Cabinet Gorge Dam Total Dissolved Gas (TDG) Spillway Modifications

During periods of spill, Cabinet Gorge Dam was generating total dissolved gas (TDG) concentrations in excess of water quality standards. Through a series of feasibility studies, design evaluations, retrofits and/or modifications, Alden engineers worked to help Avista Corporation meet their FERC licensing requirements for the hydroelectric project located in Idaho on the Clark Fork River.

In early studies, the Alden team performed a 1:50 scale physical hydraulic model study to investigate re-commissioning of diversion tunnels used during construction of the project as bypass tunnels to pass spill flows at lower TDG levels than are generated during spillway operation. Studies revealed that the reduction in TDG afforded by the bypass tunnels was less than originally expected.

Additional feasibility studies were conducted to evaluate two alternatives for TDG reduction. One idea was the creation of off-stream “gas stripping” channels downstream of the dam, which ultimately proved to be fish passage un-friendly. Ultimately, modification of the existing spillway crest was found to be most feasible.

Full hydraulic and structural engineering services from feasibility through design and construction have been completed for the modifications to five of the eight spill bays in order to reduce TDG. Construction plans and specifications were prepared, and documentation for submittal to FERC was produced. Alden also provided construction technical support.

In addition to the work done to reduce TDG, Alden engineers also performed several structural projects for Cabinet Gorge Dam:

• A Stoplog Deployment Crane was designed and fabrication-level drawings developed for the deployment crane system for lifting 10-ton stoplogs. The steel frame was designed to allow assembly and disassembly at each of the eight spill bays. Performance specifications for the 15-ton top running hoist were also provided
• The FERC Supporting Technical Information (STI) Documents were reviewed and updated for Section 6—Hydrology/Hydraulics and Section— Stability/Stress Analysis.

In further compliance measures, the development of a fish trap design in the project tailrace for the expedited transmissivity of ESA listed bull trout past the project was supported. This effort also used the 1:50 scale model for initial site selection studies. Alden also provided a fisheries and hydraulic engineering representative on a “panel of experts” who advised on the best approach for trap site selection and design. Computational Fluid Dynamics (CFD) modeling of the project tailrace was performed to confirm satisfactory location and performance of the selected trap design. The CFD model developed for the fish trap studies also confirmed compatibility of the TDG mitigation measures with the fish trap operations and to guide future development of the TDG mitigation.