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

Stormwater Management Report Everett, WA Analysis and Design of Required Runoff Control BMPs Proposed Development The proposed Lot 3C development would consist of two buildings totaling approximately 100,000 S.F. with associated parking lot and landscape areas. As designed, Lot 3C is ' 76.9% impervious. For the design of the detention pond, Lot 3D is assumed to be 90% impervious post-development. For streambank erosion control, the detention system is sized to detain the post-developed peak flow rates from the 2-yr, 24 hour storm and release at the pre-developed peak flow 2-yr, 24 hour storm. Also, the 10-yr, 24 hour and 100-yr, 24 hour post-developed flows are detained to release at the pre-developed 10-yr and 100-yr peak flow rates. A correction factor is applied to the detention volume to ' further mitigate the increased durations of flows. Figure 1-3.1 from the City of Everett's Stormwater Management Manual was used to calculate the correction factor. Lot 3C and Lot 3D were accounted for in the design of the Narbeck Creek outfall that is adjacent to the NCC, therefore, after the stormwater is detained, it will be released into the existing storm structure in the northeast corner of the Lot 3D that drains to the outfall. Sub-Basin Description For the purpose of this report, Lot 3C has been split into three basins, they are as follows: the developable portion of the site that drains to Powermill Gulch Creek; the developable portion of the site that drains to Narbeck Creek; and the steep slopes and buffers in the eastern portion of the site. The Powdermill Gulch Creek basin will not be analyzed in this section of the report since ' it has been accounted for in the design of the stormwater pond to the north. However, the stormwater system has been designed to route runoff from 3.15 acres in the post developed condition to the Powdermill Gulch Creek. This matches the pre-developed drainage basin for Powdermill Gulch Creek. The steep slopes and buffers will also not be analyzed since that portion of the property will remain untouched. ' The portion of the property that drains to Narbeck Creek will be routed to a wetpond on the NCC property to the north. The pre and post developed area onsite draining to Narbeck Creek is 3.33 acres. Additionally, the 4.0 acres Lot 3D shown in NCC Report 2 and the 0.21 acres of Lot 3D that were not accounted for in the NCC Reports will be accounted for in the design of the detention pond. This results in a 7.54 Acre drainage basin for the Narbeck Creek wetpond. Narbeck Creek Drainage Area = 7.54 acres Impervious Area = 6.35 acres (3.79 acres Lot 3D & 2.56 acres Lot 3C) CN = 98 (building roof, sidewalks & parking area) PACLAND Job# 50108001 Page 13