1019 PACIFIC AVE 2025-07-28 - Laserfiche WebLink

Developed Condition Event Output: WWHM3 Peak Flow (cfs) Area (ac) 2-yr 0.03 0.14 10-yr 0.05 0.14 100-yr 0.085 0.14 A. PERFORMANCE STANDARDS The City of Everett 2010 Stormwater Management Manual as well as the DOE 2005 Stormwater Management Manual for Western Washington were used in the design of the stormwater mitigation plan for this project. C. FLOW CONTROL SYSTEM By utilizing on-site stormwater management in the form of a swale with check dams this project will increase the peak 100-year release rate over the existing conditions by approximately 0.02 cfs. Therefore no further flow control is necessary since the interim policy requires that the project be designed in such a manner to not increase the Combine Sewer Overflows and/or basement flooding in the combined sewer area. This increase in the flow is equivalent to less than 9 gpm only during the 100-year storm event. A change in the peak flow rates from this site of that scale cannot be considered significant enough to increase the combined sewer overflows or potential for basement flooding in this basin. D. WATER QUALITY SYSTEM Since this project is in the combined storm/sewer basin treatment of the runoff will be provided at the COE treatment plant and this project is exempt from runoff treatment per interim Public Works Policy#2-11. E. CONVEYANCE SYSTEM ANALYSIS Maximum Flows from on-site developed area: For this project the entire 0.14 acres of impervious was analyzed for the peak 100-year flow rate to ensure the system as designed could handle the expected flows. Using the WWMH3 software a peak 100-year flow rate of 0.085 cfs was calculated. Using Manning's Equation the minimum conveyance capacity of the new storm pipe as designed is 0.6 cfs which is four times greater than the flows from the entire basin. Therefore, this system has more than adequate capacity for the expected flows. PSKC Nassau 12-0411 July 2012 Page-13