{"id":7315,"date":"2025-12-25T00:27:48","date_gmt":"2025-12-24T16:27:48","guid":{"rendered":"https:\/\/aquarainwater.com\/?p=7315"},"modified":"2025-12-25T08:26:54","modified_gmt":"2025-12-25T00:26:54","slug":"washington-retention-des-eaux-pluviales-decembre-2025-defaillance-du-systeme-anti-inondation","status":"publish","type":"post","link":"https:\/\/aquarainwater.com\/fr\/washington-stormwater-detention-december-2025-flood-failure\/","title":{"rendered":"washington-retenue-des-eaux-pluviales-d\u00e9cembre-2025-d\u00e9faillance-en-cas-d'inondation"},"content":{"rendered":"\n

Washington\u2019s December 2025 Floods Broke Levees and Exposed a Critical Flaw in Detention System Design<\/strong><\/h1><\/div>\n\n\n\n

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The Green River hadn’t been this high in 60 years.<\/p>\n\n\n\n

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When the Desimone levee broke near Tukwila on December 15, 2025, the river was running at levels that nobody working in King County stormwater today had ever seen. Fifteen feet of rise in a single week. Over 45,000 people under flash flood warning within hours of the breach.<\/p>\n\n\n\n

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As I watched the evacuation orders roll out, I kept thinking about every detention basin, underground vault, and flow control structure that discharges into the Green River or its tributaries. None of them could drain. The outlets were underwater. Physics doesn’t negotiate.<\/p>\n\n\n\n

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The December 2025 Pacific Northwest floods weren’t just a weather event. They stress-tested our entire stormwater infrastructure philosophy. If you design, review, or maintain detention systems anywhere in Western Washington, the results deserve a hard look.<\/p>\n\n\n\n

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\"December<\/figure>\n\n\n\n

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What Actually Happened: A Verified Timeline<\/font><\/font><\/h2><\/div>\n\n\n\n

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The details matter for understanding why the infrastructure responded the way it did.<\/p>\n\n\n\n

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December 5:<\/strong> The Center for Western Weather and Water Extremes (CW3E)<\/a><\/strong> forecasts an incoming atmospheric river at AR-5 intensity\u2014the maximum category on their scale. AR-5 events carry exceptional water vapor transport and high probability of widespread flooding.<\/p>\n\n\n\n

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December 7:<\/strong> NWS Seattle<\/strong><\/a> issues a Flood Watch covering western Washington\u2014Olympic Peninsula, Puget Sound lowlands, Cascade foothills\u2014effective through December 12. The watch warns of sharp rises on rivers draining the Olympics and Cascades, plus elevated debris flow risk in burn scar areas.<\/p>\n\n\n\n

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December 8:<\/strong> The atmospheric river makes landfall. Rainfall begins in earnest across Western Washington.<\/p>\n\n\n\n

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December 10:<\/strong> Governor Bob Ferguson declares a statewide emergency<\/a><\/strong> and activates the Washington National Guard. By day’s end, over 100 Guard members are deployed; 300 more follow by December 11. Ferguson calls the situation “extremely serious” with “lives at stake in the coming days.”<\/p>\n\n\n\n

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December 11:<\/strong> The Skagit River and Snohomish River reach record or near-record flood levels. In Stehekin\u2014accessible only by boat or aircraft\u2014debris flows from the 2024 Pioneer Fire burn scar destroy the water treatment plant and sever road access. Seven rivers across the state hit Phase 4 flooding<\/a><\/strong>, the highest warning level.<\/p>\n\n\n\n

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December 12:<\/strong> President Trump approves Washington’s request for federal emergency declaration<\/a><\/strong>, unlocking FEMA resources.<\/p>\n\n\n\n

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December 14:<\/strong> A second atmospheric river arrives\u2014rated AR-4. The region hasn’t recovered from the first system, and rivers that had begun to recede start rising again.<\/p>\n\n\n\n

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December 15 (Monday afternoon):<\/strong> The Desimone levee breaches along the Green River near Tukwila, just east of Seattle-Tacoma International Airport. The river’s water level is higher than it’s been in 60 years. More than 45,000 people are placed under flash flood warning. Crews complete a temporary repair by evening, but the damage is done.<\/p>\n\n\n\n

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December 16 (Tuesday, 1:30 AM):<\/strong> HESCO barriers along the White River fail in Pacific, about 30 miles south of Seattle. The breach extends approximately 120 feet. Between 1,300 and 2,100 residents are evacuated in the pre-dawn hours.<\/p>\n\n\n\n

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December 16:<\/strong> Governor Ferguson amends the emergency proclamation and directs $3.5 million in emergency funds to 14 affected counties. The state reports more than 1,200 rescues and evacuations since flooding began. One fatality confirmed: a 33-year-old man who drove past road closure signs into floodwater in Snohomish County.<\/p>\n\n\n\n

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The scale: approximately 5 trillion gallons of rainfall over one week. A 49-mile stretch of US-2 closed for months. Up to 100,000 residents facing potential evacuation in the Skagit River area alone.<\/p>\n\n\n\n

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Sources:<\/strong> Governor.wa.gov official press releases; CNN; NWS Seattle; Washington State Standard; Wikipedia compilation of the 2025 Pacific Northwest floods.<\/p>\n\n\n\n

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The Engineering Problem: Why Detention Systems Couldn’t Drain<\/strong><\/h2><\/div>\n\n\n\n

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ere’s what most coverage of the floods missed.<\/p>\n\n\n\n

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Detention systems work on a simple principle: store runoff temporarily, release it at a controlled rate that downstream infrastructure can handle. The outlet structure\u2014orifice, weir, or combination\u2014meters outflow based on the head differential between the water surface inside the basin and the tailwater elevation downstream.<\/p>\n\n\n\n

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That principle works under normal conditions. It works reasonably well during most storm events.<\/p>\n\n\n\n

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It stops working when the downstream receiving water rises above your outlet elevation.<\/p>\n\n\n\n

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Understanding Backwater Conditions<\/strong><\/h3><\/div>\n\n\n\n

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When river levels downstream exceed your detention outlet elevation, you get a backwater condition. The outlet becomes submerged. The head differential that drives flow through your orifice drops toward zero. Your carefully sized outlet structure effectively stops functioning.<\/p>\n\n\n\n

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During the December 2025 event, the Green River rose 15 feet over one week. When it breached the Desimone levee, it was at its highest level in six decades. Every detention facility discharging to the Green River or its tributaries\u2014every underground vault beneath a parking lot, every flow control structure sized per King County SWDM\u2014was operating with a submerged outlet.<\/p>\n\n\n\n

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Inflow from continuous rainfall kept coming. Outflow approached zero.<\/p>\n\n\n\n

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\"Underground<\/figure>\n\n\n\n

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The Failure Chain<\/strong><\/h3><\/div>\n\n\n\n

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The December floods didn’t fail because of any single weakness. They exposed a cascading sequence that builds over multi-day events:<\/p>\n\n\n\n

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Sustained AR-4 to AR-5 atmospheric river<\/strong> delivers 5 trillion gallons over one week. This isn’t a design storm\u2014it’s multiple design storms back-to-back with no recovery window.<\/p>\n\n\n\n

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Cascade slopes receive 10+ inches of rainfall.<\/strong> The runoff coefficient approaches 1.0 as soil saturation progresses. By day three or four, the ground is functionally impervious.<\/p>\n\n\n\n

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Rivers surge to record levels.<\/strong> Green River: 60-year high. Snohomish River: 33 feet with “major levee damage possible” per NWS. Skagit River: record flood levels.<\/p>\n\n\n\n

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Downstream infrastructure reaches capacity.<\/strong> Storm sewers, culverts, and channels throughout the watershed are running full.<\/p>\n\n\n\n

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Backwater conditions develop.<\/strong> River elevation exceeds detention outlet elevations across the region.<\/p>\n\n\n\n

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Upstream detention systems cannot drain.<\/strong> Orifices submerged, head differential eliminated, controlled release impossible.<\/p>\n\n\n\n

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Levees experience sustained hydraulic pressure.<\/strong> Instead of water cycling through detention and releasing, it accumulates. Pressure builds over hours and days rather than the design duration.<\/p>\n\n\n\n

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Levee failures occur in sequence.<\/strong> December 15: Desimone levee (Green River). December 16: White River HESCO barriers.<\/p>\n\n\n\n

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The critical insight: traditional detention design assumes downstream capacity exists to receive the controlled release. When an entire watershed is simultaneously at flood stage for days, that assumption fails everywhere at once.<\/p>\n\n\n\n

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What Washington Regulations Actually Require<\/strong><\/h2><\/div>\n\n\n\n

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Understanding the failure mechanism is useful. Knowing how to design for it\u2014and what the regulations require in your submittal package\u2014is practical.<\/p>\n\n\n\n

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One question worth asking: do current regulations explicitly account for multi-day AR-5 events like December 2025? The SWDM requires backwater analysis, but the tailwater assumptions in most analyses weren’t calibrated for 60-year flood levels sustained over a week. That’s a gap worth watching as guidance evolves.<\/p>\n\n\n\n

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The Statewide Framework: SWMMWW 2024<\/strong><\/h3><\/div>\n\n\n\n

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The Stormwater Management Manual for Western Washington<\/a><\/strong> published by Washington Department of Ecology establishes baseline requirements for most projects west of the Cascades. The 2024 edition is current.<\/p>\n\n\n\n

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Minimum Requirement #5 (On-Site Stormwater Management)<\/strong> establishes LID as the preferred approach. Projects must either implement specific LID BMPs via the “List Approach” or meet the “LID Performance Standard” by matching pre-developed hydrology. If LID is infeasible\u2014due to clay soils, steep slopes, or high groundwater\u2014you document the infeasibility and move to alternative flow control.<\/p>\n\n\n\n

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Minimum Requirement #7 (Flow Control)<\/strong> governs detention sizing. Storage volume calculations, outlet sizing, and performance modeling live here. The core requirement: limit post-development peak flows to pre-development levels for the range of design storms.<\/p>\n\n\n\n

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Minimum Requirement #9 (Operation and Maintenance)<\/strong> requires an O&M plan and a recorded covenant that runs with the property. The property owner is legally responsible for maintaining the facility in perpetuity.<\/p>\n\n\n\n

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One detail that trips people up: MR #5 and MR #7 are independent requirements. Satisfying the LID performance standard doesn’t exempt you from flow control. You may need both infiltration BMPs and detention depending on site conditions and downstream discharge point.<\/p>\n\n\n\n

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King County Surface Water Design Manual<\/strong><\/h3><\/div>\n\n\n\n

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If your project is in unincorporated King County, the King County Surface Water Design Manual<\/a><\/strong> applies. Chapter 5 covers flow control design:<\/p>\n\n\n\n

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Detention facility types:<\/strong> Ponds (open surface), tanks (underground, typically corrugated metal pipe), and vaults (underground, reinforced concrete). Each has specific design criteria for geometry, access, and maintenance.<\/p>\n\n\n\n

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Backwater analysis requirement:<\/strong> The SWDM explicitly requires backwater analysis for sizing detention and infiltration facilities. December’s floods raise the question: are the tailwater assumptions in those analyses adequate for extreme events?<\/p>\n\n\n\n

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Outlet structure design:<\/strong> Orifice sizing, weir configurations, restrictor plates. Detailed guidance on control structure placement and access.<\/p>\n\n\n\n

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Maintenance access:<\/strong> Vaults require access for inspection and sediment removal. Removable panels (no heavier than 5 tons) or dedicated access roads for County-maintained facilities.<\/p>\n\n\n\n

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Snohomish County Drainage Requirements<\/strong><\/h3><\/div>\n\n\n\n

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Snohomish County operates under its own Drainage Manual<\/a><\/strong> based on SWMMWW, with the 2024-2029 NPDES municipal stormwater permit in effect. Private facility owners are responsible for maintenance under SCC 7.54.<\/p>\n\n\n\n

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Seattle Stormwater Code<\/strong><\/h3><\/div>\n\n\n\n

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Seattle’s requirements are in SMC Title 22<\/a><\/strong> and the 2021 Seattle Stormwater Manual. An update is scheduled for July 2026\u2014verify which version applies at permit issuance.<\/p>\n\n\n\n

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Seattle has provisions for combined sewer basins where flow control may not be required, but if downstream drainage is “capacity constrained,” flow control requirements can still apply.<\/p>\n\n\n\n

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Design-Review-O&M Checklist<\/strong><\/h2><\/div>\n\n\n\n

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For practitioners translating regulations into a submittal package:<\/p>\n\n\n\n

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Design Phase<\/strong><\/h3><\/div>\n\n\n\n

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