10602 16TH AVE SE 2025-07-28 - Laserfiche WebLink

2. EXISTING CONDITIONS SUMMARY The project site covers approximately 3.11 acres. The proposed disturbance area of the existing site contains second growth forest. No structures have been observed on-site via aerial photos. Therefore, all of the proposed impervious surfaces will be considered new NPGIS or PGIS. The proposed improvements will convert the 3.11 acres of existing forest to roof, pavement or landscaping. The overall slope of the site is to the west and east with a high point near the middle of the proposed improvements area. The slope is flat to the west and averages approximately 2 percent. The slopes to the east vary from flat near the high point to moderate as you approach 16th Ave SE. There are some existing landscaping areas south of this project near the middle of the southern property line that drain onto this site. These areas total approximately 8,300 sf and will be collected and detained as part of the drainage design to off-set the bypass areas of the developed site, including a portion of the entrance road and fire land along with some landscaping, that cannot be collected. This project will not affect any offsite flows from passing through the site as in the existing condition. Runoff to the west will be maintained by dispersing flows from the developed site along the east edge of the critical area buffer. Figures 2 and 3 show the three on-site basin areas along with the off-site tributary areas and the developed bypass areas. It was found using the DOE stormwater modeling software that the peak discharge from the bypass area was only 0.02 cfs greater then the off-site tributary area to be collected. Therefore, by assuming the existing land cover for the off-site area is to be only landscaping, but it includes some hard surfaced paths as well, and the bypass area is a combination of pavement and landscaping the collection and detention of the off-site area is an effective off-site to the on-site bypass areas that cannot be collected. The USDA Web-Soil Survey was used to determine the existing soil conditions for the site. Refer to Figure 4. The soils are mapped as Alderwood-Urban Land Complex. The general site slopes on the site range from 2-10%. Based on the existing soil information and the setbacks of the proposed improvements from any of the property lines or critical areas there is little potential for erosion or sedimentation during construction, assuming the basic BMPs included in the SWPPP are installed and maintained properly. Refer to Figure 2 for an existing basin map. Northiake Court Townhomes 13-0609 May,2014 Page 5