The Study
In October 2018, the U.S. Army Corps of Engineers, Walla Walla District, in partnership with the non-federal sponsor, Walla Walla County, began a General Investigation (GI) study on the Mill Creek Flood Control Project.
The purpose of the study was to address the risks Mill Creek presents to the community of Walla Walla, Washington, and other communities within the Mill Creek watershed (adjacent lands). A draft Finding of No Significant Impact (FONSI) and a draft Integrated Feasibility Report and Environmental Assessment Report (FR/EA) were released for a 30-day public review and comment period on January 15, 2020.
The Flood
In February 2020, warm winter temperatures and 8 inches of rain in the Blue Mountains over 48 hours led to historic river flows throughout the region. The Mill Creek Flood Control Project was instrumental in protecting the city of Walla Walla from damage during the flood event.
The Walla Walla District sent nine teams to Waitsburg, Starbuck, Camp Wooten, Kamiah, Mill Creek, Dayton, Milton-Freewater, the city of Walla Walla water line, and Walla Walla’s bridges, to provide technical assistance in flood response operations, damage assessments and flood fight techniques. Corps officials also supplemented state and local efforts and provided 20,000 sandbags to Columbia County, 20,000 to Umatilla County, 10,000 to the city of Waitsburg and 5,000 to Walla Walla County.
The flood caused extensive erosion damage to levees throughout the region. The Corps oversaw the efforts to rehabilitate 12,000 feet of levee systems and constructed over 1,500 feet of levees before the 2021 flood season. This included repairs to 3,500 feet of levees along the Walla Walla River in Milton Freewater where riprap had been displaced and levees slopes had to be rebuilt.
The Continuing Authorities Program
The 2020 flood event helped confirm the modeling completed within the Mill Creek GI study.
“We were able to look at areas throughout the system and validate our modeling assumptions. The flood also helped us adjust our tentatively selected plan (eventual recommended plan) in locations that earlier modeling efforts did not address” said Alex Colter, project manager for the Walla Walla District.
The final report for the study was approved by Northwestern Division on September 2, 2021 and included a recommended plan for improvements to the Mill Creek Flood Control Project.
The recommended plan (Alternative 6b from the report) includes the following actions:
- Increase levee heights by up to 2.2 ft in low assurance areas along Mill Creek, to increases the system’s capacity.
- Repair and reinforce the concrete channel through downtown Walla Walla to improve the reliability of the flood control project.
- Improve aquatic habitat and fish passage conditions by increasing the floodwater diversion trigger from 1,400 cubic feet per second (cfs) to 1,700 cfs and remove a portion of the concrete channel ceiling.
The estimated cost for the recommended plan fell beneath the parameters for a typical General Investigation Study. Because of this, the project was transferred to the Continuing Authorities Program (CAP) for funding.
Once CAP funding is acquired, the Corps expects to initiate design for the recommended plan and complete it within Fiscal Year 2022. Construction for the plan is expected to begin in Fiscal year 2023. The design and construction phases will be cost-shared 50/50 with the non-federal sponsor, Walla Walla County.