Late last month Snohomish PUD submitted a preliminary permit application to investigate the
feasibility of a hydropower project at Sunset Falls on the Skykomish River.
The project would include a dam upstream of Sunset Falls that would impound 20 acre-feet of water in a reservoir that is 2 acres in surface area upstream of Canyon Falls. The project would be operated as a "run-of-river" facility (i.e. minimal storage relative to river flow).
Access to the dam and intake would be from existing residential access roads through Mt. Index Riversites on the south side of the river that would be improved as part of the proposed project. Flows would be diverted into a 2,000-foot long, unlined tunnel under Sunset Falls, to the powerhouse. A preliminary estimate of the tunnel diameter is 19 feet.
The powerhouse would be a semi-underground powerhouse housing twin 15 MW turbines and generators. A synchronous bypass bay with a 72-inch diameter fixed-cone (Howell-Bunger) valve is also assumed to be needed to control ramping rates downstream of the diversion structure in the event that one or both units shut down unexpectedly. The tailrace of the powerhouse would be approximately 15-feet high by 30-feet wide at its point of discharge to the pool downstream of Sunset Falls. The tailrace would be excavated out of sound rock, and would be concrete or steel lined as necessary.
This powerhouse would be located at the historic put-in at the base of Sunset Falls where the WDFW salmon trap-and-haul is now located. The PUD proposes to take over operation of the trap-and-haul facility, significantly upgrade the facility, and take over long-term maintenance. The PUD is considering options for public access to the pool downstream of Sunset Falls.
Total powerhouse capacity would be 2500 cfs which corresponds closely to mean annual discharge of 2455 cfs. Studies to evaluate aesthetic impacts to the falls will need to be conducted and it is likely that instream flow requirements will include a need for aesthetic flows.
This is not the first hydropower project that has been proposed at Sunset Falls. Snohomish PUD had a project (P-4786 and P-8574) in the early 1980's, Pacific Hydro submitted a permit application in October 1984 (P-8644) that they withdrew in September 1985, Sunset Falls LP submitted a permit application in September 1991 (P-11195) that they withdrew in September 1993 and Tacoma Public Utilities submitted a permit application in January 1992 (P-11216) that they withdrew in May 1992. However the river is part of the Washington State Scenic River system (RCW 79A.55.070), is within a Northwest Power and Conservation Council Protected Area from hydropower development, and has been recommended for designation as a Wild and Scenic River for its Scenic, Recreation, Fish, and Wildlife values.
American Whitewater appreciates the outreach Snohomish PUD has initiated with our organization and several other groups who are part of the Hydropower Reform Coalition. We will be soliciting feedback from our members and the community to determine how to respond to this project.
Read About the Sunset Falls Hydropower Project on the Snohomish PUD website.
Read the Preliminary Permit Application for this project (P-14295)