A redesigned water retention system on the Chehalis River just outside Pe Ell would be “tailored to the unique needs of the Chehalis Basin,” according to a new video created by the Chehalis River Basin Flood Control Zone District.
“This technologically advanced structure allows the river to flow naturally, and fish to swim up and downstream without obstacle, under all but the most severe weather conditions,” the video states.
The dam would only hold back a reservoir at times when flooding is in the forecast.
The structure’s design changes will address many of the concerns raised in state and federal environmental reviews completed over the last several years. Updates of those reviews — taking into account the district’s alterations to the dam’s design — are expected over the next year.
The video, released earlier this month, comes as progress continues on the redesigned structure, which aims to reduce flooding in the Chehalis River during rare flooding events. The video also shows a new location for the structure, which was done to “minimize impacts to a nearby area of cultural significance to the Confederated Tribes of the Chehalis Reservation,” according to a refreshed Flood Control Zone District website highlighting the changes.
The redesigned structure would reduce the size of the reservoir by 32 acres, which the Flood Control Zone District said would “result in less vegetation being impacted during operation.”
The Chehalis River Basin Flood Control Zone District will host a one hour presentation on the project Tuesday to provide updates. The presentation will be held in the Board of County Commissioners hearing room from 4:30 to 5:30 p.m. in the Lewis County Courthouse, located at 351 NW North St. in Chehalis.
A virtual option is available for residents unable to attend in person, which can be found here.
In the video, a narrator highlights the destruction that can result from catastrophic flooding and how the proposed structure would differ from a traditional dam.
“Traditional responses to flooding, like many of the aging dams across the nation, reduced flood risk to people at the expense of fish and a river’s natural flow,” the narrator states. “But modern advanced technology means we don’t have to make this trade-off anymore.”
According to the video, the structure would reduce the impact on fish, trout and other fish in the Chehalis River. Under typical conditions, the structure “mimics” the height and flow of the riverbed, allowing fish to swim up and down the river unimpeded.
“Instead of hitting a barrier, as they would at a traditional dam, fish can move unaided through the structure through open pathways called conduits,” the narrator states. “The conduits mirror the depths, flow rates and sediment fish find in other parts of the river during migration.”
During severe storms that could cause catastrophic flooding, like the flooding of 2007, the structure would close and water would be collected in a basin. According to the video, the structure would have held back water three times in the last 20 years, though the “benefits would have been substantial.”
“Thousands of structures and millions of dollars of damage to homes, farms and other infrastructure would have been avoided,” the narrator states.
The video highlights the flood of 2007, an event seared into the minds of many longtime residents of Lewis County. During the December flood, Interstate 5 was shut down for nearly a week as water covered much of the roadway while more than 1,300 homes were destroyed.
The flood and its aftermath caused more than $900 million in damage in the Chehalis River Basin.
In the video, the narrator states that “I-5 would have been protected” during severe weather events had the structure been in operation and “thousands of people” would have been able to stay in their homes.
According to a Frequently Asked Questions section on the Flood Control Zone District website, a major flooding event is defined as a “river flow of 38,800 cubic feet per second (cfs)” at the US Geological Survey gauge in Grand Mound, which typically occurs in the late fall and winter.
When activated the dam reservoir could hold approximately 62,000 acre-feet of water or 20.2 billion gallons.
In these instances, when the structure is activated, crews would utilize an “advanced trap and haul facility” to transport fish around the structure, according to the video.
“After the flood risk has passed, the conduit gates would reopen to allow fish to move freely through the dam again,” the narrator states.
According to an estimate from the Flood Control Zone District, the reservoir would drain in approximately 32 days.
Look for a full story on the proposal in the Thursday, Sept. 5, edition of The Chronicle.
