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Revamped White River flood plain passes first test following last weekend's storms

King County places large "HESCO" barriers along the White River as part of their Countyline project to increase the capacity of the river (Photo via King County)

SUMNER, Wash. -- A two-year project to help the vulnerable White River better handle the autumn and winter-time floods passed its first test during last weekend's storms.

Heavy rains brought the river just a bit over flood stage, but the new levee setback project did as expected and gave the river extra room to flood, keeping the waters from flowing into populated areas.

The $20 million project, which is just nearing completion in December, involved rebuilding a river levee farther back off the river's banks in Pacific and Sumner, which created 120 additional acres of land to become a natural flood plain. That allows more water to harmlessly fill that area instead of heading downstream into populated areas. Pierce County installed similar barriers just downstream on their side of the county line to help protect the nearby UPS and Amazon distribution centers, among other locales.

The project received its first test on Saturday when the second of two atmospheric rivers that week brought heavy rainfall to the Cascades. The White River did get some minor flooding, and successfully utilized the new flood plain:

"The river came onto the project site where our staff thought it would, and the flows fanned out across that floodplain before reentering the White at the downstream end of the project," said Doug Williams with King County Department of Natural Resources and Parks. "I think we were expecting slightly higher flows than we got from this storm event - not that we're complaining (but) everyone is happy with the way Countyline performed, and we'll definitely be out on site the next time we expect to see higher flows."

Why the project? The White River has become more flood prone recently

The reason for the needed additional flood capacity is the White River is becoming increasingly shallow due to runoff from Mt. Rainier. As the silt collects in the river bottom, its capacity to hold flood waters diminishes. The river flow can be somewhat controlled by the Mud Mountain Dam, but officials with the Army Corps of Engineers now are finding with the decreased capacity, that even minimal release flows from the dam during heavy rain events can push the river near flood stage.

The Army Corps of Engineers built Mud Mountain Dam above the river in 1948 as a way to manage the flood risk in downriver towns along the White and Puyallup Rivers such as Puyallup, Tacoma, Auburn, Sumner and Pacific. Back then, the White River could handle downstream flows up to 20,000 cubic feet per second (cfs). Rainfall events that would trigger those kind of flows on the White River are quite rare in the Northwest, and there hadn't been a big flood since the dam was built.

But that all changed during a heavy rain event in 2009. As the rains fell, the dam operators released water from the dam at 12,000 cfs -- their standard release flow at the time. But this time, it caused quite a bit of flooding in Pacific, signaling a significant change in the way the river is behaving.

Since then, engineers found even the minimal releases during heavy rain events required to keep the reservoir from overfilling are now strong enough to cause some flooding downstream in Auburn, Pacific and Sumner.

It's estimated the river's peak capacity now sits at just 7,000 cfs before it begins to flood -- a level typically reached multiple times a year in a standard fall and winter rainy season.

Why can't the Mud Mountain Dam just hold it all in and keep flows below 7,000 cfs?

To answer that question, let's dive in to how dams work. The Mud Mountain Dam is strictly used to manage flood risk in the fall and winter, so engineers will try to keep the reservoir empty to give it as much capacity to hold runoff during heavy rain events, according to Ken Brettmann with the Seattle District of the Army Corps of Engineers.

As the rains fall and the reservoir fills, corps engineers will release water as part of a delicate balancing act to follow: They want to release as little water as they can to keep downsteam levels as low as possible, yet they still to release enough so the dam doesn't fill and overflow.

Brettmann says the minimum water flow the dam can release to be effective is 6,000 cfs -- close these days to now cause minor flooding in Auburn, Sumner and Pacific. But it's a choice the corps has to make to prevent a worse scenario. If engineers don't release enough water, and the rainfall either extends and/or another heavy rain storm or two quickly follow (as has happened many times around here) they'll have to release water at a much higher rate, causing potentially life-threatening and catastrophic flooding downstream in a effort to keep the dam from overflowing.

And Brettmann wants residents to know that even after the rains stop, the dam may continue to release water that could cause minor flooding as they will be trying to get as much water out of the reservoir as quickly as feasibly possible -- again, a balancing act -- so that they have the most capacity possible for the next storm.

What's being done about it?

First of all, the National Weather Service has now developed a new river forecasting model at Auburn along the White River that will allow forecasters to predict potential flooding conditions days in advance at that point. In addition, King County has lowered the criteria on the White for it to reach higher levels on their Flood Alert System. That will allow earlier openings of their emergency operations center and their flood patrols to monitor the river's pain points.

The U.S. Army Corp. of Engineers has developed maps that will show what predicted flooding would look like in five different flooding scenarios, ranging from 8,000 cfs to as high as 20,000 cfs, to give local residents and business owners an idea of what flooding could look like based on the river's forecast.

Can't the river just be dredged?

Levee setbacks are a form of sediment management, says Jeanne Stypula, the manager of the Countyline project, and more certain way of providing flood protection.

What about both? Williams says dredging the river would require a number of permits from federal, tribal and state officials -- the river is home to some important salmon runs -- and it would be a major environmental impact for what would be a temporary solution as the river would just eventaully refill from Mt. Rainier. so it's not a feasible option.

What can you do to prepare?

With another La Nina winter likely, so too comes a higher risk of flooding events as they tend to be a bit more frequent in La Nina winters.

King County officials stress two main things you can do right now to be ready for a flood. One is sign up for their King County Flood Alerts or Pierce County Alerts that will keep you apprised of changing weather and river conditions.

The second is making sure you have Flood Insurance. All King and Pierce County residents qualify, but there is a 30 day waiting period until the policy will go into effect.

"Waiting for the rain to come is 30 days too late," said Sarah Foster with the Pierce County Department of Emergency Management.

MORE | King County Flood Prepare Tips | Pierce County Flood Prepare Tips | Take Winter By Storm Flood Preparations Checklist

You can keep tabs on the latest White River conditions at Auburn at this link from the National Weather Service which will give you a graph like this:

Flooding: Not just a White River thing...

Of course, other rivers flood too. And you can track all forecasts for other Western Washington rivers at this link.

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