Suttle Lake Dam Demolition
remove the concrete dam structures (including wing walls and fish ladder), provide year round fish passage, improve spawning and rearing habitat at the outlet, and lower the current lake water surface elevation by 0.3 feet. This slight lowering of the water surface elevation will help reduce the frequency of flooding at the Suttle Lake Lodge while maintaining established riparian vegetation and wetland features.
1) Construct approximately 220 feet of side channel
2) Construct an 0.3 acre island
3) Construct an 220 foot long riffle at 1.7 % slope
4) Reduce the lake water surface elevation approximately 0.3 feet
Designs were prepared for the constructed riffle, the island, the side channel and the engineered log jams. The following sections present the information used for the design. The design plan set (Attachment 4) has detailed drawings of the project plan.
The control for the lake level elevation shall be constructed at Station 195 which is 160 ft upstream of the current dam control at Station 355. In order to maintain the same or slightly lower lake elevation at base flow, an elevation 0.3 ft lower than the dam elevation shall be used as the elevation of the new control (control elev = 3440.9 ft). The control shall be a pool-tail crest in the mainstem of a split channel around a vegetated island. The design discharge for the split channel at the control is 70 cfs and has an estimated return interval of 1.0. Because Lake Creek, especially near Suttle Lake, does not transport a large amount of bed load, the design team believed it was more important to design the channel to provide high quality side channel habitat and riparian vegetation.
The cross sectional area shall slightly increase in the mainstem in a downstream direction by elevating the mainstem right bank and island. The island shall have a slight adverse slope from the head to the existing dam site at Station 355 that will allow the head to be inundated at approximately 40 cfs and the dam site at approximately 87 cfs. The as-built channel bed depths shall be slightly less to allow for settling of the unconsolidated bed material. Design channel dimensions for both the pool-tail crest control and the riffle and side channel are located in Attachment 4.
The constructed riffle shall be built at a 1.7 % gradient which is within the range of other riffle slopes in the system. The riffle shall be constructed using approximately 980 cubic yards of materials in the approximate quantities: sand/silt (21%), gravel (32 %), cobble (41 %), and boulder (6%). The USFS COR(s) or Inspector(s) shall direct the contractor in mixing of above mentioned materials. Round boulders shall be embedded into the surface to promote a low flow fish passage channel and to provide velocity breaks for fish to hold and rest in while migrating. Sand/silt, gravel and cobble for the riffle will come from Black Butte Pit on USFS lands located 5.3 miles from the project sit. Small round boulders will come from a source off forest road 2066 located 2 miles from the site (See Attachments 5 and 6 for mileages, directions and locator map).
Approximately 30 cubic yards of concrete will be removed from the 1960âs dam which shall include the dam, wing walls and the fish ladder. Another 15-20 cubic yards from the 1930âs dam shall also be removed and disposed of. All removed material shall be disposed of off National Forest Lands.
Several whole trees shall be buried into the island at the head of the island, at the hydraulic control of the lake outlet, and at the head of the existing pool at the downstream end of the project just above the vehicle bridge. Individual trees shall be buried into the island and shall extend into the riffle and/or the side channel to create additional habitat features. A total of approximately 70 trees will be needed for the entire project. Trees for the log jams are 30-50 feet in