2202 BROADWAY DUTCH BROS COFFEE 2017-04-05

Dutch Bros. Coffee Stormwater Site Plan Everett, Washington Table 5— Developed Peak Flow Rates Without Flow Control Event Flow Rate (cfs) 2-year 0.058 10-year 0.098 100-year 0.158 The project site is considered one basin as all on-site stormwater conveys to the detention cell and flow control structure at the northeast corner of the site. Without flow control facilities, the developed peak flow rates exceed the existing rates; therefore,.the detention cell and flow control structure must be sized accordingly in order to match the existing peak discharge rates. The flow control structure properties are based on the StormShed 3G modeling outputs for the basin in order to match up to the 2-year, 10-year, and 100-year storm events. D. Stormwater Flow Control Plan The project proposes to create more than 5,000 square feet of new impervious area. Per Minimum Requirement #7, the project will need to provide flow control for stormwater runoff. Typically, this would require peak runoff control such that the 2-year developed peak rate matches 50% of the existing 2-year peak rate, while the 10-year and 100-year peak rates match as well. However, per the City of Everett Public Works Policy#2-11 for Stormwater Control in the Combined Sewer Area, the project site is simply required to match peak flows for the 2-year, 10-year, and 100-year storm events, thereby ensuring that the resulting stormwater flow does not increase Combined Sewer Overflows (CSOs) in the combined sewer area. Detention Cell The proposed flow control system consists of a single detention cell with a flow control structure located at the northeast corner of the site. The flow control system was sized in order to determine the required storage capacity for the detention cell. As stated earlier, the detention cell and flow control structure were sized to match the existing discharge rates for the 2-year, 10-year, and 100-year storm events. Table 6 summarizes the modeled detention cell's properties: Table 6— Detention Cell Properties Required Provided Provided Bottom of Top of Pond Detention Detention Volume Bottom Area (sf) Pond Elevation Elevation Volume (cf) (cf) 300 340 137 87.0' 88.5' DA CI Anon 17-...,, 1')