p-ISSN: 0972-6268 Nature Environment and Pollution Technology (Print copies up to 2016) Vol. 19 No. 2 pp. 739-745 2020 An International Quarterly Scientific Journal e-ISSN: 2395-3454

Original Research Paper Originalhttps://doi.org/10.46488/NEPT.2020.v19i02.029 Research Paper Open Access Journal Effects of Reservoir Withdrawal on Generation and Dissipation of Supersaturated TDG

Lei Tang*, Ran Li *†, Ben R. Hodges**, Jingjie Feng* and Jingying Lu* *State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, 24 south Section 1 Ring Road No.1 ChengDu, SiChuan 610065 P.R.China **Department of Civil, Architectural and Environmental Engineering, University of Texas at Austin, USA †Corresponding author: Ran Li; [email protected]

Nat. Env. & Poll. Tech. ABSTRACT Website: www.neptjournal.com One of the challenges for hydropower dam operation is the occurrence of supersaturated total Received: 29-07-2019 dissolved gas (TDG) levels that can cause gas bubble disease in downstream fish. Supersaturated Accepted: 08-10-2019 TDG is generated when water discharged from a dam entrains air and temporarily encounters higher pressures (e.g. in a plunge pool) where TDG saturation occurs at a higher gas concentration, allowing a Key Words: greater of gas to enter into solution than would otherwise occur at ambient pressures. As the water Hydropower; moves downstream into regions of essentially hydrostatic pressure, the gas concentration of saturation Total dissolved gas; will drop, as a result, the mass of dissolved gas (which may not have substantially changed) will now Supersaturation; be at supersaturated conditions. The overall problem arises because the generation of supersaturated Hydropower station TDG at the dam occurs faster than the dissipation of supersaturated TDG in the downstream reach. Because both generation and dissipation of TDG are functions of water temperature, there is an opportunity to affect the TDG process through selective withdrawal structures at a reservoir. Using a combination of field observations, and hydrodynamic modelling, we analysed the dependence of the water temperature difference on TDG generation from different-elevation release structures of high- dam reservoirs. By using of the dissipation model coupled with TDG and temperature, the evolution of supersaturated TDG from different withdrawal structures was simulated and compared in a natural river reach. It showed that warmer withdrawals result in reduced generation of TDG and enhanced dissipation of TDG.

INTRODUCTION fish mortality (Weitkamp et al. 2003, Liang et al. 2013, Xue et al. 2019). Many studies have been performed on Water temperature is an amazing controlling factor that the evolution of TDGS and its mitigation strategies (Pol- affects water quality and aquatic system in many aspects, itano et al. 2012, Urban et al. 2008, Yingzhu et al. 2018). such as the activity of microorganisms, the mass transfer Li et al. (2009) proposed that two processes are involved of dissolved gas, and the suitability of habitats for aquatic in the TDGS problem of large dams. The first process is organisms. The increasing number of dam constructions and the generation process of TDGS, in which excessive air is operations are causing distinct spatial and temporal changes dissolved under high pressure. The second consequent pro- in the water temperature (Deng et al. 2011). Temperature cess is the dissipation process, in which the supersaturated stratification is one of the dominant characteristics of large TDG dissipates from water in the downstream river. Shen and deep reservoirs. According to the literature (Deng 2003), (2014) conducted experiments to explore the effect of water the temperature difference between the surface and the bottom temperature on supersaturated TDG dissipation in static and layer in a reservoir may reach as high as 17°C. To reduce the turbulent conditions. Ou (2016) carried out experiments in negative effects on aquatic organisms, various mitigation both a straight flume and a converted flume to study the measures, such as stoplog intake and eco-regulation strategies, relationship between the dissipation coefficient and temper- have been implemented (Deng et al. 2011, Chen et al. 2016). ature in flowing water. However, before the present , The temperature-related responses have aroused more public no investigation has provided insight into the quantitative concern, among which is the response of supersaturated total response of TDGS to the water temperature stratification dissolved gas (TDG) to temperature changes. related to the reservoir operation. Determining the rela- TDG supersaturation (TDGS) is an adverse impact tionship between TDGS and water temperature variation caused by spill discharge from reservoirs, which is widely is not only a practical but also a theoretical challenge for understood to cause bubble disease and thereby increase improving high dam operations. 740 Lei Tang et al.

With respect to the two processes of TDGS, both air Effect of Water Temperature Regulation on the dissolution and degasification are closely related to the water Generation of TDGS temperature. Herein, the paper will describe the response of TDG supersaturation to water temperature changes in each In engineering practice, discharges from different elevations process. of a reservoir usually exhibit different . Herein, we consider two typical reservoirs as examples to analyse RESPONSE OF SUPERSATURATED TDG the contribution of the water temperature difference to TDG GENERATION TO TEMPERATURE CHANGES generation. Pubugou reservoir in Daduhe River: The Pubugou hydro- The generation extent of TDGS is closely related to the air station is located at the middle of the Daduhe River. solubility at a specific pressure and temperature in a plunge The maximum height of the dam is 186 m, and the backwater pool. Therefore, we first examine the air solubility depend- length is 72 km. The authors conducted field observations ence on temperature and then discuss the effect of water tem- on the water temperature and the TDG from 2012 to 2013. perature regulation on the generation of supersaturated TDG. Fig. 2 illustrates the distribution of water temperature in Air Solubility in Terms of Temperature the Pubugou reservoir in August. The water temperature is vertically stratified obviously. It is well understood that the solubility of dissolved gas The open spillway and the bottom spillway tunnel are decreases as the temperature is increased under specific pres- the main release structures of the Pubugou hydropower sure and salinity (Colt 1984). The air solubilities of oxygen, station, for which the crest elevations are 833 m and 795 m, nitrogen, and carbon dioxide, in units of mg/L, as a function respectively. According to the observed results of the water of temperature are presented in Fig. 1. The data indicate that temperature in 2012, the temperatures corresponding to the the air solubility varies with the change in temperature. For inlet of the open spillway and the bottom spillway tunnel are example, the solubility of oxygen at barometric pressure 21.4°C and 17.3°C, respectively. The temperature difference is 14.6 mg/L, 9.1 mg/L, and 6.4 mg/L at 0°C, 20°C, and is approximately 4.1°C. 40°C, respectively. The total dissolved gas concentration at barometric pressure is 39.6 mg/L, 25.1 mg/L, and 18.0 A TDG supersaturation level of 127% was observed mg/L at 0°C, 20°C, and 40°C, respectively. The percentage during the discharge of the bottom spillway tunnel, with a of dissolved gas saturation level is defined as the ratio of flow rate of 2195 m3/s. As the flow rate of 2195 m3/s didn’t the actual gas concentration to its solubility at a specific occur during the operation practice of the open spillway, temperature. The gas concentration (in mg/L) is expected to the TDG level under the same discharge rate was failed to be different, even if the per cent TDGS level under different obtain. To compare the sole temperature effect on TDGS, it water temperature conditions is equal. is reasonable to assume that the open spillway exhibits the

45 O2 N2 Ar CO2 TDG 40 35 30 25 20 15 Dissolubility(mg/L) 10 5 0 0 10 20 30 40 Temperature(℃)

Fig. 1: Solubility of the air compositions and the total dissolved gas versus temperature. Fig. 1: Solubility of the air compositions and the total dissolved gas versus temperature.

Vol. 19, No.Effect 2, 2020 of • W Natureater Environment Temperature and Pollution Regulation Technology on the Generation of TDGS

In engineering practice, discharges from different elevations of a reservoir

usually exhibit different temperatures. Herein, we consider two typical reservoirs as

examples to analyse the contribution of the water temperature difference to TDG

generation.

Pubugou reservoir in Daduhe River: The Pubugou hydropower station is

located at the middle of the Daduhe River. The maximum height of the dam is 186 m,

and the backwater length is 72 km. The authors conducted field observations on the

water temperature and the TDG from 2012 to 2013. Fig. 2 illustrates the distribution

of water temperature in the Pubugou reservoir in August. The water temperature is

vertically stratified obviously.

The open spillway and the bottom spillway tunnel are the main release structures

of the Pubugou hydropower station, for which the crest elevations are 833 m and 795

m, respectively. According to the observed results of the water temperature in 2012,

the temperatures corresponding to the inlet of the open spillway and the bottom

spillway tunnel are 21.4℃ and 17.3℃, respectively. The temperature difference is

5 model (SKLH 2013). Fig. 3 depicts the temperature distribution in the reservoir in

September of a dry year. The predicted water temperature corresponding to the depths

of the surface orifices and the bottom orifices are 24.8℃ and 20.8℃, respectively.

According to Fig. 1, at barometric pressure, the air dissolubility at 24.8℃ and

20.8℃ is 22.93 mg/L and 24.67 mg/L, respectively. The difference in TDG

concentration is 1.74 mg/L, and the relative difference is approximately 7.6%.

According to the predicted results of TDG supersaturation caused by the release

structures (SKLH 2008), the percentage level of TDG supersaturation at the exit of

the plunge pool will be as high as 140% when the dam discharges. Assuming that the

supersaturation value of 140% is valid for both the surface-orifice discharge and the

bottom-orifice discharge, the equivalent TDG contents at 24.8℃ (surface orifice) and

20.8℃ (bottom orifice) are 32.10 mg/L and 34.54 mg/L, respectively. The difference

of the TDG concentrations for the surface-orifice discharge and the bottom-orifice

discharge is as high as 2.45 mg/L due to the temperature difference, which is

equivalent to 7.6% of the TDG level at the surface orifice. For this reason, discharge

from the EFFECTSsurface orifices OF HYDROPOWER is more advisable RESERVOIR than thatWITHDRAWAL from the bottom TEMPERATURE-orifices. 741

850 820 830 780 810

790 740

770 700 750

Water Level(m)Water 660 Water Level(m)Water 730 620 710

690 580 8 12 16 20 24 28 12 14 16 18 20 22 24 26 Temperature(℃) Temperature(℃) Fig. 2: Observed vertical temperature 1400-m of upstream Fig. 3: Predicted vertical temperature upstream of the dam Fig.the dam2: Observed in the Pubugou vertical Reservoir temperature (Mid-August, 1400 2012)-.m Fig. in3 :the Predicted Baihetan Reservoir vertical (Mid-September, temperature Dryupstre year).am same TDG level, 127%, with the bottom spillway tunnel. will be 182 km. Six surface orifices and seven bottom orifices Hence the TDG concentration in mg/L can be obtained by will be included on the on-dam release structures for flood multiplying the TDG solubility at the corresponding temper- discharge, and the crest elevations will be 810 m and 714 m, ature by the TDG level of 127%. According to Fig. 1, the respectively. The predicted temperature distributions in the TDG solubility at 21.4°C is 24.4 mg/L and that at 17.3°C 7 reservoir for different typical years were obtained by the use is 26.4original mg/L. Thus, value the TDG when concentrations the water in temperature mg/L of the risesof a laterally 10℃, averagedresulting numerical in very simulation different model (SKLH open spillway and the bottom spillway tunnel are determined 2013). Fig. 3 depicts the temperature distribution in the res- to be 31.0 mg/L and 33.5 mg/L, respectively. The absolute ervoir in September of a dry year. The predicted water tem- TDGdissipation difference is states2.5 mg/L, of the which supersaturated is equivalent to TDG. 8.1% perature corresponding to the depths of the surface orifices of the TDG quantity of the open spillway. The reason for and the bottom orifices are 24.8°C and 20.8°C, respectively. the differenceModel V iserification the TDG solubility difference caused by According to Fig. 1, at barometric pressure, the air dis- the temperature difference. According to previous studies, solubility at 24.8°C and 20.8°C is 22.93 mg/L and 24.67 a higherA flevelield ofobservation TDG generation on causesthe dissipation a larger adverse of supemg/L,rsaturated respectively. TDG The was difference conducted in TDG concentrationby is impact on TDG (Li et al. 2013). Therefore, with respect to 1.74 mg/L, and the relative difference is approximately 7.6%. TDG generation, discharge from the open spillway is more Sichuan University on the river reach from theAccording confluence to the of predicted the Yalongjiang results of TDG River supersaturation advisable than that from the spillway tunnel. caused by the release structures (SKLH 2008), the percentage Baihetan reservoir in Jinshajiang River: The Baihetan hy- level of TDG supersaturation at the exit of the plunge pool dropowerand stationthe Jinshajiang is a high-dam River hydropower to Longjie station currentlyferry. The will observed be as high reach as 140% is whenapproximately the dam discharges. 100 Assuming under construction on the Jinshajiang River. The maximum that the supersaturation value of 140% is valid for both the heightkm. of theThe dam observed will be 289 results m and were the backwaterused to verify length thesurface-orifice model as de picteddischarge in andFig. the 4. bottom-orifice discharge,

114

112

110 Calculated results 108 Observed result s 106 0 25 50 75 100 TDG percent saturation (%) Distance from Yalongjiang confluence (km)

Fig. 4: Comparison of the TDG level between the calculated and observed results in the Jinshajiang River. Fig. 4: Comparison of the TDG level between the calculated and observed results in the

JinshajiangNature River Environment. and Pollution Technology • Vol. 19, No. 2, 2020

Case Description

Using the discharge of the Baihetan hydropower station as an example, the

morphological data were employed to simulate the dissipation process of TDGS at

different temperature conditions. The simulation cases and the boundary conditions

are listed in Table 1.

Table 1: Simulation cases and boundary conditions of the Baihetan hydropower station.

Discharge Discharge temperature TDG level in per TDG level Case No. Structure (℃) cent (%) (mg/L)

1 Surface orifice 24.8 140 32.10

2 Bottom orifice 20.8 140 34.54

10 upstream of the dam in the Pubugou Reservoir of the dam in the Baihetan Reservoir

(Mid-August, 2012). (Mid-September, Dry year).

RESPONSE OF THE SUPERSATURATED TDG DISSIPATION PROCESS TO TEMPERATURE CHANGES

During the transport of the TDG-supersaturated discharge flow downstream, the water

temperature changes as it travels due to the effects of solar and transfer

with air. In response to the change in water temperature, the gas solubility and the

dissipation rate of supersaturated TDG changes, consequently resulting in different

dissipation processes of the TDG.

Using the discharge flow from the Baihetan hydropower station as an example, a

1-D model was employed to simulate the dissipation process of TDGS at different

temperature conditions.

Prediction Model

The model couples hydrodynamics, temperature and TDG. The governing equations

are as follows：

Continuity equation:

AQ L …(1) txq upstream of the dam in the Pubugou Reservoir of the dam in the Baihetan Reservoir equation： (Mid-August, 2012). (Mid-September, Dry year). upstream of the dam in the Pubugou Reservoir of the dam QQin the Baihetan 2 Reservoir  Z Q ()( gA  S ) L  0 …(2) t xA  xfq A 742 (Mid-August, 2012). Lei Tang et al.(Mid-September, Dry year). RESPONSE OF THE SUPERSATURATED TDG DISSIPATION PROCESS TO Temperature equation: TEMPERATUREthe equivalent TDG CHANGES contents at 24.8°C (surface orifice) and Temperature equation: 20.8°C (bottom orifice) are 32.10 mg/L and 34.54 mg/L, ()QT T b respectively. The difference of the TDG concentrations for n RESPONSE OF THE SUPERSATURATED TDG DISSIPATION () PROCESSADL  TO …(3) …(3) the surface-orifice discharge and the bottom-orifice discharge x x  xC During the transportTEMPERATURE of the TDG-supersaturated CHANGES discharge flow downstream, the water p is as high as 2.45 mg/L due to the temperature difference, TDGTDG equation: equation: temperaturewhich is changesequivalent as to it 7.6% travel ofs the due TDG to the level effects at the ofsurface solar radiation and heat transfer orifice. For Duringthis reason, the transportdischarge offrom the the TDG surface-supersaturated orifices is discharge flow downstream, the water ()()AC  QC C with moreair. Inadvisable response than to that the from change the bottom-orifices. in water temperature, the gas solubility and  theAD L  KT() C8 C sT() …(4) temperature changes as it travels due to the effects of tsolar radiation  xx and heat transfer  x RESPONSE OF THE SUPERSATURATED TDG dissipation rate of supersaturated TDG changes, consequently resulting in different …(4) -1 DISSIPATIONwith PROCESSair. In response TO TEMPERATURE to the change in water temperatureIn the above, the gasequations, solubility x and= the the l ongitudinal distance (m). t = (s ). T = In the above equations, x = the longitudinal distance (m). CHANGES -1 3 dissipation processdissipationes of the rate TDG of . supersaturated TDG changes,t =consequently time (s ). T = resulting Temperature in different (°C). Q 3= the flow rate (m /s). 2 Temperature (℃). Q = the flow rate2 (m /s). A = the area of the cross-section (m ). DL= During the transport of the TDG-supersaturated discharge A = the area of the cross-section (m ). DL= the turbulent diffu- flowUsing downstream, the discharge the flowwater temperaturefrom the Baihetan changes hydropoweras it travels sivitystation (m as2/s). an example,= the water a density (kg/m3). C = the specific dissipation processes of the TDG. the turbulent diffusivityr (m2/s).  = the waterp density (kg/m3). C = the specific heat due to the effects of solar radiation and heat transfer with heat of water [J/(kg·°C)], b = the width of the cross-section p 1-D air.model In response was employed to the change to simula in waterte temperature,the dissipation the gasprocess (m) of. j TDGS= the heatat different flux between water and air (W/m2). C = Using the discharge flow from the Baihetan hydropowern station， as an example, a ， of water [J/(kg·℃)] b = the width of the cross-section (m) n = the heat flux between solubility and the dissipation rate of supersaturated TDG the TDG concentration in mg/L. Cs(T) = the TDG solubility temperaturechanges, conditions. consequently1-D model wasresulting employed in different to simula dissipationte the dissipation capacity process in mg/L of with TDGS respect at different to the local temperature T. KT 2 processes of the TDG. water= andthe first-orderair (W/m transfer). C = the rate TDG coefficient concentration with respect in mg/L. to the Cs(T) = the TDG solubility PredictionUsing Model thetemperature discharge flow conditions. from the Baihetan hydropower local temperature T (1/s). station as an example, a 1-D model was employed to simulatecapacity j inis mg/L estimated with according respect to thethe meteorological local temperature data from T. K T = the first-order transfer n The themodel dissipationRESULTS couplePrediction sprocess hydrodynamics, AND Modelof TDGS DISCUSSIONS at temperature different temperature and TDG. Thethe weathergoverning stations equations in Qiaojia in the Yunnan province and conditions. rate coefficientHuize in the with Sichuan respect province. to the local temperature T (1/s). are as follows：The model couples hydrodynamics, temperature and TDG. The governing equations PredictionFig. Model5 shows that the water temperature increasesIn previousslowly studies,along thedegasification river reach. was Themodelled using first-ordern is estimated kinetics according for the TDGto the concentration meteorological (USACE data from the weather stations are as follows： TheContinuity modeltemperature couples equation: hydrodynamics, increment within temperature 200 andkm TDG. is 0. 8℃2005). for Liuthe (2013)bottom conducted discharge a series and of0.97℃ TDG dissipation The governing equations are as follows: in Qiaojiaexperiments in the atYun differentnan pr ovincewater temperatures and Huize inand the developed Sichuan province. Continuity equation: the relationship of the TDG dissipation coefficient versus AQContinuityfor the equation:surface discharge. The temperature increase along the river reach is very small Lq water temperature…(1) that is described as follows: tx AQ In previous studies, degasification was modelled using first-order kinetics for the Lq …(1) …(1) due to thetx large flow rate and the high water depth. ThisK = resultK × 1.062 indicates(T – 20) that the …(5) Momentum equation： TDG concentrationT (USACE20 2005). Liu (2013) conducted a series of TDG dissipation Momentum equation: Momentum equation： Where, K in equation (5) can be determined by the impact of the temperature difference fromexperiments different release at different20 structure waters continuetemperatures fors and a developed the relationship of the 2 observed values analogically or can be determined accord- QQ  Z Q …(2) ()( gA  S2 ) L  0 …(2) QQ fq  Z Qing to the following equation derived by Feng et al. (2014): tvery xA long distance  x()( from gA the Adam  S. ) L TDG 0 dissipation c oe fficient … (versus2) water temperature that is described as follows: t xA  xfq A Temperature equation: (T  20) …(5) Bottom discharge KKT 20 1.062 Temperature26.0 equation: Surface discharge ()QT T25.0 bn  ()AD()QTL   T b  Where , …K20(3 in) equation (5) can be determined by the observed values     n x x xC24.0 ()ADp L …(3) x x  xC p 23.0 analogically or can be determined according to the following equation derived by

22.0 Feng et al. (2014): 21.0 8 8 uH 20.0   * 2.02 1.73

TDG saturation level(%) K 3600 ( ) Fr …(6) 0.0 50.0 100.020 RR150.0 200.0 Distance downstream the dam(km)

Fig. 5: Evolution of the temperature in the JinshajiangAccording River downstream to equation of the Baihetan (5), the Dam. TDG dissipation coefficient will increase to 1.4 Fig. 5: Evolution of the temperature in the Jinshajiang River downstream of the Baihetan Dam. the original value when the water temperature rises 5℃ and 1.8 times the Vol. 19, No. 2, 2020 • Nature Environment and Pollution Technology As a function of velocity and water depth, the TDG dissipation coefficient of the

Jinshajiang River downstream of the Baihetan Dam was calculated according to 9 equation (6), as derived by Feng et al. (2014), and the results are shown in Fig. 6. The

coefficient values fluctuate in the range of 0.03 h-1~0.09 h-1 for the surface discharge

and 0.02 h-1~0.07 h-1 for the bottom discharge. The maximum difference of the

coefficients of the two discharges is 0.018 h-1 at the section 123-km downstream of

the dam.

11 TDG equation:

()()AC  QC C  ADL  KT() C C sT() …(4) t  xx  x

In the above equations, x = the longitudinal distance (m). t = time (s-1). T =

3 2 Temperature (℃). Q = the flow rate (m /s). A = the area of the cross-section (m ). DL=

2 3 the turbulent diffusivity (m /s).  = the water density (kg/m ). Cp = the specific heat

， ， of water [J/(kg·℃)] b = the width of the cross-section (m) n = the heat flux between

2 water and air (W/m ). C = the TDG concentration in mg/L. Cs(T) = the TDG solubility

capacity in mg/L with respect to the local temperature T. KT = the first-order transfer

rate coefficient with respect to the local temperature T (1/s).

n is estimated according to the meteorological data from the weather stations

in Qiaojia in theEFFECTS Yunnan OF pr HYDROPOWERovince and Huize RESERVOIR in the Sichuan WITHDRAWAL province TEMPERATURE. 743

In previ0.09ous studies, degasification was modelled using first-order kineticssurface for dischargethe 0.08 bot t om discharge TDG concentration (USACE 2005). Liu (2013) conducted a series of TDG dissipation 0.07

experiments0.06 at different water temperatures and developed the relationship of the

0.05 TDG dissipation coefficient versus water temperature that is described as follows: 0.04

Dissipation coefficient(1/h) 0.03 (T  20) …(5) KKT 20 1.062 0.02 0 20 40 60 80 100 120 140 160 180 200 Where, K20 in equation (5) can be determined by the observed values Distance downstream the dam(km)

analogically or can be determined according to the following equation derived by Fig. 6: TDG dissipation coefficient of the Jinshajiang River downstream of the Baihetan Dam. Fig. 6: TDG dissipation coefficient of the Jinshajiang River downstream of the Baihetan Dam. Feng et al. (2014): The evolution of the TDG per cent saturation level for each discharge case is uH RESULTS AND DISCUSSIONS K  3600 * ( )2.02Fr 1.73 …(6) …(6) presented20 inRR Fig. 7. The dissipation processFig. of 5 showsTDG thatfrom the the water surface temperature discharge increases is slowly According to equation (5), the TDG dissipation coef- along the river reach. The temperature increment within ficient willsignificantly increase to 1.4fast timeser than the originalthat of value the bottomwhen 200 discharge. km is 0.8°C The for thepercentage bottom discharge of the and TDG 0.97°C for According to equation (5), the TDG dissipationthe surface coefficient discharge. will The increase temperature to 1.increase4 along the the water temperature rises 5°C and 1.8 times the original river reach is very small due to the large flow rate and the value whensa turathe watertion leveltemperature of the rises surface 10°C, discharge resulting in decreases 27.9% within 200 km, whereas that times the original value when the water temperaturehigh water rise depth.s 5℃ This and result 1.8 indicatestimes thethat the impact of very different dissipation states of the supersaturated TDG. the temperature difference from different release structures of the bottom discharge decreases 24.0% within 200 km. The maximum difference of Model Verification continues for a very long distance from the dam. As a function of velocity and water depth, the TDG dis- A field observationthe TDG on level the dissipation at 200-km of supersaturated downstream TDG of thesipation dam is coefficient 3.9%, which of the isJinshajiang equivalent River to downstream16% of 9 was conducted by Sichuan University on the river reach from the Baihetan Dam was calculated according to equation (6), the confluenceof the of TD the GYalongjiang dissipation River of andthe the bottom Jinshajiang discharge. as derived by Feng et al. (2014), and the results are shown in River to Longjie ferry. The observed reach is approximately Fig. 6. The coefficient values fluctuate in the range of 0.03 100 km. The observed results were used to verify the model h-1~0.09 h-1 for the surface discharge and 0.02 h-1~0.07 h-1 as depicted in Fig. 4. 150.0 for the bottom discharge.Bottom The discharge maximum difference of the coefficients of the two discharges is 0.018 -1h at the section Case Description Surface discharge 140.0 123-km downstream of the dam. Using the discharge of the Baihetan hydropower station as an The evolution of the TDG per cent saturation level for example, the morphological130.0 data were employed to simulate each discharge case is presented in Fig. 7. The dissipation the dissipation process of TDGS at different temperature process of TDG from the surface discharge is significantly conditions. The simulation120.0 cases and the boundary conditions faster than that of the bottom discharge. The percentage of are listed in Table 1. the TDG saturation level of the surface discharge decreases

TDG saturation level(%) 110.0 0.0 50.0 100.0 150.0 200.0 Table 1: Simulation cases and boundary conditions of the Baihetan hydropower station. Distance downstream the dam(km)

Case No. Discharge Structure Discharge temperature (°C) TDG level in per cent (%) TDG level (mg/L) 1 Fig. 7Surface: Evolution orifice of the TDG 24.8saturation level in the Jinshajiang River140 downstream of the Baihetan32.10 2 Bottom orifice 20.8 140 34.54 Dam.

The water temperature is a governing factor in both TDG generation and Nature Environment and Pollution Technology • Vol. 19, No. 2, 2020 dissipation. Spills from different elevations result in different generation and

12 0.09 surface discharge 0.08 bot t om discharge

0.07

0.06

0.05

0.04

Dissipation coefficient(1/h) 0.03

0.02 0 20 40 60 80 100 120 140 160 180 200

Distance downstream the dam(km)

Fig. 6: TDG dissipation coefficient of the Jinshajiang River downstream of the Baihetan Dam.

The evolution of the TDG per cent saturation level for each discharge case is

presented in Fig. 7. The dissipation process of TDG from the surface discharge is

significantly faster than that of the bottom discharge. The percentage of the TDG

saturation level of the surface discharge decreases 27.9% within 200 km, whereas that

of the bottom discharge decreases 24.0% within 200 km. The maximum difference of

the TDG level at 200-km downstream of the dam is 3.9%, which is equivalent to 16%

744 of the TDG dissipation of the bottomLei discharge. Tang et al.

150.0 Bottom discharge Surface discharge 140.0

130.0

120.0

TDG saturation level(%) 110.0 0.0 50.0 100.0 150.0 200.0

Distance downstream the dam(km)

Fig. 7: Evolution of the TDG saturation level in the Jinshajiang River downstream of the Baihetan Dam. Fig. 7: Evolution of the TDG saturation level in the Jinshajiang River downstream of the Baihetan

27.9% within 200 km, whereas that of the bottom dischargeDam from. different release structures at different water tempera- decreases 24.0% within 200 km. The maximum difference tures were simulated employing a proposed numerical model. of the TDG levelThe at 200-km water downstream temperature of the isdam a is governing3.9%, Two factordifferent in dissipation both TDG processes generation of TDG were and obtained. which is equivalent to 16% of the TDG dissipation of the The difference of the TDG per cent saturation levels at 200- bottom discharge. km downstream of the dam is 3.9%, which is equivalent to dissipation. Spills from different elevations result in different generation and The water temperature is a governing factor in both TDG 16% of the TDG decrement of the bottom discharge. This generation and dissipation. Spills from different elevations result demonstrates that water temperature plays an important result in different generation and dissipation processes of role in the TDG dissolubility, both in the dissipation rate TDG. With respect to TDGS, the use of release structures 12and the per cent saturation level. The response of TDG to at the surface is recommended to discharge floods due to the water temperature changes implies that water temperature higher temperature near the surface of the reservoir. This changes should be taken into account in the study of the TDG result demonstrates that TDGS can be mitigated further by supersaturation problem of high dam discharges. exploring temperature-based operational regulations. Discharges from different elevations exhibit different TDG generation and dissipation processes. With respect to CONCLUSIONS TDG supersaturation, the use of release structures at higher elevations is recommended for discharge due to the elevated The development of hydropower stations and reservoirs temperature near the surface of the reservoir. This recom- causes distinct spatial and temporal changes in water tem- mendation implies that the temperature-based operational perature. For high dam hydropower stations, the release regulation is an effective approach to minimize the effect of structures are designed to be at different elevations. Due to TDG supersaturation. the temperature stratification for large and deep reservoirs, the spill from different release structures may exhibit dif- ACKNOWLEDGEMENTS ferent water temperatures. This temperature difference can not only result in different TDG generation levels but can The research was supported by the National Key R&D Pro- also affect the dissipation rate and per cent saturation level gram of China (Grant No. 2016YFC0401710) and the Open in terms of dissolubility. Fund of the State Key Laboratory of Hydraulics and Moun- According to the temperature stratification results of the tain River Engineering, Sichuan University (SKHL1702). Pubugou and Baihetan reservoirs, the water temperature dif- ference from different release structures was approximately REFERENCES 4°C. The relative difference of the TDG generation from Chen, D., Chen, Q., Leon, A. S. and Li, R. 2016. A genetic algorithm par- the different release structures of the Pubugou and Baihetan allel strategy for optimizing the operation of reservoir with multiple reservoirs was analysed to be approximately 7.6% and 8.1%, eco-environmental objectives. Water Resources Management, 30(7): respectively, due to the temperature difference. 2127-2142. Colt, J. 1984. Computation of dissolved gas concentration in water as a By using the discharge of the Baihetan high dam as an function a temperature, salinity and pressure. American Fisheries example, the dissipation processes of supersaturated TDG Society Special Publication, No 14. Bethesda, USA.

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