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

Stormwater Management Report Everett, WA 1 Preliminary Conditions Summary 1 Existing Conditions The existing land coverage is entirely pervious and made up of second growth conifer and deciduous trees with moderate ground cover. The USGS defines the site soils as ' Alderwood Gravelly Sandy Loam (hydrologic soil group C) and Everett Gravelly Sandy Loam (hydrologic soil group A). The curve number relating to the existing ground cover is 81 for C soils and 55 for A soils. The soils in the onsite area that is contained in the steep slopes and associated setbacks that will not be disturbed as a part of this project are defined as Alderwood-Everett Gravelly Sandy Loams. The soils in the Narbeck Creek drainage basin are entirely Alderwood Gravelly Sandy Loam. Therefore all vault sizing ' calculations utilize group C soils. The NCC site that has been included in the pond design for the Narbeck Creek portion of ' the site will be designed primarily based on the following two reports (included in Section VI): ' NCC Report 1 . STORMWATER SITE PLAN — Northshore Christian Church Campus; by On-Purpose Engineering, Inc, dated February 15, 1999. NCC Report 2. Northshore Christian Church Campus — Supplemental Calculations; ' by Trepanier Engineering, dated April 4, 1999. Per NCC Report 2, 4.0 acres of Lot 3D drains to Narbeck creek. The following is the ' definition of the drainage basin per NCC Report 2: Western Boundary— Natural ridge line that separates the Powdermill Gulch Creek ' drainage and the Narbeck Creek drainage Eastern Boundary — 25-foot steep slope setback associated with Narbeck Creek ' Northern Boundary— 130-foot residential buffer* Southern Boundary — Lot 3C property line * Since the buffer for the pond is only 75 feet, the additional 0.21 acres between the 130- ' foot buffer to the 75-foot buffer will be accounted for in the pond calculations. ' Therefore, 4.21 acres of Lot 3D are accounted for in the design of the Narbeck Creek detention pond. 90% of Lot 3D is assumed to be impervious in the developed condition. Lot 3C is 10.66 acres; however a large portion of the site is occupied by Narbeck Creek and the associated NGPA. The area of the development is 6.48 acres. ' There is an existing storm drainage easement along the eastern side of the developable portion of the property for the existing storm system that outfalls into Narbeck Creek. Runoff from the eastern portion of Lot 3C will be routed through a parallel system to the ' offsite detention pond before connecting to the existing drainage system and being routed to the Narbeck Creek outfall. PACLAND Job# 50108001 Page 4