3101 CEDAR ST STORM DRAINAGE DESIGN 2025-07-25

MAGNUSSON KLEMENCIC 111 ASSOCIATES ■ . Stormwater Minimum Management Manual Requirement# Description Vol. 1 Section Applicability 7 Flow Control 2.2.7 Appl i es 8 Wetlands Protection 2.2.8 Does not apply 9 Operations and Maintenance 2.2.9 Appl i es The mitigations for the applicable Minimum Technical Requirements are discussed below and in later sections of this Plan. MINIMUM TECHNICAL REQUIREMENT NO.4—PRESERVATION OF NATURAL DRAINAGE SYSTEMS AND OUTFALLS The pre-developed site has no dedicated storm drainage system, as stormwater would pond and, over time, evaporate or overflow to the adjacent Thirteen Street (with minor and localized infiltration). As a temporary parking facility,the project improves the drainage system by removing the potential for surface ponding while providing detention to minimize downstream impacts. PERMANENT STORMVVkTER CONTROL PLAN Given that the project is eventually proposed to be re-developed in accordance with the PEMC master plan, a project objective is to keep the impacts within the boundaries of the site and minimize impacts outside of the footprint. The only storm drainage facility available on the site is the existing combined system pipe that flows to the east underneath the dead-end of Twelfth Street. The design focuses on this pipe as the ideal connection point for the proposed drainage system. OFF- SITE ANALYSIS The downstream combined system that the project proposes to tie into was recently constructed by the City during the overall PEMC campus development. Based on the visual evidence and on conversations with the PEMC ownership and site contractors,the existing site floods during rain events. There is no evidence to suggest that the existing combined system piping has been the cause of the flooding. The project does not propose to increase the flow into the existing combined system pipe,therefore no downstream or offsite impacts are anticipated. No erosion is visible around the perimeter of the site, so side slopes are considered to be stable. City staff did inform the project team that utility trenches installed across the site's consolidated soils tend to act as conduits for groundwater flow in very unpredictable ways. The project has mitigated this City concern by proposing to use filter fabric lining for utility trenches to keep trench backfill fines from migrating out of the trench section. During a September 26, 2012 community meeting regarding the project, neighbors of the project site mentioned periodic flooding of the residential yards nearby (which are significantly higher in elevation than the project site). It is likely that the yard flooding is caused by poor infiltration of local soils. No Storm Drainage Design �. s_ ,.> w..r „ ... . rs . >... 5 Utility Plant Expanded Parking Facility, Everett,Washington