Last week’s historic flash flood of Mill Creek was devastating for many of our neighbors in the Walla Walla Valley, but the City of Walla Walla was again spared from the ruinous damage of 6,000 cubic feet per second of floodwater raging through the city. As we recover from the storm and prepare for the future, we should pause to remember the vision of city leaders such as Virgil Bennington, the ingenuity of an earlier generation of U.S. Army engineers, and the dedicated professionals who operate today’s Mill Creek Project.
After the Mill Creek Flood of 1931 severely damaged the city center, the Walla Walla Chamber of Commerce President Virgil Bennington and community leaders petitioned the U.S. Congress to design and construct a flood control project to protect the city. Congress authorized the Mill Creek Project in the Flood Control Act of 1938, and in 1942 completed the inventive system of diversion dams and off-stream storage. Walla Walla thrives as a city today in its current location, in part, because of this project.
The Mill Creek Project is simple in concept; minimize flood risk to the downtown by moving water quickly through the city using efficient concrete channels and divert any flows above 3,800 cubic-feet-per-second into a man-made lake (now called Bennington Lake) for enough time to let the flash flood subside. Engineers analyzed earlier Mill Creek flood events and learned that while the flows are furious, they are relatively short in duration. Ninety years after this system’s design, the Mill Creek Project continues to function as designed (despite not having the hydrologic modeling tools we have today).
Corps’ projects of this nature mitigate flood risk, they don’t eliminate flood risk. This recent flood was dangerous with flows above town exceeding 6,000 cfs. We came within 11 hours of reaching the diversion capacity of Bennington Lake, which was 80% full on Friday afternoon when Mill Creek finally subsided below 3,500 cfs. If the rain and warm temperatures had continued into late Saturday, we would have filled the reservoir and had no choice but to send flows from 5,000 cfs to 6,000 cfs through the city, overwhelming the channel and flooding hundreds of homes and businesses.
In 2019, Walla Walla’s civic leaders continued to honor the vision of Virgil Bennington by partnering with the U.S. Army Corps of Engineers to initiate a study that will recommend a cost-effective way of rehabilitating and improving the Mill Creek Project and the flood control channel through downtown Walla Walla. In a remarkable coincidence, the public meeting to discuss this study is Wednesday, February 12th, at 5:30 PM at the Marriott Courtyard Hotel on Rose Street. I invite you to join us, share your comments, and influence future Walla Walla flood protection.
Lieutenant Colonel Christian Dietz, Commander, Walla Walla District, U.S. Army Corps of Engineers.