Influence of Channel Regulating Structures on the Transportation and Dissipation of Supersaturated Total Dissolved Gas

Bubble dissolution during the ﬂood discharge creates high total dissolved gas (TDG) concentration zones downstream of the dams. The dissipation of supersaturated TDG is a very slow process. Thus, the elevated TDG may remain through the water body for hundreds of kilometers downstream and lead to gas bubble disease (GBD) and even mortality in ﬁsh. To improve the navigation conditions of waterways, dikes (i.e., a solid structure) of varied sizes and shapes are commonly constructed. However, this would aﬀect the dissipation and transportation of the supersaturated TDG. It would signiﬁcantly change the turbulence intensity and hydropressure of the ﬂow, which dominates the dissipation of TDG. Therefore, TDG distribution in the waterway diﬀers from that in the natural river. In this study, a numerical simulation of the TDG at the Yangtze River’s upper reaches (one of the inland waterways in China) was conducted with the establishment of a two-dimensional TDG dissipation model. The eﬀect of the dikes’ size and shape was analyzed to assess the inﬂuence of the regulation structures on the dissipation and transportation of the supersaturated TDG. Meanwhile, simulation in the study area with the natural topography was also set as blank control. Based on that, impact evaluation of TDG supersaturation on ﬁsh under diﬀerent simulation scenarios was made. This study can provide a scientiﬁc basis for reducing the adverse eﬀect of supersaturated TDG in ﬁsh and the construction of ecological waterway therefore.


Introduction
A waterway is known as any navigable body of water. Along with the rapid pace of the Golden Waterway of Yangtze River and inland waterway construction, more and more giant cascade hydropower stations are built or to be built. However, during the discharge process, excess air would be entrained into the water and cause supersaturation of total dissolved gas (TDG) due to the variation of deep pressure head. Dissipation of supersaturated TDG is known as a slight process [1]. It will exist along the river for quite a long time, leading to gas bubble disease (GBD) or even death to fish [2][3][4]. During dam sluice of the ree Gorges, it was observed that TDG in section 600 km downstream, the dam was high above 117% [5]. e other side of channel construction is waterway regulation by dredging, reef explosion, and the construction of regulating structures. It may significantly change the flow condition and lead to a difference in TDG dissipation compared with the natural river [6][7][8]. Until now, there are plenty of research studies on the environmental effects of waterway regulating structures [9,10], but few focused on the effect of channel regulating structures on the transportation and dissipation of supersaturated total dissolved gas. ere are numerous studies performed on the dam and reservoir hydroenvironment models. Different studies are conducted for the environment sustainability concerns of things happening on rivers [11][12][13][14][15], hydrobased energy [16][17][18][19][20], soil [21], water [22], decontamination [23,24], air/carbon-emission implications [25][26][27][28][29][30][31][32][33], precipitation [34], and evaporation [35][36][37][38][39][40]. Some geo-hydro-environmental-based studies that have been taken recently are tabulated in Table 1.
Lots of research works have been carried out for TDG dissipation [58][59][60][61]. It was found that the release of supersaturated TDG is related to the water depth. TDG dissipation can be accelerated with the decrease of water depth. Based on prototype observation of TDG dissipation in several rivers of China (e.g., Yangzi River, Yalong River, and Langtang River), Feng et al. [62] computed each river's release coefficient using the first-order kinetic process. It was found out that the release coefficient of supersaturated TDG downstream the Zipingpu Dam in Min River was 0.563 h −1 ～0.650 h −1 , which was larger than that in the river reach downstream the ree Gorges in the Yangzi River, 0.014 h −1 ～0.020 h −1 .
e water depth downstream of the ree Gorges during the flood discharge period was much deeper than that of the Min River. It was also observed that TDG observation in the reservoir of Dachaoshan and the natural reach downstream was 0.04%/km and 0.26%/km, respectively, which means variation of water depth has a significant effect on TDG dissipation. Water temperature is also a key factor in TDG dissipation. TDG supersaturation is an unstable nonequilibrium state. e excess gas in the water will be released slowly to regain the equilibrium state. Temperature is one of the critical factors influencing gas solubility. Ou et al. [54] researched the influence of temperature on the release of supersaturated TDG. It was found that, under certain conditions of pressure and turbulence intensity, the coefficient of 28°C water temperature was about four times under 4°C. Moreover, wind can significantly promote the release of supersaturated TDG, and the quantitative relation of release coefficient and wind speed was developed by Huang et al. [63]. Besides, turbulence intensity, water-sediment concentration, and river morphology also significantly influence the release rate of TDG [64].
Based on research results of the release coefficient of TDG, a series of calculation models for TDG release were established and were used to simulation TDG dissipation in the natural river. Perkins and Richmond [65] developed a depthaveraged 2-D model to study TDG saturation distribution downstream the Bonneville Dam and the Ice Harbor Dam. Ma et al. [53] studied operation regulation of water replenishment to deduce supersaturated TDG through a 1-D unsteady TDG model. Shen et al. [66] established a depth-averaged, two-dimensional model of TDG dissipation at a river confluence and explored shelter construction for fish at the confluence of a river to avoid the effect of TDG supersaturation. Feng et al. [62] carried out a width-averaged 2-D TDG model for numerical simulation of water temperature and TDG distribution in a large reservoir based on the 2-D water temperature model. Among those studies mentioned above, the river reaches were gentrified, while only the natural topography was considered. However, the channel regulating structures will change the topographic condition to a greater extent, and the flow condition would not be the same anymore. For now, little research was conducted about the effect of the channel regulating structures on TDG dissipation. e present work examined the distribution of supersaturated TDG near the regulating structures in a numerical simulation. Potential intervention for enlarging the area of low TDG was studied in the model.

Mathematical Model
2.1. Numerical Model. A depth-averaged, 2D model applying the Reynolds-averaged, hydrostatic (shallow-water) Navier-Stokes equations was used to simulate the transport of TDG in a waterway with the contribution of the regulating structures: where ζ is the difference between the surface elevation and the mean depth, t is the time, h is the mean water depth, u and v are the depth-averaged flow velocity components in the x-and y-direction, respectively, g is the acceleration of gravity, ρ is the water density, v t is an eddy viscosity coefficient, τ s and τ b are the surface wind stress and the river bottom friction, G is the concentration of TDG, and S C � −k TDG G is the sink dissipation of TDG, where k TDG is the dissipation coefficient of the supersaturated TDG.

Model
Verification. e numerical model for hydrodynamics in our work was validated by Wang et al. [4]. To validate the scalar transport model, we developed a simulation according to a laboratory experiment by Kang [15]. In this experiment, salt concentrations were used as a conservative tracer to identify tributary water. e model grid used 19577 grid cells in an unstructured triangular mesh, as shown in Figure 1. Experimental data and the simulation results are compared in Figure 2. e error values at the measurement points between model and experiment range from 2.9% to 9.8%, which is a reasonable agreement.

Study Site.
As a study site, we use a reach of the Jialing River (China) 3 km downstream of the Caojie Dam and stretches 4 km. ere exist the typical dry rapids, Gouzuwan.

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To improve the navigation condition in this reach, waterway regulations were designed in 2009, consisting of cutoff works, reef explosion, dredging, and groins construction, as shown in Figure 3.

General Conditions and Boundary Conditions.
e Jialing River is itself a tributary of the Yangzi River in Chongqing Province, China. e dam height of the Caojie Navigation-Power junction is 56.0 m. Its average water level is 203.00 m above the sea level. e typical storage capacity is 7.54 × 10 8 m /s. e flood discharge structures include five scouring sluices and 15 spillways with the use of bottom-flow dissipation. For the maximum navigable discharge, 15,000 m 3 /s, the supersaturated TDG level is 131% [15]. Power flow rate is 3,054 m 3 /s with TDG saturation of approximately 100%. Because of the flow rates' disparities, full mixing of the floods discharge with the power flow reduces the 131.0% predicted TDG supersaturation down to 125%. Water flow with supersaturated TDG high above 110% can be lethal to fish. us, the effect of waterway relation on the dissipation of TDG is desirable. Field measurements for velocity, mixing, and TDG in the study are not available. So, our work focuses on comparing a baseline numerical simulation of the known river morphology with the simulation of waterway regulation to examine how these works change the TDG distribution.

Domain and Mesh Division.
e computation domain (

Parameter Determination.
e Smagorinsky coefficient used for the turbulence model was 0.28, and the Prandtl constant value was 1. e Manning coefficient for bottom roughness was set as 0.03. ese values are the same as those used in the validation experiment (Section 2.2). e dissipation coefficient of the supersaturated TDG was set as 1.72 × 10 −5 s −1 , which matches field observation results in the Yangzi River.  Figure 4. A noticeable difference in water depth and velocity occurred due to the topographical boundary change, especially in the area where three groins were constructed. Water depth before the one # groin and the four # groins increased to 14.5 m and 14.2 m, respectively, while those before the regulation were 13.1 m and 13.0 m. Due to water contraction induced by the groins, the mainstream was narrowed, and the maximum velocity increased to 4.8 m/s, which was 1.6 m/s larger than that before the regulation. e area downstream of the groins turned into the recirculation zone, and the velocity decreased significantly. e recirculation zones in A-1 and A-2 were 109,466 m 2 and 75,145 m 2 , which could increase the detention time of the supersaturated TDG and provided shelter for fishes. Figure 5 shows the simulation results of TDG distribution in the regulated waterway compared with that in the natural river. TDG saturation of the mainstream was only reduced to 122.1% in the natural river while that after the regulation was 122.0%, which was nearly the same. TDG dissipation is a slight process; with a large flow rate, TDG saturation in the mainstream is dominated by the inflow boundary. However, a significant difference occurred in the area where the groins were constructed. Due to the water contraction and the recirculation zones induced by the groins, the mainstream was narrowed. e diffusing width of the polluted zone with TDG saturation less than 120% enlarged to 219 m from 173 m, which extended to 45.7% of the outlet section's width.

Effect for Fish.
According to the abovementioned simulation results, there will not be a significant difference in TDG saturation in the mainstream in the computational domain due to the large flow rate. However, the recirculation zones and the riverbank, as shown in Figure 5, increase the detention time of supersaturated TDG, which was beneficial for the release process. us, the waterway regulation's construction enlarged the area of low-saturation along the riverbank where it can provide a shelter for fish. It can protect the fish from the damaging effects of TDG supersaturation. According to the area statistics of TDG saturation at a different level before and after the waterway regulation, as listed in Table 2, the size of the saturation regions of TDG saturation less than 110%, 115%, and 120% increased 36,679 m 2 , 56,477 m 2 , 161,135 m 2 , respectively. Based on research results of fish tolerance to fish, the river reaches the Jialing River after the waterway regulation was expected to meet the space requirements necessary for fish to avoid the supersaturation damaging effect of TDG. e numerical simulation study shows that waterway regulation may be beneficial to the river's ecological function as far as TDG supersaturation is concerned. e fundamental idea is installing groins along the riverbank to control the distribution of low TDG water downstream the dikes and create a low TDG refuge that otherwise might not occur because of the high-water flow rate or velocities of the natural river. Note that the waterway regulation varies with the waterway topography and operation features of the hydropower station nearby. e effect of waterway regulation on the flow field is different from the distribution of TDG. To reduce the adverse effect of waterway regulation on the river ecosystem and maximize its benefits, we need to investigate how 3D turbulence at the local area where the regulation measures conducted affects the dissipation rate of supersaturated TDG. It is of theoretical value and practical significance in developing eco-environmentally friendly Advances in Civil Engineering waterway. In this sense, knowledge of how the transportation and dissipation of supersaturated TDG can be controlled is vital for protecting the fishes from the destructive effect of TDG supersaturation. Due to the complexity of the inland waterway, the effect of waterway regulation needs to be further studied in combination with waterway regulation design and an assessment of local fish survival.

Conclusions
A depth-averaged, two-dimensional model for TDG transportation and dissipation was developed in this paper. A flume experiment verified the model, and the results matched well. A numerical simulation of TDG in the Jialing River's river reach, where the waterway regulation measures were constructed, was conducted. Besides, simulation in the study area with the natural topography was also set to analyze the effect of the waterway regulation on the transportation and dissipation of TDG. e simulation results showed reef explosion and dredging in the study site did not have a noticeable effect on the distribution of TDG since TDG release is a slight process and the inflow boundary condition dominated that of the mainstream. However, the groins' construction narrowed the mainstream, and the recirculation area was formed downstream of the dam in a wide area. It can increase the detention time of water flow with supersaturated TDG and allowed the low-saturation region to remain in a particular range. us, the area with low saturation of TDG was enlarged. is area could provide refuge space for fish to avoid the damaging of supersaturated TDG. is study provides a scientific basis for waterway regulation on the river ecosystem and some mitigation measures to reduce TDG supersaturation.

Data Availability
e data used to support the findings of this study are currently under embargo while the research findings are commercialized. Requests for data, 6/12 months after publication of this article, will be considered by the corresponding author. Disclosure e paper was presented in the 13th International Conference on Hydroscience and Engineering Proceedings.

Conflicts of Interest
e authors declare that they have no conflicts of interest.