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STORMWATER DESIGN MEMO Thoroughbred Express Wash, Woodburn 1299 N. Pacific Highway Woodburn, OR 97071 Prepared for: Blitz Holdings, LLC 506 Second Ave, Suite 1009 Seattle, WA 98104 October 17, 2025 Our Job No. 23947 ---PAGE BREAK--- PROJECT OVERVIEW The proposed commercial development includes construction of a 2,944-sf car wash with associated site improvements on one lot (Map Tax Lot # 051W08DB02400) encompassing 0.95 acres. The site is located at 1299 North Pacific Hwy, Woodburn OR 97071 at the SW intersection of Alexandra Ave. and North Pacific Highway (99E) as seen in Figure 1 & 2. This site contains a Commercial land use designation and is zoned as Commercial General (CG). The properties to the NW are zoned Residential (RS), with all other surrounding properties to the N/S/E zoned Commercial General (CG). The property is currently vacant after the demolition of the previous fast-food restaurant and now consists of a large asphalt parking lot, building foundation remnants and unmaintained grasses/trees. The proposed improvements to the site include a 2,900 SF car wash, vehicle vacuum units, paved parking lot, curb & gutter, interior/exterior landscaping and a storm drainage system. The northern property boundary fronts the city owned Alexandra Ave. and the eastern property boundary abuts state owned N. Pacific Hwy (99E) as an ODOT Right-of-Way. Existing site conditions collect all drainage on-site through a system of catch basins and storm pipes to convey it to the city owned storm drainage system at a catch basin on Alexandra Ave. at the NE corner of the lot. The existing storm drainage system does not include a detention system or a designated treatment system, and discharges the collected stormwater run off directly to the City's stormwater system. STORMWATER MANAGEMENT PLAN A site composition analysis of the pre and post-development surfaces was performed to compare existing and proposed surface type and runoff patterns. Impervious surfaces included all concrete/asphalt areas and rooftop drainage. Pervious surfaces accounted for all landscaped areas. Tabulated data from the analysis is shown in Table 1 below. In a video call meeting with City Engineer Dago Garcia and Brion Scott at ODOT on 10/02/25, it was stated that it is City policy that any increase of impervious area greater than 0.25 acres over the existing site condition would trigger stormwater detention requirements. As shown in Table 1, the proposed post- development site design reduces the impervious area by 13%, resulting in an overall net reduction of stormwater runoff compared to existing conditions. As proposed, the design does not exceed this threshold and therefore no stormwater detention system is required or proposed. Table 1: Contributing Area Development Summary Dev. Phase Impervious Pervious Total (SF) Pre-Dev. 32,302- 78% 8,899- 22% 41,201 Post-Dev. 26,430- 65% 14,035- 35% 40,465 Net Change 5,872-(13%) 5,136-13% *Pre/post total areas are different due to Right-of-Way dedication The stormwater runoff from the site will be collected using a combination of rooftop drains and surface sheet-flow to direct drainage to onsite catch basins and conveyed through a series of storm drainage pipes. Prior to discharging the drainage off-site, all stormwater runoff is routed into a proposed type 2 storm drainage manhole. This structure employs a sump area and downturned elbow invert to prevent sediment from entering the City system at the connection point on Alexandra Ave. The storm drainage system will significantly improve upon existing conditions. All onsite components of the storm drainage system prior to connection with city infrastructure will be privately owned, operated and maintained. Please refer to Exhibit 1 showing the proposed storm drainage collection and conveyance system. ---PAGE BREAK--- Figure 1: Vicinity Map Figure 2: Aerial Image ---PAGE BREAK--- C C C C C C OHW OHW OHW OHW OHW OHW OHW OHW SD SD SD SD SD SD SD SD SD SD SD SD E E E E E E E UU UU C C C C C C C C C C C C C C C E E E E E C C C C SD SD SD SD SD SS UU Sheet Job Number of Horizontal Designed Drawn Checked Scale: For: Vertical Approved Date Title: P:\23000s\23947\engineering\Storm\z23947-EXBT-Basin Map-Combined.dwg 10/17/2025 8:56 AM RGAUL 23947 BLITZ HOLDINGS 506 SECOND AVE, SUITE 1009 SEATTLE, WA 98104 1299 N. PACIFIC HIGHWAY WOODBURN, OR, 97071 10/17/25 NJM NJM RSG RSG WOODBURN THOROUGHBRED EXPRESS WASH Revision No. Date By Ckd. Appr. 1 N/A DRAINAGE BASIN MAP SS SD SD SD SD SD SD SD SD SD VAC VAC VAC VAC VAC VAC VAC VAC VAC SD SD SD SD IMPERVIOUS SURFACE PERVIOUS SURFACE SCALE: 1"=20' 0 40' 20' 10' DEVELOPMENT STATUS TOTAL AREA (SF) PERVIOUS AREA (SF) % PERVIOUS IMPERVIOUS AREA (SF) % IMPERVIOUS 1 EXHIBIT 1: DRAINAGE BASIN MAP PRE-DEVELOPMENT SITE POST-DEVELOPMENT SITE