As U.S. Army Corps of Engineers’ flood response teams were supporting local communities in Skagit, Whatcom and Snohomish counties over the weekend, Seattle District water managers were busy regulating flows at five dams to reduce downstream flood risk. Without those efforts, levees in Mount Vernon would have overtopped, leading to catastrophic flooding.
Seattle District’s Reservoir Control Center worked 24-hour operations during the flood events to monitor and regulate Howard Hanson, Mud Mountain, Upper Baker, Ross and Wynoochee dams. The Corps owns and operates Howard Hanson and Mud Mountain dams and directs operations of the utility-owned projects during flood events per established agreements.
In the Skagit River Basin, Corps water managers began actively directing operations at Puget Sound Energy-owned Upper Baker Dam and Seattle City Light-owned Ross Dam November 14.
Corps operations at these two upper Skagit River dams stored inflows and decreased peak flow by 40 percent, reducing the peak river stage at Concrete, Washington, by more than eight feet and preventing the overtopping of levees in Mount Vernon, Washington.
The flood event included back-to-back atmospheric rivers – long, flowing columns of condensed water vapor that act like a conveyor belt, carrying vapor for thousands of miles from out over the ocean. When an atmospheric river hits the West Coast, it can generate a series of storms, with each storm producing inches of rain.
This flood with its corresponding atmospheric rivers brought heavy precipitation to the region and broke inflow records in the upper Skagit River. Inflows to Ross Dam were the highest on record at approximately 60,000 cubic feet per second (cfs) and have a 0.2 percent chance of occurring any given year. Inflows to Upper Baker Dam also broke records with inflows of 41,000 cfs, which has a 1 percent chance of occurring any year. Outflow from both dams were significantly reduced during the peak of the flood, lowering downstream flows by as much as 80,000 cfs.
Ross Dam and Upper Baker Dam are mandated to provide storage space in their reservoirs for use by the Corps during the flood season in Western Washington. Both PSE and SCL are proactive partners and often provide additional non-required storage space. During this event, Ross Dam was able to provide more than twice the required storage space which was integral to reducing the peak flood flow. Additionally, Corps water managers proactively conserved critical reservoir space early in the event to be able to reduce flows during the peak of the flood. Without this additional storage and careful reservoir management, the flood peak at Mount Vernon would have overwhelmed the levee and flood wall system.
The reservoir at Ross Dam peaked within 3 inches of the normal full level and the reservoir at Upper Baker Dam peaked within 9 inches of the normal full level. Both reservoirs are currently lowering as water stored during the flood is released. “This causes the Skagit River level to stay more elevated than it would naturally,” said Sonja Michelsen, Seattle District’s Western Washington senior water manager. “However, flows will remain much lower than the peak flow seen on Monday, November 15.”
The Corps began regulating City of Aberdeen-owned Wynoochee Dam November 15 for approximately 8 hours before returning control to Tacoma Public Utilities, which operates the dam. During peak operations the inflows to the dam were almost 10,000 cubic feet per second (cfs) and Corps water managers held back some of the flow, releasing 7,000 cfs. The peak river flow observed at Montesano was 13,800 cfs.
At Howard Hanson Dam, Green River inflows rose above 10,000 cfs and Corps water managers held back over 5,000 cfs during the event’s peak. Mud Mountain Dam inflows for the White River were above 13,600 cfs and managers targeted releases below 6,000 cfs. Releases are being managed at both dams to evacuate stored water from last week's flood event.
“Public safety is our number one priority, and our next concern is reducing the reservoir pools,” said Michelsen. “It’s important to quickly empty the storage space in the reservoirs after a flood to remain prepared for future flood events.”