5830 EVERGREEN WAY 2016-01-01 MF Import

i I ( • . Housing Hope 3hort Plat Drainege Analysis Page 2 I IExisting Runaff Patterns ' Due to Pigeon Croek, the site is divided inro two diainage sub-basins (see Figuro A). I__ Basin A (Southwest region including Lots 1-3)contains b.39 ac:es which drain to the northeas4 I . The area draining in Basin A travels as sheet flow and dischazges into Pigeon C¢ek. Adjacent lots � on the southwest side of Basin A have slopes which appear to direct some of its flow towards Basin A (sec Figure�B), fBasin B (Northeast region including L.ot 4)contains 0.82 acres which drain to the west. The area draining in Basin B initially travets as shept flow from Evergreen Way and appears to become shallow concentrated as it discharges on the northwest side. Drainage from adjacent lots on the nonh side of Basin B appear to not enter the basin and flow parallel to tl�e north property 3ine. ICurtently, Pigeon �reek,serves as a stonn drainage system and originates from a reservoir on the ( east side of Evergreen Way and discharges into Possession Sound. The creek enters the property on the soudieast side and exits on the northwest cornec Existing 60-inch and 24-inch culverts outfall into the creek on tlie sautlieast side of the propeny and midway through the site an existing { 42-inch culvert d'u•ects the flow under an old driveway. The creek exits the property througli two, 48-inch concrete cu(verts under Fleming Street. ,. , _. , , _ , , „ � Pigeon Creek consisis of earth lined sides ranging from near vertical to 1:1 slopes. The bottom of � the creek is lined with cobbles,concrete<md rock. The width of the creek varies between 6 feet and 10 feet and the gredient is approximately 2.94 pe��:ent. 'l Tl�e City oi Bve��ett has modeled the expected peak tlows in the creek. Tl;e 25-year and 100-year flows are 170 cfs and 207 cfs, respectively. Given tliese flows, the estimated maximum depdi of � the creek is 3.83 feet(see calculations in Appendix). The side slopes of the channel appear to be stable and of adequate capaciry to convey the design sto�m without overtopping or tlooding I adjacent a��eas. During large storni events, Uiere may be some flooding but diis overtopping should � not affect a�iy of die buiidings or ac�jacent propeiYies in the vicinity. 'fhe design of the detendon Ifaciliry on site would provide attenuati�n of sromi ivnoff up to the 100 year retum Crequency, I , .