On Friday, representatives from the Chehalis River Basin Flood Control Zone District toured the site of a proposed dam in the upper reaches of the Chehalis River Basin near Pe Ell.
The land where the dam would be built is owned by Weyerhaeuser and is part of its 190,000-acre tree farm in western Lewis County.
The district accepted sponsorship of the dam earlier this year, meaning it will be the agency to apply for permits throughout the early steps of the process, District Administrator and Lewis County Public Works Director Erik Martin said. Ownership of the dam has yet to be determined, though it could be the property of the district, the state or another agency.
“This is a big step for us,” County Commissioner Edna Fund said.
Fund, along with commissioners Bobby Jackson and Gary Stamper, were on the tour Friday. In addition to their roles as commissioners, they are also acting as the supervisors of the district.
The district is ready to start the permitting process, which will trigger environmental reviews from the U.S. Army Corps of Engineers, Martin said. During that process, the Army Corps will determine what the environmental impacts are, if and how they can be mitigated or whether other projects could accomplish the same goals.
Early studies show the proposed dam would have reduced the flood level on the Chehalis River at Mellen Street in Centralia by 1 foot had it been in place during the 2007 flood, Martin said. Interstate 5 would have been closed for 24 hours and not three days, he said.
HDR Engineering, the Olympia-based engineering firm hired to design the project, provided the district with three options for a dam.
The first would be for flood retention only. The reservoir would normally be dry except during flood events, HDR Engineer Keith Moen said. The Chehalis River would flow through it normally until the flood gate is shut to hold back a flood. It has the capacity to store 65,000 acre-feet of water at a time. The reservoir created would be 6.8 miles long at full capacity, covering 863 acres of land.
This option would allow for flood storage up to 32 days. The majority of the trees in the reservoir would be able to survive a short time under water, Moen said. Debris is to be expected in any flooding event, so crews will have to clean it out every few years to allow water to flow and to prevent damage to the dam.
Fish would be able to pass through the dam except when the reservoir is in use, he added. As part of the project, a fish trap would be put in place on the downstream side of the dam to store fish traveling up stream. From there, they would be transported to the top of the dam. It will only be operated when the reservoir is in use.
This option costs between $277 million and $416 million, according to the HDR report. The final design of the dam has not been done so an exact cost is not known.
The second option is a larger dam to control flooding and regulate the flow of the river. The 65,000-acre-foot reservoir would be permanent and have the capacity for an additional 65,000 acre-feet of storage. The length of the reservoir when full would be 7.5 miles, and the amount of land underwater at its 130,000 acre-feet max capacity would be 1,344 acres.
A fish collection and transportation system would have to be operated year round, Moen said. A fish ladder would not work in this situation because the reservoir would vary in levels depending on the season, but the water could be stored to be slowly let out during the drier summer months to improve water quality and habitat.
The estimated cost of this option ranges from $454 million to $662 million.
The third option is a flood retention only dam that can be expanded to a flood retention and flow regulation dam at a later date. Moen said the only difference between this option and the first option is it would be built with a wider foundation to support possible expansions. The estimated cost of the first phase of construction is between $339 million and $484 million with the estimated cost of the expansion being between $179 million and $266 million.
“We believe the expandable version is the best option for us,” Fund said.
The funding for the dam could come from several different places, she added, including from the state capital budget or from the federal government. The Office of the Chehalis River Basin was established by the state Legislature, but an expected allocation of more than $50 million in funding didn’t come through due to a lack of an agreement on the state capital budget.
Currently, the district has some funds available for the permitting processes, Martin said.
“We probably would have been further along if we had a state capital budget,” he added.
In preparation for the project, HDR has conducted extensive geological and seismic research and tests to determine if a dam would be safe to build and how to build it so it can withstand these events, Moen said. The rock layer under the dam is solid and shafts will be drilled into it for concrete to be poured down to form the foundation.
The spillway over the top of the dam would be designed like a ramp, which would cause the water to head straight up into the air and then down, digging a hole and defusing its energy, Moen said. It would then continue downstream at a normal rate.
Fund said she has received comments from constituents voicing their concerns about the safety of the dam.
“This would be one with current technology,” Fund said.
Moen said the technology for building dams has improved a great deal over the last 30 years or so, and extensive research has been done before the permitting process begins. The reasons why older dams have failed in the past have been identified and corrected, he said.
The permitting agencies, such as the Army Corps of Engineers, have a lot of influence over the project during the permitting process, Martin said. If they find areas of concern, they will identify and address them.
Building a dam is one option being considered to reduce flooding in the region. Other options include more extensive habitat revitalization and moving residents out of the floodplain, along with more traditional projects such as levees.
Habitat conservation and revitalization projects would be done by the district and other agencies after the dam is built, Fund said, adding those duties reflect about half of the district’s workload.