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Description of Drainage Basins: (existing and developed basin maps are attached, <br /> Figures 2 &3). The development area lies along the west side of Seahurst Ave. The <br /> eastern portion of the project site near the existing house drains to the north and east with <br /> the remainder of the site flowing west. The new imperious surfaces in the eastern portion <br /> of the site will be fully infiltrated and the existing pervious conditions will not be affected so <br /> there will be no change or a reduction in flows to the north. The portion of the site flowing <br /> west also has a significant amount of new impervious surfaces being infiltrated as part of <br /> this project so the net result to the downstream system will also be a reduction in the peak <br /> flows. The runoff from both portions of the site eventually drain into and are tributary to <br /> Glenwood Creek. <br /> The site is irregular in shape and composed of one lot with an existing SFR and detached <br /> garage along with landscaping. <br /> Schematic of Proposed Drainage System: (See Figure 3) <br /> Downstream drainage analysis or summary: (See Figure 5) Runoff from Basin C which <br /> includes all of the dispersed flows from this project flow west and leave the site along the <br /> western property line. The total peak flows from this basin has actually been reduced by <br /> 0.02 cfs in the 100-year condition. After leaving the site as sheet flow the flows continue <br /> west and are eventually collected with in a joint driveway drainage system approximately <br /> • 150-feet from the site. The slopes in the area west of this project are steep and densely <br /> vegetated. After the flows are collected they are conveyed within a pipe conveyance <br /> system further west and into an existing stormwater pond near the intersection of <br /> Glenwood Ave and Mukilteo Blvd. <br /> Once leaving the pond the runoff enters the conveyance system within the road R.O.W.for <br /> Glenwood Ave and then flows into the Mukilteo Blvd system via catch basins and pipes. <br /> After entering the Mukilteo Blvd R.O.W. the flow turn west and then north to the north side <br /> of the roadway and eventually the conveyance system discharges into Glenwood Creek <br /> along the north side of Mukilteo Blvd. Due to the depth of the ravine that contains <br /> Glenwood Creek the piped outfall was not located during the site visit. Glenwood Creek <br /> flows north for approximately 700-feet before passing under the rail road and into Puget <br /> Sound. <br /> Please note that in the case of any overflow for either infiltration system proposed on this <br /> site the downstream flow path would be the same as Basin C above. <br /> Thompson Short Plat 11-012 <br /> May 2011 Page 5 <br />