stormfilter Sizing and MGSFlood Calculations. The water quality design flow rate when detention facilities are not required is the rate at or below which 91 percent of the 24-hour runoff volume will be treated as estimated by an approved continuous runoff model using the 15-minute time step. The continuous runoff model chosen for this project was MGS Flood. See Appendix C for MGS Flood model results. I 6. CONVEYANCE ANALYSIS 1 Conveyance Analysis was prepared for the piping system as follows: The conveyance capacity was checked for the pipe segment downstream of the catch basin 5 where most of the treated runoff from the impervious surfaces drain to;the reach of pipe downstream of catch basin 10,which collects and transports stormwater I runoff from the playground and the future building areas; and the segment of pipe downstream of catch basin 11,which collects drainage from upstream pipes, trench systems, and a conveyance trail side ditch and routed to SDMH#10008,which discharges via the city's storm ouffall.Also included in the conveyance capacity calculations are the pipe segments downstream of catch basin 7, 8, 8A, and 9. 7. SPECIAL REPORTS AND/OR STUDIES The following reports were prepared as part of this project: • Final Geotechnical Report(HWA GeoSciences Inc., September, 2017) • Biological Assessment and Habitat Management Plan (ESA, 2009) I • Everett Riverfront Public Amenities Railroad Corridor Trail Wetland and Stream Determination Report (ESA, 2012) • Everett Riverfront Public Amenities Railroad Corridor Trail Wetland Mitigation Plan (ESA, 2012) KPFF Consulting Engineers 4