2220 GIBSON PL 2020-01-21 - Laserfiche WebLink

, L , Kirk Brooks From: Erik Emerson Sent: Wednesday,July 19, 2017 11:41 AM To: Kirk Brooks Cc: Heather Griffin Subject: Hummer SFR drainage review comments. Kirk, I have reviewed the Drainage Report and submitted plans for the Hummer Single Family Residence (2220 Gibson PI)and have the following comments: 1. The project must comply with the most current version of the City of Everett's Stormwater Management Manual (SWMM) and Design and Development Standards(DCSS).The City currently uses the 2014 Ecology Stormwater Management Manual for Western Washington,with modifications and additions contained in the DCSS,as the SWMM. 2. Section 6-On-site Stormwater Management references a Project handout for selection of BMP options. I am unaware of any such handout for the City of Everett.The analysis which follows appears consistent with the List #2 approach contained in the SWMM Volume 1 Section 2.5.6. 3. Section 4-4.4 of the DCSS modifies the thresholds contained in the SWMM for flow control. Based on the impervious areas listed in the drainage report flow control is required for this project.Section 4-4.4 of the DCSS reads as follows: 4-4.4 FLOW CONTROL Projects which are required to meet Minimum Requirement#7 shall provide flow control facilities in full compliance with the requirements of the Surface Water Management Manual. Flow control BMP design is described in detail in the Stormwater Management Manual.Additional requirements for specific types of BMPs are included in the following subsections. In accordance with Section 14.28 of the City of Everett Municipal Code the threshold requirements contained in Section 2.5.7 of the Stormwater Management Manual are replaced with the following: The following circumstances require achievement of the standard flow Control requirement for western Washington: • Projects in which the total of effective impervious surfaces is 5,000 square feet or more in a threshold discharge area, or • Projects that convert%acres or more of vegetation to lawn or landscape, or convert 2.5 acres or more of native vegetation to pasture in a threshold discharge area, and from which there is a surface discharge in a natural or man-made conveyance system from the site, or • Projects that through a combination of effective hard surfaces and converted vegetation areas cause a 0.15 cubic feet per second increase in the 100-year flow frequency from a threshold discharge area as estimated using the Western Washington Hydrology Model or other approved model using 15-minute time steps. The determination of the amount of increase in flow shall be based on the site conditions at the time of application and the anticipated runoff at the completion of the proposed improvements. All other requirements of Section 2.5.7 of the Stormwater Management Manual are applied as given. 1 J,)