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Report -7 032. S( D"oin Unitus Credit Union Employee Amenity Area Improvements 7632 SW Durham Rd Tigard, OR 97224 Storm Drainage Memo RECEIVED JUL 3 2023 CITY OF TIGARD BUILDING DIVISION 6814 Greenwood Ave.N T 206.522.9510 Seattle,WA 98103 www.pacland.com Prepared By: Willis Liu Prepared: May 2023 Revised June 2023 Purpose of Memo The proposed improvements as part of this project include the construction of an employee amenity area for the Unitus Community Credit Union facility, along with landscaping and limited modifications to existing utilities and stormwater facilities. This memo is intended to show that the proposed improvements located in the NW corner of the lot, located in Tigard, OR, are compliant with the amount of impervious surface area approved for the site under the original development of the campus. In reviewing the Clean Water Services (CWS) standards, it does not appear that the project scope triggers the Hydromodification Assessment Requirement nor Water Quality Treatment. Per Section 4.03.2.b of the CWS standards, the following is stated: Unless specifically waived in writing by the District, a Hydromodification Assessment is required of all activities described in Section 4.03.1, unless the activity meets any of the following criteria: a. The project results in the addition and/or modification of less than 12,000 square feet of impervious surface. b. The project is located in an area with a District approved subbasin strategy with an identified regional storm water management approach for hydromodification. Per the as-builts for the original development of the site (found in Appendix A), water quality and detention facilities were designed and constructed as approved by the City for the contributing basin areas located on the property (satisfying Item B). The project does not alter nor change the existing drainage patterns of the site as runoff will continue to be captured by the existing storm conveyance system and routed thru the water quality treatment and detention pond. Furthermore, the project effectively removes over 4,000 sf of existing impervious area within this basin by converting it to landscaping. Per Section 4.04.2 of the CWS standards, the following is stated: A water quality approach shall be implemented on-site unless, in the judgment of the District or City, any of the following conditions exist: 1. Due to topography, soils or other site conditions, implementation of on on-site approach is impractical, ineffective or results in the inefficient use of District or City resources for long-term operations and maintenance;or 2. There is a more efficient and effective regional approach within the subbasin that was designed to incorporate the development, or there is an approach in the subbasin which is demonstrated to have the capacity to treat the site. As described before, the parcel has a water quality/detention pond that was designed to incorporate the proposed employee amenity areas.These existing facilities already have the capacity to treat the project extents and the proposed scope of work reduces the amount of stormwater runoff by reducing the overall impervious surface area that drains into the existing storm system.The project also converts existing parking lot pavement to surfaces subject only to pedestrian traffic, thereby reducing the pollutant loads on the existing treatment system. Conclusion The proposed improvements for employee amenity areas within the existing parking lot should be considered compliant under the overall stormwater design for the Unitus Community Credit Union parcel and that this project does not trigger the Hydromodification Assessment Requirement nor Water Quality Treatment. As the project is located in an area with an approved regional stormwater management approach for hydromodification and this approach has been demonstrated to have the capacity to treat the site, per correspondences with the City Engineer, this is an approved approach to demonstrating compliance with CWS standards. ti . u 2j �TMvr . 4 - _ _ assn v r _ y�i! s. w IRT- ESATMEDPADA ATOM,AD ©1 SIDEwALN SEGRal.s.aw.ma CURB SECTION ...� \� . :4� ''� � "` 9 _ S .au°an i4`14x. EwsrNC3 eUEana ) N`', 4 - ''� ,a _ _._. I 1NII ale `. .yamivf< g \` .. 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