Fact Sheet: Snoqualmie Flood Reduction Project
(also known as Snoqualmie 205 Project)

The following information is provided by the U.S. Army Corps of Engineers.

The Project:

This Corps of Engineers Project is being developed under the continuing authority contained in Section 205 of the 1948 Flood Control Act, as amended. The project sponsors are King County and the City of Snoqualmie. The authorizing report for this project was completed in December 1999. The plan recommended in this document was known as the "3 element plan" consisting of the following major elements: 1) about 340 linear feet of right bank widening of the Snoqualmie River just upstream of Snoqualmie Falls, 2) about 500 linear feet of left bank widening of the Snoqualmie River just downstream of the Highway 202 bridge, and 3) removal of an abandoned railroad bridge just downstream of the city. Please refer to attached map.

Project Benefits:

Upstream flood damage reduction benefits in and near the city of Snoqualmie would be achieved by the removal of constrictions in the Snoqualmie River and improved flood conveyance of waters. The 3 element plan would reduce the flood peak for all levels of flooding. For instance, in the heart of the city, the flood peak would be reduced by about 0.7 feet for the fairly common 5-year flood, about 1.1 feet for the 50-year flood, and about 1.2 feet for the rare 100-year flood. The flood reduction benefits would be diminished upstream of the city, and would not be significant upstream of the "3 Forks" area.

Downstream Impacts (1999):

The December 1999 report stated (within the Executive Summary, the body of the report, and the Finding of No Significant Impact) that the 3 element plan, by improving flood conveyance in the area between the Falls and the Highway 202 bridge, would result in a very slight increase in the flood peaks downstream of the Falls and would result in the flood peak arriving slightly sooner in the downstream communities. The December 1999 report estimated that for the 100-year flood the downstream flood peak would increase by about 1 inch from Snoqualmie Falls downstream to Carnation. For the more common smaller floods, the flood peak would increase by a fraction of an inch. The December 1999 report stated that with construction of the project the flood peak would arrive about 15 minutes sooner in Fall City and about one hour sooner in Carnation. Downstream of Carnation there should be no measurable impacts from the 3 element plan.

Downstream Impacts (2001):

Studies conducted in 2001estimate that the downstream 100-year flood peak would increase by about 0.1 foot at Fall City and less than 0.1 foot at Carnation. The 100-year flood peak was estimated to arrive 15 to 20 minutes sooner at both Fall City and Carnation under project conditions. The impacts from the project lessen with downstream distance from Snoqualmie Falls and are estimated to be zero downstream of Duvall.

For questions about the Snoqualmie 205 Project, please contact Tom Bean, Senior Engineer, River and Floodplain Management Section.