Corps revises 205 project downstream impacts
October 2, 2008 · Updated 2:19 PM
CARNATION - If work begins this summer on a project to widen the Snoqualmie River near the Falls, involving the excavation of nearly 50,000 cubic yards of dirt and rock from the right and left banks, the impact of the project to downstream residents would be minimal.
That's according to a new study conducted by the Army Corps of Engineers, which is working with the city of Snoqualmie and King County to design and build the Snoqualmie River Flood Damage Reduction Project. Scheduled to begin this summer, the flood-reduction effort, also known as the 205 project, is expected to reduce flooding in downtown Snoqualmie during a 100-year flood by 1.2 feet, and by more than half a foot during a five-year flood. It would also save an average of more than $800,000 per year in flood-related damages to businesses and houses.
The Corps of Engineers undertook the study to double-check the expected downstream impacts first outlined in its December 1999 final report on the 205 project, "to make sure that we haven't missed something," said Dennis Mekkers, a hydraulic engineer with the Corps of Engineers' Seattle District. That report had concluded that if the 205 project was built, residents living in and around the cities of Fall City and Carnation would see flood waters rise by 1 inch during a 100-year flood. The new study, however, revises that figure to 0.1 feet, or a little more than 1 inch.
Many Lower Valley residents, though, don't trust those numbers. At a meeting Feb. 14 at Carnation Elementary School, they said the study should take into account other factors when it comes to flooding, such as developments going in upstream and the effect of tributaries on the entire river system.
"But that's not the only thing that's happening," said Van Strom, who lives on the West Snoqualmie River Road in Carnation, of the potential impact of the 205 project. "It's not your little project, it's everything that's being done."
The $3 million project - of which the Corps would fund 65 percent, with the city and county splitting the remaining 35 percent - contains four elements to address flooding in Snoqualmie. The first one involves excavating 340 linear feet of rock and dirt from the right bank of the river, just upstream of Snoqualmie Falls. Then 500 linear feet of material would be excavated along the left bank, just downstream of the State Route 202 bridge. The Corps also plans to remove what remains of a railroad bridge downstream of the city and reinforce the right bank where the river bows downstream of the SR 202 bridge.
Paul Cooke, the Corps' project manager, said by widening the banks, the water would move more quickly through Snoqualmie, reducing the level of flood waters.
"What this [the 205 project] does is to make this bottleneck here ... a little less of a bottleneck," he said. "All of the flood waters have to go through this bottleneck, and that's what backs up flood waters into the city of Snoqualmie."
The project, he added, is not a cure-all, and floods would still affect the city once the project is completed.
"They'll still get wet in floods," Cooke told Lower Valley residents. "But when it does flood, they shouldn't get hurt as much as they're getting hurt now."
In its 1999 report, the Corps estimated downstream impacts of the 205 project based on previous flood-reduction plans, including the Snohomish Mediated Agreement of 1981, which proposed excavating 710,000 cubic yards of material to widen the river through Snoqualmie. Mekkers said the agreement would have destroyed large riparian areas around the city, and it also called for a series of setback levies in North Bend and a dam for the North Fork of the Snoqualmie River. That plan was expected to reduce flooding during a 100-year flood by two to three feet in Snoqualmie.
A similar proposal was put forth in 1998, but did not include the dam on the North Fork. The downstream impacts from it were expected to be 2 1/2 inches more water downstream during a 100-year flood, with the flood wave arriving one to two hours sooner at Carnation.
Using those two proposals as a model, the Corps estimated in its 1999 report that an inch of water would be added downstream during a 100-year flood if the 205 project was built, but the further one got, the less of an effect the project would have. Downstream of Carnation, Corps officials said, the project would not have a noticeable impact.
With the new study, the impact downstream was updated to a tenth of a foot of additional water during a 100-year flood, and the peak of the flood would arrive 15 minutes sooner at Fall City and an hour sooner at Carnation.
Steve Barton, a hydraulic engineer with the Corps, said in the event of a flood, the 205 project would not add a large amount of water to an already swollen river, and the velocity of the river would not increase substantially.
"One could conclude that the change in velocity over the existing conditions would be slight," he said.
But with the flood wave reaching the Lower Valley sooner than normal, Steve Hallstrom, who lives on the Tolt River Road in Carnation, said flood waves from the nearby Tolt and Raging rivers wouldn't have a chance to dissipate, adding "Every small flood is going to bring these waters together."
Strom said the impacts of everything happening upstream must be taken into account when making calculations on how the 205 project would affect downstream residents, because he's already feeling those effects.
"Flooding is becoming more frequent," he said. "Last year, I bet there was seven or eight floods where I could not get out of our property."
To analyze the Corps' own analysis of its estimates, the city of Snoqualmie hired Northwest Hydraulic Consultants of Seattle to go over the new numbers, which agreed with the Corps' findings.
"The most significant impact of this project is the larger flood, the 100-year," said consulting-firm engineer D.G. Mutter. "At lesser and lesser floods than the 100-year, the impact is going to be significantly less."
The 205 project is scheduled to begin this summer, but it must first make its way through a lengthy permitting process and win approval from the Metropolitan King County Council this spring. Several Lower Valley residents - at the meeting Feb. 14 and at a similar meeting in January - questioned why Snoqualmie, the county and the Corps are rushing to build the project. Last week, Dave Clark, manager of the Rivers Section of the county Water and Land Resources Division, admitted that a lot must be completed before construction can begin, and time was running out.
"The likelihood of that [finishing the project this year] is getting less by the day," he told the audience.
The Feb. 14 meeting was the third in a series of four to discuss downstream impacts of the 205 project. The last meeting will be held at 7 p.m. Feb. 28 at the elementary school, and officials plan to discuss potential mitigation work to be done in the Lower Valley, such as raising homes.