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Walla Walla's Mill Creek Channel performed as designed; flood recovery support underway

Walla Walla District
Published Feb. 12, 2020
Lt. Col. Christian Dietz, Commander of the Walla Walla District Corps of Engineers greets Walla Walla City workers who are monitoring the Mill Creek Channel.

Lt. Col. Christian Dietz, Commander of the Walla Walla District Corps of Engineers greets Walla Walla City workers who are monitoring the Mill Creek Channel.

An excavator removes debris from the Mill Creek Channel.

An excavator removes debris from the Mill Creek Channel.

Mill Creek Diversion Dam near Rooks Park.

Mill Creek Diversion Dam near Rooks Park.

Alex Hammond, Dam and Levee Safety Manager for the Walla Walla Corps of Engineers, overlooks the Mill Creek Channel.

Alex Hammond, Dam and Levee Safety Manager for the Walla Walla Corps of Engineers, overlooks the Mill Creek Channel.

Mill Creek Channel.

Mill Creek Channel.

Bennington Lake

Bennington Lake

Flooding in residential area off Mill Creek Rd.

Flooding in residential area off Mill Creek Rd.

Twenty four years to the day in 1996, the U.S. Army Corps of Engineers Walla Walla District and City of Walla Walla, Washington, engaged in one of the most significant flood fights to occur along the Mill Creek Channel.

Walla Walla means the “place of many waters” and that significance wasn’t lost on Lt. Col. Christian Dietz, USACE Walla Walla District commander, whose men and women, last week, navigated through warm winter rainstorms and the highest peak inflows ever recorded at the Mill Creek Project to keep the City of Walla Walla dry as flood waters hit the surrounding communities of Dayton, Waitsburg, Milton-Freewater and Touchet.

“Our staff live and work in these communities so we are very much a part of that interwoven fabric,” Dietz said. “We have a partnership with these communities which we take to heart during a flood fight like we just endured.”

That partnership was on full display as the District’s Readiness staff organized the Corps’ crisis action team and began engaging with their Walla Walla County partners while Walla Walla County Commissioners Todd Kimball and Greg Tompkins saw first-hand that flood fight effort while touring the Mill Creek Project and Bennington Lake on Friday morning. They learned how the Corps manages flood water by operating flood gates and diversion channels to redirect excess water to Bennington Lake and ensure that water flowing down the Mill Creek Channel stays off the streets of Walla Walla.  

Major flood events have occurred in Walla Walla in 1931, 1964 and 1996. It was the destructive flood of 1931 in which Mill Creek flowed through Walla Walla that led community leaders to petition Congress to build the Mill Creek Flood risk reduction project and downstream channel in 1941 to protect the city from future floods

Warm winter temperatures and 8 inches of rain in the Blue Mountains over 48 hours Feb. 6-7, 2020 inundated the region and led Corps officials to begin preparing for a flood fight.

“At that point we had two very specific public safety goals,” Dietz said. “Avoid fatalities and injuries and keep Walla Walla dry.”

For about 36 hours from Thursday morning to Friday night, Corps officials continuously monitored water flows up and down the Mill Creek Channel and throughout the Walla Walla Basin. They increased Bennington Lake diversions Thursday night, focused on effectively managing the amount of water going through town with those into Bennington Lake.

Mill Creek Project personnel, supplemented by engineering staff were on-site diverting water into Bennington Lake, adjusting gates as needed and monitoring the surrounding infrastructure. As rains continued, it was a tense moment for hydraulic engineer Jonathan Roberts who stayed at the Mill Creek project assessing project inflow, quickly changing rain forecasts, and decreasing storage space.   

“On Friday morning, around 2 a.m. we saw the inflow hydrograph jump up significantly at Kooskooski, passing the 1996 flood levels with more rain in the forecast,” Roberts said.”

“At that point, balancing how much water to push down the Mill Creek Channel versus what to divert into the reservoir became critical.  We coordinated with John Heitstuman, the District Hydrology Chief, who helped us navigate through the 1996 flood. At times like these there is no substitute for having experienced hands on the helm, and the decision was made to raise the channel’s flows to accommodate the incoming flood water,” he said.

The Corps increased flows through the Mill Creek Channel from 1,500 cubic feet per second  to 3,800 cfs by Friday morning. Overnight Bennington Lake levels rose from 15% to 65% full, so Corps staff began diverting flows of 100 cfs into Russell Creek, a secondary outlet in order to conserve as much space in Bennington Lake as possible should the rain continue.   

By noon, Bennington Lake was approaching 72% full, with the Corps maintaining flows of 3,800 cfs through the Mill Creek channel through the City of Walla Walla.

By 5 p.m. on Friday, Feb. 7, Bennington Lake had reached 80% capacity, and though weather forecasters were anticipating another inch of rain, the rain had stopped and inflows decreased. At that point the Corps stopped diverting water into the lake and were able to slow total inflows into the Mill Creek Channel to about 3500 cfs, which reserved approximately 20% storage space for uncertain precipitation. 

Throughout Friday night flows began to decrease and by Saturday morning they had dropped to around 2,000 cfs through the channel. In addition, the Operating project started returning flows from the lake into the Mill Creek Channel. By noon Saturday, Bennington Lake levels dropped to 70% of its capacity.

Ultimately, the Corps’ Mill Creek flood management system functioned as designed.

“People normally enjoy the Mill Creek Project and Bennington Lake on sunny cloudless days, sometimes wondering why the Corps operates parks and recreational areas,” said Justin Stegall, Mill Creek Project manager.

“Events like this demonstrate that the project’s primary function is flood damage reduction to protect the City of Walla Walla. With 78 years in service protecting Walla Walla and the downstream community, the Mill Creek Project and Bennington Lake performed as designed. That allowed us to open Rooks Park to general public access on Saturday afternoon so people could see the project at work and enjoy some well-needed rest and recreation following a tense 36 hours.”

“With our maintenance crews continue working diligently to release stored flood water we will return recreation areas to full access as soon as it’s safe to do so, but this overall effort really speaks to the dedication and commitment of the Corps staff and our partners,” he added.

The next phase of Corps engagement includes helping the surrounding communities like Milton-Freewater, Waitsburg, Touchet, Dayton, and others recover from the flood’s impacts.

“We’ve been to some of the surrounding communities directly impacted by flood waters,” Dietz said. “In the coming days we will be visiting more, and start working with local public officials and community leaders to further assess the damage and to help them identify their needs and the emergency response resources available to help start the recovery process,” Dietz said.

“Meanwhile, I have a tremendous amount of pride in the dedication and actions of our Corps staff and I’d like to thank our entire District team for the superb job they did for this great community. We were fortunate it didn’t rain more, but in many instances, flood fights are won before they’re ever fought because of outstanding planning and preparation,” he said.