5105 CHINOOK DR 2016-01-01 MF Import

; . , 3. OFFSITE ANALYSIS AND MITIGATION This site proposes to construct approximately 3,917 sf of new impervious surfaces. The following table summarizes the new PGIS and NPGIS proposed both on and off-site for this project. PGIS NPGIS Total (sf) (sf� sf _ Existing 0 0 0 Proposed 1 ,207 2,710 3,917 _ Replaced o 0 0 _ New 1,207 2,710 --— 3,917 _ The new impervio�s surfaces will be mitigated using on-site stortnwaler management BMPs iricluding two level spreaders, soil management and driveway transition zones. A. UPSTREAM ANALYSIS During the site visil it was observed tha; ttie only runoff entering ihis site is from the south in the low area or along lhe east side of the southern property line. As a result none of these oHsite (lows will be aHected by lhe proposed improvements on Ihis site. B. DOWNSTREAM ANALYSIS Runoff from the proposed disturbance area of the existing site sheet flows over the northern or eastern property lines. The runoH is collected in a low "draw" that drains lo the north. This low area drains north for approximately 200-feet where it is intercepted and collected by a long driveway. The runoH on the driveway then flows east over the asphalt surfacing and is coilected by a slotted drai� approximately 50-feet to the east. This drain appears to be connected to Ihe piped conveyance system within Ocean Ave to ihe east. However, if this s�otted drain were to become plugged and overflow the runoft would pass over the landscaping on the property to the north and eventually be collected by lhe same drainage system within Ocean Ave The Ocean Ave. drainage system conveys the runoH b the north and combines with Ihe conveyance system within Wesl Mukilteo Blvd. The downslream system was lost at ihis pomt. During the sile visit it was not clear d the conveyance syslem directed the ru�off norih or east However, the drainaqe system m this area has been long established and maintained to accommodate the area's runoff So whether the runoN is conveyed to Powder Mill Gulch Creek pnor to entering Puget Sound or takes another route the downsiream system is assumed lo be capable of handhng Ihe very slight increase of runofi expected due lo this pro�ect. Based on the tad that the new NPGIS and PGIS runoff will be dispersed with a minimum flow palh of 25' before the runoH leaves the site and that this project only proposes lo increase lhe total impervious sur(aces on the site by less than 4,000 sf the developed flows will be slightly higher lhan lhe existing conditions for ihis project a�d ihe offsite drainage syslems in the downstream path for this site are large enough and accommodate such a large existing basin, any minor change in ihe flow rale irom this project wiU not Chinook SFR .. .. -- —.. . 12-013 � Jan 2012 Page 5 �f/7 / (y J(-s \�