2115 55TH ST SE 2018-12-31 - Laserfiche WebLink

0"-4" Topsoil 4"— 12"Red/brown Silty Sand 12"—36"Grey Brown Medium Sand A simple percolation test gave an infiltration rate of 2 minutes per inch There is not adequate space for full dispersion. There is no storm drainage in the area so a perforated pipe connection is not practical. Erosion Control can be provided by a construction entrance and silt fence around the south and west sides of the property. Section 3 Site Development The site was visited on March 1, 2017. The downstream drainage system was field verified and the drainage patterns and ground cover were observed. The site consists of a vacant lot with grass, brush, ivy and few trees. The site slopes down to the northwest at 20%to 40%. To the north and east is the Pacific NW Traction Company ROW and a substation. To the South is Lowel Drive,which has no drainage, curbs or gutters and slopes down to the east. There is a SFR to the south which does not appear to drain any significant amount of water onto the property. To the west is the gravel alley that does not appear to drain onto the site. The site sloped down to the northeast. The site will be developed with a new 1,838sf Single Family Residence and a 688sf driveway for a total impervious area of 2,726sf or 24.56%. See site plan. A quarter mile downstream inspection was performed. The inspection concluded no visual impacts or potential problems will occur from the development of the subject site. The downstream course consists of a steep (40% )wooded hillside and then the flat grassed Pacific Northwest Traction Company ROW. The driveway and roof downspouts will be routed to catch basins which will drain into 21' x 2'x2' and 56'x2'x2'gravel filled infiltration trenches. A catch basin in the driveway will collect runoff from the driveway which will drain to the 21'x2'x2' infiltration trench in the front yard. This is the only practical method of storm water control as the site is not large enough for dispersion and the slopes are too great and there is no storm drainage in the area to connect to. Section 4 Stormwater Pollution Prevention Plan The site does not require any special stormwater pollution prevention. See SPPP. Section 5 Source Control of Pollution Due to the small size of this project and residential nature of this project no significant amount of Pollution will be generated. The roof downspouts will be tightlined to a catch basin with a downturned elbow which will drain to a 21' x 2' x 2' infiltration trench. Section 6 Special Reports and Studies No special reports or studies are available on this site. The SCS Soil Map lists the soils as Alderwood (5). A soil hole dug by Anstey Engineering on March 1, 2017 found the soil to be a medium sand. Therefore the Stormwater can be infiltrated at a rate of 301f of 2x2' trench per 1,000sf per the DOE manual. 2 AE2017-27