Posted on August 16, 2018
A long-delayed final environmental impact study for protecting lower Cowlitz River cities from flooding related to Mount St. Helens has been released for public review and comment, marking a “milestone” for community safety, according to the U.S. Army Corps of Engineers.
The plan consists of three basic elements proposed earlier: Raising the sediment-retaining dam on the north fork of the Toutle River, dredging the Cowlitz as needed, and building structures in the upper Toutle River Valley to trap additional debris.
However, new to the plan is a proposal to drastically reconstruct the fish trap just downstream of existing sediment retaining dam, which was built in the mid-1980s to intercept returning adult salmon so they could be trucked above the 125-foot-tall dam. It has never worked to the satisfaction of fish and wildlife biologists.
A preliminary estimate for the fish trap modification is $20 million, and the fisheries project will include a new fish release site on Deer Creek above the dam.
Comments on the plan must be submitted by Sept. 30.
The plan is a blueprint for controlling the flow of volcanic silt, which washes off the giant debris avalanche that filled the upper Toutle Valley during the volcano’s eruption on May 18, 1980. The document has been under development and negotiation for almost a decade and had to overcome opposition from federal fisheries officials and the Cowlitz Tribe, which have now signed onto it.
Mike Turaski, the Cowlitz-Toutle project engineer for the Corps Portland District, said the study and the soon-to-come record of decision are a “milestone” that will help the corps plan and budget for the future.
“This is the culmination for a lot of planning and engineering and talking with and hearing from the public and other resource agencies about how we continue to meet the flood-risk reduction mission in the Cowlitz Basin while protecting other natural resources,” Turaski said on Monday.
He noted that fish and wildlife concerns “are more prominent now than they were in the immediate aftermath of the eruption.”
The corps completed a sediment-retaining dam in 1989 just east of Kid Valley. The earth-and-rock structure has trapped hundreds of millions of cubic yards of debris and prevented it from flowing into the lower Toutle, Cowlitz and Columbia rivers. But the dam filled up within a decade and no longer blocks silt effectively.
As silt flows into the lower rivers, it reduces their flood-carrying capacity, though flood protection levels were at federally mandated levels when the corps last checked two years ago, Turaski said.
The corps has not monitored the flow of silt into the river for a couple years and is hoping to resume doing so next year, depending on whether it gets the necessary funding. Based on the findings, it will design and schedule the modifications to the fish trap and the spillway raise of the dam, which will increase its silt-storage capacity.
The corps raised the spillway seven feet in 2012 and could raise it an additional 16 feet, though a raise of 10 to 13 feet may be more likely.
Monitoring the flow and build-up of silt in the rivers “is important. It helps us make sound engineering judgments on the size and timing of the spillway raise,” Turaski said. However, the corps will start engineering and designing the project in case studies show there is an urgent need for action, he said.
No dredging of the Cowlitz River is anticipated to maintain the authorized levels of flood protection, he said.
Corps officials have emphasized the need for developing a flexible plan to adapt to the irregular nature of the silt flow, which is highly dependent on the intensity of storms that strike the upper Toutle Valley.
Source: TDN.com
