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.
Dive in Deeper
Read more about Total Dissolved Gas at High Head Dams in this three-part blog series.
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Civil InfrastructureNatural Resources & Environmental Planning
Services
Hydraulic Structure Engineering DesignFish Passage Design, Modeling and Testing
Fish Protection Design, Modeling and Testing
Related Projects
An existing roof vent arrangement was allowing rainwater to enter the Pot Room. Alden supported efforts to develop a roof vent geometry to eliminate the intrusion of rain water. The purpose of the CFD study was to ensure that the roof vent modification did not increase pot room temperature levels beyond specified limits for workers in the plant.
To evaluate the existing and proposed Pot Room arrangements, thermal and fluid flow profiles in the immediate vicinity of the pots were determined based on air flows through the plant floor and wall mounted vents. The detailed CFD model was developed from plant drawings to include all major basement, pot room and roof venting geometries. The surrounding ambient environment was included with quiescent atmospheric conditions and average ambient temperature. Thermal losses form the pots to the pot room air and from the pot room to the environment were included in the analysis. The results of the CFD modeling showed that the proposed modification to the roof venting arrangement was acceptable and would not increase the temperature in the worker-occupied spaces by more than 2 degrees F.
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Civil Infrastructure
Smelter Pot Room Roof Ventilation System
Read how a CFD study ensured that a roof vent modification did not increase pot room temperature levels beyond safe levels
Plant McDonough, owned and operated by Southern Company, has experienced excessive siltation at the makeup water intake. The intake uses cylindrical wedgewire screening within an intake originally designed for much larger, once-through cooling water flows. Flow modeling was performed to provide a viable passive solution to reducing the sediment accumulation at the intake. To model the geometric details of the system accurately, a field survey was performed prior to the flow modeling efforts. The flow study included both CFD modeling and scale physical modeling.
For this investigation, Alden developed a 1:20 scale live bed physical model. This model was extremely well tuned to reproduce the behavior of bed load sediment. Even with the very fine crushed walnut shell particles, however, it was challenging to reproduce the behavior of suspended load. The use of a high fidelity CFD model, therefore, proved extremely useful for this project, in that suspended load is generally very accurately tracked with CFD models, which are not well validated for bed load simulation. By using the two together, the two extremes of sediment transport are captured, and developing a solution that covers this range has a high likelihood of success.
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Civil Infrastructure | Hydrology Hydraulics and Fluids
Plant McDonough Intake Modification
CFD and physical model study to assist in the evaluation of a solution to reduce the sediment accumulation.