USACE begins analysis of Mill Creek flood risks

By on Sunday, February 3rd, 2019 in Local News

The Mill Creek Channel in downtown Walla Walla during flood stage in December 2017. MYCB file photo, courtesy of Cole Massey.

WALLA WALLA, Washington – The U.S. Army Corps of Engineers is partnering with Walla Walla County, as well as other local, state and federal agencies to conduct a general investigation study of the overall flood risks Mill Creek poses to Walla Walla and surrounding communities.

According to the Corps, the Mill Creek flood-risk management system consists of four main components:

1) Bennington Dam, which impounds Bennington Lake, an off-channel reservoir

2) Diversion Dam, which diverts flood water through a channel into Bennington Lake

3) Leveed channel (upstream and downstream of Walla Walla),

4) The concrete channel through downtown Walla Walla

In 1996, a flood caused significant damage to the levee system, flooding upstream and downstream of Lathe channel and damaging and nearly overlapping parts of the concrete channel. The flood also exposed scour issues and raised embankment erosion concerns.

Some of the components of the system have degraded, resulting in decreased capacity and reliability, the Corps states in a fact sheet about the Mill Creek General Investigation Study.

The Mill Creek GIS will consider the reduced capacity, performance and reliability of several components of the existing system. It also will develop a range of alternatives that could be implemented to reduce those risks.

“The local governments will say these are the elements we believe are important from an operational standpoint, from the access standpoint – whatever the elements are,” Port of Walla Walla Director Pat Reay said. “Once those elements are provided to the Corps, they they’ll determine a cost benefit analysis. There has to be a federal interest.”

Reay adds there’s a 35 percent local contribution so whether the improvement ideas are big or small, if it’s not in the federal government’s best interests, locals will contribute 100 percent of the costs to any improvements not approved by the Corps.

The Corps said the study process will take about three years to complete and will include outreach activities with citizens and stakeholders across the community to gather their thoughts and ideas related to Mill Creek.

“There’s a lot of elements I know folks want to see incorporated into the channel – stream naturalization, access to recreation – all these things that are great ideas, great concepts, but the reality of the federal government paying for them is not going to happen,” Reay said. “This is a flood control project, that’s the reason this infrastructure was built – to protect life and property.”

The Port of Walla Walla is going to provide some concepts and ideas of some elements to the plan to the Corps. Reay said the Corps will then model the idea and do some cost analysis to depend whose is paying for what portion of the project.  A coalition of local governments meets monthly with the Corps. Right now, the Corps is conducting detailed analysis and federal reviews. Reay anticipates a major update on the study this summer.

Alternative solutions could include a rehabilitated channel or a new bypass channel to reduce the risk of flooding and infrastructure collapse within the population and commercial case of Walla Walla.