9929 EVERGREEN WAY HORIZON BANK 2018-01-01 MF Import

i t Introduction: - This drainage analysis examines existing and developed storm runoff rates for the proposed commercial development located in the southwest quarter of Section 13, j Township 28 North, Range 4 East, in the city of Everett, Washingrton. The site address is 9939 Evergreen 1Nay. The 1.08-acre site is bordered by 100 " Street S.W. on the south, Evergreen Way on the west, 4T" Avenue W and an apartment complex on the � east and commercial development to the north. See Attachment 1- Vicinity Map. 1 The following evaluation will consider the existing and developed storm water runoff conditions. Storm water runoff rates were calculated using the SBUH Method, as computed by the Stormshed software by Engenious Systems. Existinq Conditions: The site is currently occupied by Mutual Materials and has been developed for approximately 50 years. The entire site is covered with buildings and pavement, basically 100% impervious coverage. The site slopes to the south and east with very flat slopes. Runoff from the paved area adjacent to Evergreen Way sheetflows into the roadway drainage system. Runoff from paved storage area north of 100T" Street sheetflows south towards 100T" Street where there is a low area with no defined drainage outlet. Flows from the eastern portion of the site are collected in a catch basin and routed to the drainage system in 4T" Avenue. There does not appear to be any offsite flows entering the site. For drainage calculations, the existing basin will be considered 100% impervious as indicated in the City's pre-application comments for this project. The storm intensities used to determine storm water runoff rates, can be found in Table 1. Table 1. Storm Water Intensities Storm 2-year 24 hour storm f 10-year, 24 hour storm 100-year, 24 hour storm 6-month, 24 hour storm 1.55 2.25 The existing soil on site is Alderwood Urban Land Complex, with a Hydrologic Group "C" rating. The existing time of concentration for the existing basin, as shown on Attachment 2, was determined as follows: �M:1Stlg12J041259�SDG-04 - FSI - Horizon Bank in EverettlDRqINIDrainage Analysis-259.doc � 3