2102 ROSS AVE 2018-01-02 MF Import

l SITE INFORMATION EXISTING DRAINAGE The site is mounded with about 10 feet of river dredge spoils. The Snohomish River bank bisects the site in a northwest to southeast direction, such that the river bank slope is approximately 17 percent and the slope from top of bank toward Ross Avenue is approximately 3 percent. Drainage can be characterized as sheet now on each side of the bank. If surface water accumulates on the northeast side of the hank it will flow northward to the ditch along Ross Avenue. Stormwater within the ditch will flow in a northwest direction and eventually discharge into the Snohomish River. Due to the sandy nature of the soil on site, a large part of the surface water will infiltrate into the soil before entering the collection system on Ross Avenue. DEVELOPED DRUUNAGE The proposed project involves the construction of a metal fabrication building and office encompassing a gross building area of 33,750 W with associated asphalt paving and landscaping. Due to sites location adjacent to the Snohomish River no detention is proposed. This report will address all Minimum Requirements for New Development or Redevelopment as listed in the Department of Ecology (DOE) 2005 Stormwater Management Manual for Western Washington with the exception of Minimun>Requiremcnt #7 Flow Control. // tI.Q nvrnhet5 w )orrZor add up . 1'hc 9.44 acre site consists of 33,750 square feet (0.77 acre) o area and 109,235 square feet (2.51 acres) of pavement used fr.r parking and storage. The remaim' g 5.34 acres consists of pervious landscaping, buffer enhancement and untouched riverside. The impervious a7e�divided into pollution generating sutiaces and non -pollution generating surfaces. Pollution generating surfacing will be routed to one of several DOE approved Contech Stormwater StormFilters sized to treat a small tributary basin. Additionally, the City of Everett requires pretreatment in the form of a wet vault or any DOE approved pretreatment carrying the GULD rating. Because of elevation constraints, a wet vault was chosen to store 30 percent of the runoff volume from the 6-month, 24-hour storm event, see Appendix for calculations. The project will discharge at the ordinary high water elevation of the existing drainage ditch on Ross Avenue in order to mimic predevelopment hydrology and drainage patterns. The rooftop area is a non -pollution generating surface and will be discharged directly into the bioswale/ditch system along Ross Avenue. See Appendix for WQ Analysis and Design CONVEYANCE SYSTEM wNALYSIS The conveyance system is analyzed using Bentley CulvertMaster program, by Bentley Systems, Inc. The 25-year peak runoff rate for Basins A and F were calculated using StormShed hydrology modeling software by Engenious Systems Inc. and was found to be 0.30cfs. The proposed onsite storm conveyance system is affected by the required fall for the Contech stormfilters (2.3') and the ordinary high water elevation of 9.0' of the bioswale along Ross Avenue. The tailwater elevation is approximately 1 1.5, or the water quality vault water surface and given that the nearest catchbasin rim elevation is 12.5, for Basins A and 17, there is only 1 foot of freeboard. The City of Everett requires a minimum of 0.5 feet of freeboard from the headwater surface and the top of the structure from the 25-year developed storm. This analysis shows that both catchbasins for Basins A and F, have sufficient freeboard for the 25-yr storm event. See Appendix for Culverti\laster and StormShed printouts Drainage Report page 4 McClean Ironworks May 8, 2008 Revised July 8, 2008