9311 18TH AVE W A,B 2018-01-02 MF Import

u 1f I. Project O��cn�ic�s The proposed project is a 0.85 acre site located at 9327 18'�' Ave. W in Gverett, Washington. The proposed project consists of developing the site into four 2-wiit condominiums with associated driveways, s[orm drainage and utility features. One single Family residence, one garage and one shed exists on site. These structures are proposed to be removed. Vegetation on the site consists of grass, trees .wd shrubs. A weUanel and type 5 stream that is a tribulary of Swamp Creek exists :dong the eastern portion of the property. It appears that downed trees tangled with wedand plants and ve�etation contribute to strrim flow restrictions that result in the wedand area. The developed site conditions will incluJed approximately 7,500 sq. feet of pavement and frontagc improvements. Pervious arcas are modeled with a CN of 86 (grass). Runoff will be collected in catch basins and trai�sported through pipes to three undergrowid detention pipes (GO inches in diameter) located under the driveway are�i in the western portion of the site. Draivage from these pipes discharges to CI3 2 in the center of tlie site. Prom hare, drainage (lows to the cast through a 36" pipe designed to provide three feet of dead storage. Additional de�d storage of two f'eet is provided in the catch basins. Drainage from the pipe continues through the conU�ol structiue and will be treated through a Storm Pilter manhole by Stornnvater 360 (formerly Storm Water M.�nagement Inc.). (See Drainaae Plan). Upon release from the Storm �ilter drainaoe will enter a 12" CPP and dischnrge through a level spreader to thc exisling wedancl to Ihe east. Ali ioof drains and impervious areas will contribute to the detentiou pipes. [I. 1'reliminary Conditions Two drainage basins exist on si�c. Basin A is the smaller ef the t��o basins. It exists along ihe western portion of Ihe property and runs nortlnvesterly. Some bypass from 18"' Avc. and the driveway will help preserve dr;iinage to this basin Basin I3 is or ihe castern portion of tlie site and it drains toward the east, and heads south through a wetland stream tl�ait is a tributary to Swamp Creek. The vast majority of the sitc wiil drain to this basin under devcloped, detained and tmated conditions. llI Ot�'-Sitc Analysis [3asin A Upstream drainage from [3asin A appears to be limited ro runofl'that sheet Flows over 18°i Ave. W and heads north. Some runoff from the western portion of the property sheet ilo�ti�s townrd Ihe east side of 18'�' Ave. W where it is picked up in closed drainage system. Downsu�eam: 1=i�om the nortlnvestern portion of the property, drainage flows nortl»vest downhill toward 18"' Ave. W. It travels north in a 12" pipe nlong the eastern side of the roadway. Then drainage t;avels west under 18'�' Ave. W. Upon exiting the culvert,