5901 23RD DR W SEAWAY CORP 3C 2025-07-29

I Immediately south of the site though, catchbasins and stormsewer pipes on 23rd Drive W and 60thStreet SW pick up drainage and direct it to Powder Mill Gulch where it is released. This creates a barrier to offsite drainage entering the site on the southwest half. As can be seen on Figure 2, most of the area south of the site and east of 23`d Drive W drains to Narbeck Creek and is "buffer" which will not be developed. Therefore, drainage can only enter the site on the south property line in the area between 23rd Drive W and the basin boundary line, which is about 250 feet of frontage. III. Off-Site Analysis Drainage flowing off the west side of the property enters into Powder Mill Gulch which flows to the north/northwest. Adjacent to the site and downstream, Powder Mill Gulch is a defined channel in an incised ravine. The ravine is at least 20 feet deep and 100 feet wide. Powder Mill Gulch flows under 25th Avenue W in a 25" wide arched pipe. Powder Mill Gulch eventually flows into Puget Sound. Drainage flowing off the north side of the property either sheetfiows onto the lots adjacent to the site or is picked up by two stormsewer stubs provided to the subject site. All drainage exiting the north side of the property ends up in the detention facility for the adjacent residential plat. Drainage picked up in either of the existing stubs flows for 700' to the north in 18" concrete pipes, where it is released to a grass lined swale. Drainage flows east for 200' in the grass lined swale, then enters into a vault and is released to the detention pond. From the pond, drainage is released at a controlled rate to the north and continues flowing in pipes. Figure 2 shows the drainage channels, conveyance systems, and receiving waters, to a point %. mile downstream through the adjacent plat. There are no known existing problems in the downstream network. IV. Analysis and Design of Required Erosion and Sediment Control BMPs Figure 4 is a reduced copy of the Large Parcel Erosion and Sediment Control Plan (LPESCP). The LPESCP illustrates the following BMPs and strategies for controlling erosion and sediment on the construction site: 1. 20' x 100' Stabilized Construction Entrance (BMP E2.10) 2. —3,000 If Filter Fence (BMP E3.10) 3. Minimum 75' Buffer Zone around wetland (BMP E1.30) 4. Temporary Sedimentation Pond (BMP E3.40) 8,925 sf surface area, 2 ft settling depth (1.2(7.14)/0.00096 = 8,925 sf) 5. Riprap at pond outfall (BMP E2.75) 6. Storm Drain Inlet Protection (BMP E3.30) 111 7. Permanent Seeding (BMP E1.35) I 1 2 I