Total Dissolved Gas Concentration Research Articles

Experimental and Simulation Investigation of Total Dissolved Gas Prediction in Supersaturated Water Treatment: Focusing on Source Calibration and Combining with Bubble Coalescence

Supersaturation of total dissolved gas (TDG) is a common occurrence in high dams during the spill process, which can lead to fish bubble disease and threaten aquatic organisms. As a result, many scholars have taken a keen interest in this issue. Research into the gas-liquid mass transfer mechanism has improved the accuracy of TDG prediction models. This article investigates the factors affecting TDG dissipation through laboratory experiments. Especially, the mass transfer coefficient across the bubble interface based on the slip penetration model was calibrated, and the coaxial bubble coalescence added to the coefficient was studied. The results show that the aeration rate has the most significant impact, followed by water depth, and then the aeration aperture. The TDG dissipation rate increases with the parameter β, and the parameter of 0.35 shows the highest correlation with the experimental data. Furthermore, the concentration change rate after coalescence is lower than before, suggesting that aggregation negatively impacts mass transfer. This study improves TDG concentration prediction accuracy and proposes measures for mitigating supersaturation to avoid fish bubble disease.

Predicting Total Dissolved Gas Concentration on a Daily Scale Using Kriging Interpolation, Response Surface Method and Artificial Neural Network: Case Study of Columbia River Basin Dams, USA

Total dissolved gas (TDG) is an important factor for aquatic life and can cause gas bubble trauma in fish if the concentration is higher than 110%. Dissolved gas is entrained in the water over the spillways of dams. Generally, total dissolved gas is simulated and predicted using models based on fluid mechanics, hydrodynamics and mass exchange processes. In the present study, two novel data-driven techniques, namely kriging interpolation method (KIM) and response surface method (RSM), were proposed for predicting total dissolved gas, measured on a daily scale at the upstream and downstream of spillways at four different dams’ reservoir sites located in Columbia River, USA. For developing models, we selected several input variables, namely water temperature, barometric pressure, spill from dam and discharge; in addition, total dissolved gas measured as percent of saturation (%) was selected as the predicted variable. Results obtained from the newly proposed models were compared with those obtained with the standard feedforward neural networks (FFNN) model to assess their performances. The proposed models were developed and compared with each other based on several input combinations. Four statistical indexes were utilized to evaluate models’ performances: coefficient of correlation (R), Nash–Sutcliffe efficiency (NSE), root-mean-squared error (RMSE) and mean absolute error (MAE). The results obtained clearly show that: (1) the KIM model is better than the RSM and FFNN models at three dams and FFNN is the best for the fourth; (2) the RSM model is ranked in the third place and provided the lowest accuracy; and (3) the highest R and NSE in addition to the lowest RMSE and MAE are obtained when the models include all the four input variables. The R, NSE, RMSE and MAE of the best KIM model among the four dam’s reservoirs are 0.973, 0.941, 1.462 and 1.122 while the corresponding values of the best FFNN (RSM) model are 0.962 (0.952), 0.926 (0.906), 1.643 (1.848) and 1.297 (1.426), respectively.

A parameter analysis of a two-phase flow model for supersaturated total dissolved gas downstream spillways

A high concentration of the total dissolved gas (TDG) in a flow downstream high dams may cause the gas bubble disease in fishes. To better understand the spatial distribution of a supersaturated TDG, a numerical simulation approach for determining the TDG concentration is shown to be effective and convenient; however, the determination of the model parameters relies to a great extent on the observed field data, which are scarce but are very sensitive to the accuracy of the simulation. In this regard, determining the source parameter in the TDG transport equation is the primary concern of this paper. Observed field data from six different spillways in China are used to calibrate the source parameter. A relationship between the source parameter and the hydrodynamic characteristics is established. The inclusion of this relationship in the predictive relationship will enable an accurate and rapid estimation of the source parameter and may help in developing mitigation measures for the TDG supersaturation downstream the spillways.

A laterally averaged two-dimensional simulation of unsteady supersaturated total dissolved gas in deep reservoir

Elevated levels of the Total Dissolved Gas (TDG) may be reached downstream of dams, leading to increased incidences of gas bubble diseases in fish. The supersaturated TDG dissipates and transports more slowly in reservoirs than in natural rivers because of the greater depth and the lower turbulence, which endangers the fish more seriously. With consideration of the topographical characteristics of a deep reservoir, a laterally averaged two-dimensional unsteady TDG model for deep reservoir is proposed. The dissipation process of the TDG inside the waterbody and the mass transfer through the free surface are separately modeled with different functions in the model. Hydrodynamics equations are solved coupling with those of water temperature and density. The TDG concentration is calculated based on the density current field. A good agreement is found in the simulation of the Dachaoshan Reservoir between the simulation results and the field data of the hydrodynamics parameters and the TDG distribution in the vertical direction and their unsteady evolution with time. The hydrodynamics parameters, the temperature and the TDG concentration are analyzed based on the simulation results. This study demonstrates that the model can be used to predict the evolutions of hydrodynamics parameters, the temperature and the TDG distribution in a deep reservoir with unsteady inflow and outflow. The results can be used in the study of the mitigation measures of the supersaturated TDG.

Evaluation of operational strategies to minimize gas supersaturation downstream of a dam

Bubble dissolution downstream of spillways may create zones of high total dissolved gas (TDG) concentration, which can be detrimental to fish. This paper presents the use of a numerical model to identify dam operational strategies that mitigate elevated TDG production. A mixture model takes into account the effect of the bubbles on the hydrodynamics. The model calculates bubble dissolution considering bubble size change due to dissolution and pressure. The model is validated using field velocity and TDG data.Several simulations are performed to understand the physical phenomena leading to supersaturated water under different operational configurations. According to the model, concentrating the spillway flow in one bay causes bubbles to travel closer to the free surface and thus lower TDG production and more degasification. In order to obtain the lowest TDG concentration, it is best to concentrate most of the flow in a central spillway bay. If additional water needs to be spilled, the use of a western bay is recommended. An additional simulation using compliance conditions indicates that, using the proposed configuration, the dam meets TDG water quality standards. Moreover, statistical analysis performed on inert particles released in the spillway and turbines demonstrates that, after 2h, 83.9% of the time particles are exposed to TDG values lower than 110%. If hydrostatic compensation is considered, particles are in undersaturated water 70% of the time.

Investigation into the Total Dissolved Gas Dynamics of Wells Dam Using a Two-Phase Flow Model

Bubbles entrained by spilled water at hydroelectric projects increase the concentration of total dissolved gas (TDG), which may lead to gas bubble disease in fish. In this paper, the TDG dynamics downstream of Wells Dam are investigated using a two-phase flow model that accounts for the effect of the bubbles on the flow field. The TDG is calculated with a transport equation in which the source is the bubble/liquid mass transfer, a function of the gas volume fraction and bubble size. The model uses anisotropic turbulence modeling and includes attenuation of normal fluctuation at the free surface to capture the flow field and TDG mixing. The model is validated using velocity and TDG field data. Simulations under two plant operational configurations are performed to gain a better understanding of the effect of spill operations on the production, transport, and mixing of TDG. Model results indicate that concentrated spill releases create surface jets that result in the lowest TDG concentration downstream. On ...

Influence of the Hyporheic Zone on Supersaturated Gas Exposure to Incubating Chum Salmon

AbstractHydroelectric dam operation causes total dissolved gas (TDG) to be seasonally elevated in the lower Columbia River, as surface water concentrations approach 120%. Federally protected chum salmon Oncorhynchus keta embryos incubating in nearby spawning areas could be affected if depth‐compensated TDG concentrations within the hyporheic zone exceed 103%. The objective of this study was to determine whether TDG of the hyporheic zone in two chum salmon spawning areas—one in a side channel near Ives Island, Washington, and another on the main‐stem Columbia River near Multnomah Falls, Oregon—was affected by the elevated TDG of the surface water. Depth‐compensated hyporheic TDG did not exceed 103% at the Multnomah Falls site. However, in the Ives Island area, chum salmon redds were exposed to TDG greater than 103% for more than 300 h. In response to river depth fluctuations, TDG varied significantly in the Ives Island area, suggesting increased interaction between the hyporheic zone and surface water at that site. We conclude from this study that the interaction between surface water and the hyporheic zone affects the concentration of TDG within the hyporheic zone directly via physical mixing and indirectly by altering water chemistry and thus dissolved gas solubility. Consideration of these interactions is important when estimating TDG exposure within egg pocket environments and will enable resource managers to optimize recovery strategies.

A multiphase model for the hydrodynamics and total dissolved gas in tailraces

Elevated supersaturation of total dissolved gas (TDG) has deleterious effects in aquatic organisms. To minimize the supersaturation of TDG at hydropower dams, spillway flow deflectors redirect spilled water horizontally forming a surface jet that prevents bubbles from plunging to depth in the stilling basin. A major issue regarding the prediction of the hydrodynamics and TDG in tailraces is the effect of the spillway bubbly surface jets on the flow field. Surface jets cause significant changes on the flow pattern since they attract water toward the jet region, a phenomenon called water entrainment. Bubbles create interfacial forces on the liquid, reduce the effective density and viscosity, and affect the liquid turbulence increasing the water entrainment. Most numerical studies on dams use standard single-phase models, which have demonstrated to fail to predict the hydrodynamics and TDG distribution. In this paper, an anisotropic two-phase flow model based on mechanistic principles capable of predicting water entrainment, gas volume fraction, bubble size and TDG concentration is presented. Good agreement between model results and field data is found in the tailrace of Wanapum Dam. The simulations capture the measured water entrainment and TDG distribution. The effect of the bubbles on the hydrodynamics and TDG distribution is analyzed.

Modeling Total Dissolved Gas Concentration Downstream of Spillways

Dams are often operated to facilitate downstream juvenile anadromous fish migration over the spillways, but such operation can cause high dissolved concentrations of oxygen and nitrogen that can be harmful to fish. The concentration of total dissolved gas (TDG) in the flow changes with distance downstream of the spillway crest and depends on the geometric configuration of the spillway and on hydraulic and operating conditions. A model is presented that simulates the physical processes of gas transfer with the goal of having an accurate and more widely applicable TDG model for plunging spillway discharges. Bubble transfer is dominant in the stilling basin, while water surface transfer is dominant downstream. Sensitivity analyses suggest which physical processes are important for accurate total dissolved gas predictions. Instantaneous bubble coalescence and breakup based upon local turbulence conditions is an appropriate assumption. Vertical bubble profiles do not need to be simulated in this type of model. Water surface roughness provides a significant increase to surface transfer. Tailwater depth is important to downstream TDG concentrations. Finally, a 10% difference in air entrained at the plunge point causes relatively minor differences in TDG of 1.4 and 3.1% at low and high discharges, respectively.

Air Binding of Granular Media Filters

Air bubbles that form in water treatment filters create headloss and can form whenever the total dissolved gas pressure exceeds the local solution pressure. The location of potential bubble formation in filters can be predicted based on measurements of the clean bed headloss with depth, flow rate, and the influent total dissolved gas concentration. Bubble formation within filters can be reduced by increasing the pressure within the filter via greater submergence (water head above the media), lower hydraulic flow rate, or using a more porous media. Bubbles trapped in the bed can be released by “burping,” which can reduce the extent of headloss buildup. Burping is more significant at lower flow rates and within a lower density, higher porosity, hydrophobic anthracite layer.

Modeling total dissolved gas production and transport downstream of spillways: Three‐dimensional development and applications

Challenges a resource manager encounters concerning water quality downstream of dam spillways are well documented. The plunging and air‐entraining nature of spillway flows can lead to supersaturated water downstream of the dam. A multi‐dimensional numerical model is developed for predicting total dissolved gas (TDG) production and transport downstream of a hydropower dam spillway. The model is based on the Reynolds averaged Navier‐Stokes equations for flow hydrodynamics and a total dissolved gas model that incorporates the gas production, exchange and transport physics. A three‐dimensional version of the model is calibrated using one set of available field data with a moderate flow discharge. The applicable range of the model is established by applying the calibrated model to different spillway discharges and comparing results with field data. The model is then validated with TDG field data available for Wanapum Dam and applied to two practical engineering problems. The simulations successfully reproduce the convection dominated TDG levels downstream of spillways as well as the variation of TDG concentration across the channel.