3429 15TH ST 2018-02-23 - Laserfiche WebLink

_ Detention facility depth includes one foot of freeboard and one foot of sediment storage for a total depth of 7.1 feet. The facility was modeled with vertical side slopes, as it will be built with a retaining wall to reduce pond footprint. The final pond footprint modeled matches the average surface area of the designed detention pond. The primary outlet consists of a flow control structure with a riser pipe designed using WWHM2012. Three orifices are set at increasing elevations along the riser pipe to maintain steady discharge and meet flow control requirements. An emergency overflow spillway provides redundancy through a secondary outlet designed to allow overflow of the post-developed 100-year flow at a depth of 4-inches. OverFlow from the spillway would flow into the existing ditch on the east end of the property. Calculations supporting the design of the spillway may be found in Appendix <<E„ Maintenance access is provided to the pond from the parking lot to the west end of the facility. The 12-foot wide access ramp will be constructed of gravel pavement at a 15 . percent grade. The W1NHM2012 printout documenting the pond design may be found in Appendix "E". Conclusions This report presents the methodology and standards used to design the storm water system conveyance and pre-treatment systems for the BNSF Everett Yard Office project. Minimum design standards are the City of Everett Stormwater Managment Manual (2010) and the 2012 Western Washington Hydrology Model. Information contained in this report shows that proposed stormwater improvements have been designed per City of Everett requirements. March 05, 2015 � Hydrology Report — BNSF Everett Yard Office Page 8 of 8