Mitigating Total Dissolved Gas at Boundary Dam
This article was contributed by Nikou Jalayeri, Water Resources Engineer at HATCH
The Boundary Dam is located on the Pend Oreille River in northeastern Washington. The project consists of a 340 ft. high concrete arch dam, seven low level sluiceway outlets, two high level overflow spillways (Spillway 1 and Spillway 2), and an approximately 1,003 MW authorized capacity powerhouse. The spillway and sluiceway discharge at the Boundary Hydroelectric Development have been shown to produce high total dissolved gas (TDG) concentrations in the tailwater of the spillway and the river reach downstream. Studies were commissioned to determine modifications to the project’s spillway structures to help mitigate this gas production. Resolution of many of the hydraulic design issues for the study relied heavily on the results of numerical hydraulic models. These modifications were constructed and tested in the field. The CFD model that was developed in support of these studies was used to simulate flows through a number of the project’s seven sluice gates and two overflow spillways. This model was also used to simulate the entry and movement of these flows through the project’s downstream plunge pool and powerhouse area.
FLOW-3D was selected for the analysis given its ability to simulate free falling jets, and its unique algorithm for simulating air entrainment by turbulence at the free surface. These capabilities make the program very well suited for simulating the varied and complex flow conditions in the project tailrace. The FLOW-3D models developed for the Boundary Dam study have primarily been used to develop an understanding of the governing hydraulic and hydrodynamic processes driving gas exchange in the tailrace of the existing project under spill conditions. In addition, these models been used to develop the designs of structural TDG mitigation alternatives (including estimation of the hydraulic loads expected on proposed appurtenances), and in combination with the TDG predictive model, to predict the TDG performance of proposed TDG mitigation alternatives.
To do so, representative air bubbles were released on the spillway in the model and tracked as they were entrained into the plunge pool and tailrace, circulated within the plunge pool, and eventually exhausted at the surface. The model tracked the pressure- and time-histories associated with each of these representative air bubbles. This data was then used as input to a TDG predictive tool to help predict total dissolved gas production in the tailrace. The overall predictive performance was successfully calibrated and validated to actual prototype (field) TDG data. TDG predictions were made for the project using a two-step process: the CFD model was first applied to assess the plunge pool hydraulics and flow patterns, and then the hydraulic output of the CFD model was imported into the Plunge Pool Gas Transfer (PPGT) model, which was developed using Excel.
The model was first run to simulate flow conditions for the existing or base case scenario with flows of 10,000, 13,000, and 20,000 cfs through each of the Project Spillways. The simulated hydraulic conditions for this test were analyzed. Bubble particles were then added to this model, the run was re-started, and the particles were tracked until they were able to reach the surface, and exhaust back into the atmosphere.
Following the base case runs, various CFD simulations were conducted to assess the hydraulic conditions that would result from the introduction of Roughness Elements (REs) on the downstream end of the spillway chute. The introduction of these REs helps to break up the jet at the end of the chute more quickly and efficiently, accelerating boundary layer growth and resulting in the formation of small “packets” of water entering the plunge pool rather than coherent streams/jets. This accelerated breakup of the jet will help to reduce overall plunge depths, and reduce gas transfer. Given concerns for potential cavitation damage on the spillway chute floor and on the REs themselves, additional runs were undertaken to test the effect on flow conditions at the REs if a ramp were to be installed immediately upstream of the roughness elements. The Spillway 1 RE geometry is presented in Figure 1.
The final model results were used to help assess the impact that the addition of these modifications would have on TDG levels downstream of the project under a range of flows. CFD runs were made with identical flow releases through the spillways under both existing and modified conditions, bubble histories were extracted from the CFD results and input to the TDG predictive spreadsheet model. The results showed that the proposed RE configuration for Spillway 1 is effective at reducing TDG production, but appears to deliver the greatest TDG reduction when operating at a flow of approximately 10,000 cfs. For higher flows, the ability of the roughness elements to break up the jet appears to be reduced, since the jet begins to override the roughness elements. This results in the formation of a more competent jet core that is able to penetrate the plunge pool to a greater depth. Figure 2 illustrates the difference between the baseline (existing) case and the modified Spillway 1 for flows of 10,000 cfs and 13,000 cfs respectively.