4620 SEAHURST AVE 2020-05-20 - Laserfiche WebLink

6. OFFSITE DRAINAGE ANALYSIS - UPSTREAM From field observation and review of the available aerial topography, it appears that the only location, which could contribute offsite flows onto this site, is from the property to the. south. However, based on the site improvements on this property along with the site grades very little runoff is expected to enter this site from that property. Refer to figure 5 for map of the surrounding area. 7. OFFSITE DRAINAGE ANALYSIS - DOWNSTREAM The project lies in a basin that is tributary to Glenwood Creek. From a field investigation by J. Smeby in February 2011 and from field topographic mapping it was determined that in the existing condition there is one on-site basin. This basin mostly slopes west with some areas sloping north. The majority of the site runoff leaves the site to the west and any flows leaving along the north property line will eventually be recombined with the western flows at the intersection of Glenwood Ave and Mukilteo Blvd. The westerly flows drain over the western property line and are collected by a conveyance system approximately 150-feet west of the site. The slopes in this area are steep and densely vegetated. After the flows are collected they are conveyed within a pipe conveyance system further west and into an existing stormwater pond near the intersection of Glenwood Ave and Mukilteo Blvd. This system of pipes and catch basins collect runoff from the adjacent lots and a steep joint driveway. There were no signs of any erosion or capacity problems in this area. Once leaving the pond the runoff enters the conveyance system within the road R.O.W. for Glenwood Ave and then flows into the Mukilteo Blvd system via catch basins and pipes. After entering the Mukilteo Blvd R.O.W.the flow turn west and then north to the north side of the roadway and eventually the conveyance system discharges into Glenwood Creek along the north side of Mukilteo Blvd. Due to the depth of the ravine that contains Glenwood Creek the piped outfall was not located during the site visit. Glenwood Creek flows north for approximately 700-feet before passing under the rail road and into Puget Sound. Please note that in the case of any overflow for either infiltration system proposed on this site the downstream flow path would be the same as Basin C above. During the site visit no signs of excessive erosion or lack of capacity issues were observed in the downstream path. Refer to figure 5 for a map of the drainage systems in surrounding area. Also refer to Appendix C for photos of the downstream system. Thompson Short Plat 11-012 May 2011 Page 12