1450 80TH ST SW 2018-01-02 MF Import

Existina Conditions: The site has previously been developed as an overflow parking area for the site to the east. The northern portion of the site is paved. A gravel surfaced parking area extends i south from the paved area. Apr •oximately the south one third of the site is covered with I tall grass and brush. A wetland area borders the site on the west with the buffer extending onto the site. A cedar rail fence was erected along the buffer boundary as part of the previous development. The fence will remain to delineate and protect the wetland and buffer. A drainage system consisting of an ecology block wall detention/treatment pond, control structure and collection/conveyance system was also constructed on the site. Flows from the site are conveyed to the public drainage system in 80TH Street S.W. Slopes on the site range from 2 to 20% with the steeper slopes being associated mostly with the wetland and buffer areas. ' There may be minimal flows entering the site from the landscaped area bordering the site to the east. There are no flows entering the site from the north, south or west. For I drainage calculations, the existing basin will consist of the development area of 2.39 acres. This area excludes the wetland and buffer areas to be undisturbed. This site is I subject to re -development standards per the city of Everett. Therefore, the existing conditions will be considered as second -growth forest with a CN value of 81. The storm intensities used to determine storm water runoff rates, can be found in Table 1. Table 1. Storm Water Intensities Storm Intensity Inches/Hour 2-year, 24 hour storm event 1.55 10- ear, 24 hour storm event 2.25 100 ear, 24 hour storm event 3.25 The existing soil on site is Alderwood Urban Land Complex, with a Hydrologic Group "C rating. The existing time of concentration for the historical basin, as shown on Attachment 2, was determined as follows: 120 If sheet flow through trees/brush S = 0.040 100 If shallow flow through trees/brush S = 0.040 Total Existing Tc= 28.72 min. I The storm water runoff rates for the historical basin can be found in Table 2 - Historical Peak Storm water Flows and the Waterworks calculations can be found in Appendix A — Basin Summaries. IM.1SDG12001177-SDG-011docs0ralnege Analysis77.doc 4 C./