Study on Distribution Characteristics and Mitigation Measures of Dissolved Oxygen Super-saturation in the Lower Reaches of High Dam

Transcription

1 Study on Distribution Characteristics and Mitigation Measures of Dissolved Oxygen Super-saturation in the Lower Reaches of High Dam WANG Lei, ZHANG Jun-peng (China Three Gorges Corporation, Chengdu , Sichuan) Abstract: With dissolved oxygen (DO) in the water downstream of a Class II combined power station on a large river as an object of study, this paper, through monitored data on saturation between April of 013 and July of 014, garners information on saturation state, spatial and temporal distribution characteristics, and reduction law of DO in downstream water; analyzes the impact of different means of discharge of the dam on DO saturation in downstream water; and offers mitigation measures and suggestions. The paper concludes that: (1) High dam discharge leads to DO super-saturation in downstream water; () Different discharge means impact DO saturation in downstream water differentially, among which, spillway tunnel discharge brings the greatest impact, while discharge of generating unit has no impact; (3) The reduction process of DO saturation tends to be lengthy, declining around 6-8% for every 100 km; (4) Water catchment from tributaries is able to dilute saturation of DO in downstream water, with these tributaries featuring ample water and low DO content being better able to dilute DO; (5) DO super-saturation in downstream water can be effectively mitigated by reduced surplus water flow and increased flow for power generation resulting from optimizing operation of the power station, changing discharge means of the dam, and making full use of capacity of the reservoir. Keywords: High Dam; Dissolved Oxygen (DO); Super-saturation; Distribution; Mitigation Study on Distribution Characteristics and Mitigation Measures of Dissolved Oxygen Super-saturation Downstream of High Dam WANG Lei, ZHANG Jun-peng (China Three Gorges Corporation, Chengdu , Sichuan) Introduction Super-saturation of downstream DO, a problem resulting from high dam discharge, has been ranking among hot issues in the sector of hydro power environmental protection both at home and abroad. As early as in 1960s, Beiningen, K. T. and Ebel W. J [1] from the U.S. detected the presence of super-saturation of dissolved gases on the downstream of water conservancy projects. The super-saturation affected fishes in Columbia River. The United States Environmental Protection Agency (EPA) set a total air pressure in excess of 110% in the water as the criteria for harm to fish. [] Domestically, when Gezhouba Project saw initial impoundment in 198, fish fries or juvenile fish showing symptoms of gas disease were taken from downstream dam, and some of these fish had been dead. [3] During , [4] The Sanjiang Mouth of Gezhouba Project witnessed death of fish fries, a phenomenon which, after preliminary analysis, was deemed in connection with 1

2 super-saturation of dissolved gases in the water. [5] At present, a consensus has been reached regarding super-saturation of gases triggered by discharge of high dam. With dissolved oxygen (DO) saturation in the water downstream of a Class II combined power station on a large river as an object of study, this paper ascertains DO saturation state and spatial and temporal distribution characteristics in the water discharged from high dam; analyzes the reduction law of DO saturation; highlights the impact of different means of discharge of the dam on the DO saturation in downstream water; and offers mitigation measures and suggestions, in an effort to provide reference for prevention and evaluation of the impact of hydropower stations on aquatic ecosystem. 1 Monitoring of DO Saturation 1.1 Sample Collection Monitoring Time and Frequency Monitoring Duration is between April of 013 and July of 014 and the monitoring includes monthly routine monitoring of DO saturation in main stream river ways; special monitoring of DO saturation in the water downstream of dam during the flood season (0 days in total, August 17-September 5, 013); and special monitoring in stationary points of DO saturation in the reservoir area (39 days in total, June 15-July 5, 013) Monitoring Section and Layout of Sampling Point The two dams are approximately 150km apart, with two tributaries falling into the main stream respectively 1.8km and 180km downstream of the dam. The monitoring of this study includes reservoir tail, reservoir area, sections before and below the dam, as well as river ways at the confluent areas of large tributaries surrounding the main stream. Specifically, surface, middle, and lower layers of water is sampled on the middle section of the reservoir area and the section before the dam; and only surface sampling is done at reservoir tail, section below the dam, and river ways at the confluent areas of large tributaries surrounding the main stream, given relatively shallower water as compared with river ways in the middle section and before the dam, sufficient exchange between upper and lower water layers, and absence of a marked layering characteristics in these sections. 1. Monitoring Index and Test Method The Monitoring Index includes DO (content and saturation), water temperature, and atmospheric pressure. (1) Monitoring Method for DO: iodometry (GB ) and electrochemical probe method are used; () Monitoring Method for Water Temperature: temperature data are collected on the site in accordance with GB thermometer method; (3) Monitoring Method for Pressure Intensity: pressure intensity and percentage of oxygen in the air are measured using a barometer. 1.3 Quality Assurance and Quality Control During the sampling process, for the same sampling point, two samples are collected to serve as parallel samples and repeated monitoring is carried out using the same method; and during the test process, all indexes are tested strictly in accordance with relevant national standards. DO is checked using iodometry and electrode method and

3 verified depending on different air pressures. Meanwhile, the applicability and effectiveness of the two methods are promptly readjusted according to check results. Results and Discussion.1 Characteristics of DO Saturation in the Water below the Dam According to Henry's Law, oxygen's saturation solubility in the water is calculated under the sampling conditions using Formula (1), oxygen concentration in the sample S ( measuremen t) is measured in the lab, and DO saturation in the sample is O gained through Formula (). Where: SO S P X (1) O O M KO HO -oxygen's saturation solubility in the water (mol/l) M-oxygen's molecular weight (g/mol) P-on-site atmospheric pressure (KPa) X -percentage of on-site oxygen in the air O K -the Henry's constant of oxygen in the water (KPa L/moL) O HO DO saturation (%) = S (measuremen t) O S O Super-saturation was encountered between April and October of 013 and in June of 014, with the maximum value of % observed on June 30, 013, referencing Table 1, the corresponding discharge means used was deep hole (780m 3 /s); and the minimum value of % was observed on October 9, 013, with the discharge means being deep hole (979m 3 /s) and discharge of generating unit (850m 3 /s). DO saturation was normal between November, 013 and June, 014, during which discharge of generating served as the only discharge means. Table 1. Table of Correspondence between DO Saturation in the Reservoir Area Downstream of Power Station and Discharge Means of Upstream Dam Time Discharge Means Overall Conditions of DO Saturation Exceptional Cases Diversion Underport Deep Hole + Generating Unit DO Super-saturation 1. The maximum value of % was observed on the reservoir section of downstream power station on June 30 of 013, with a discharge means of deep hole discharge (780m 3 /s);. The minimum value of %, however, was observed on the section below the upstream dam on October 9 of 013, with a discharge means of (979m 3 /s)+ discharge of generating unit (850m 3 /s). Generating Unit Normal DO Saturation () Spillway Tunnel + Generating Unit DO Super-saturation The DO saturation on the section below upstream dam stood at 16.05%, with a discharge means of spillway discharge (450m 3 /s)+ generating unit (750m 3 /s). 3

4 . Cause Analysis of DO Super-saturation DO Saturation % Total Surplus Flow Figure 1. Correlation between DO Saturation in the Water below the Dam of Upstream Power Station and Discharge Flow DO saturation in the reservoir area of upstream dam ranged from %, with DO background value basically staying stable. The correlation between discharge flow of upstream dam (excluding discharge of generating unit) and DO saturation below the dam is shown in Figure 1, where DO super-saturation below the dam is observed in the presence of dam discharge. DO Saturation % Flow for Power Generation Figure. Correlation between DO Saturation in the Water below the Dam and Flow for Power Generation The correlation between DO saturation in the water below the dam of the two power stations and the flow for power generation of the dam is shown in Figure, where DO saturation in the water below the dams is normal only in the presence of the discharge of generating unit. In view of this, DO super-saturation in the water below the dam is associated with the discharge of the dam. During the discharge of the dam, a violent gas-water exchange led to a significant rise in dissolved gases in the downstream water, which had an impact in a wider range downstream with the flow of water current. [6].3 Impact of Dam Discharge Means on DO Saturation in the Water Downstream 4

5 There were four discharge means on the upstream dam during the monitoring period: diversion underport discharge, deep hole discharge, spillway tunnel discharge + discharge of generating unit, and discharge of full generating unit. (1) Diversion Underport Discharge: In April and May of 013, the flow recorded 1760m 3 /s, and the increment of saturation recorded 6.86%, causing DO super-saturation in the section below the dam. () Deep Hole Discharge: From June to October, 013, the discharge flow registered m 3 /s, and the saturation variable stood at -4.5~0.91%, resulting in DO super-saturation in the water below the dam. Despite the peak value of % of DO saturation observed on June 30, overall, deep hole discharge tends to trigger less impact compared with other means of discharge, and the high DO saturation value on a single day remains to be further monitored. (3) Spillway Tunnel Discharge + Discharge of Generating Unit: in July, 014, the flow was 450m 3 /s + discharge of generating unit of 750m 3 /s, and the DO increment stood at 34%, leading to DO super-saturation in the section below the dam. (4) Discharge of Full Generating Unit: From October of 013 to June of 014, the saturation variable experienced zero-growth or negative growth, and the DO saturation in the water below the dam stayed stable, which further indicates that discharge of generating unit has no adverse effects on DO saturation in the water below the dam. According to the monitoring results, the discharge means of spillway tunnel impacts DO saturation most; the means of deep hole discharge impacts DO saturation considerably; while the means of discharge of generating unit has no effects on DO saturation. The study results is consistent with the conclusions drawn by PENG Qi-dong. [7], etc. of super-saturation of dissolved gases in the water of downstream river ways since impoundment of the Three Gorges Reservoir..4 Preliminary Analysis of the Impact of Water Depth on DO Saturation in the Water The correlation between DO saturation in the water of section before the dam of upstream power station and water depth in October, 013 is shown in Figure 3, where, with the increase in water depth, DO saturation decreases rapidly, and the reduction tends to become smaller with increase in water depth: down from 1m to m, the saturation decreases by 9.77%; while from 40m to 100m, the saturation experiences a reduction of only 11.5%. Saturation % Saturation Water Depth Figure 3. Correlation between Water Depth and DO Saturation.5 Section Reduction Law of DO Saturation 5

6 Once river water experiences super-saturation, it will be a long time and distance before the water becomes normal again. As shown by monitoring results of section release prototype of super-saturation TDG on downstream river ways carried out by Sichuan University, the saturation of super-saturated water below the dam of Three Gorges Power Station decreases only 6% after a distance of 656km; and the saturation reduction of super-saturated water below the dam of Ertan Power Station is only 6.3% after a distance of 99.5km. [6] As calculated by this observation and study, DO saturation in the river water declines roughly 6-8% for every 100km. The distribution of DO saturation along the section of the water below the dam in July of 014 is shown in Figure 4, where, the reduction law of DO saturation in the river ways of the reservoir area of downstream power station is not obvious; however, as a result of two confluent tributaries, the DO saturation in the water below the dam of downstream power station shows an apparent reduction: the saturation of Tributary I is 10.1%, and after the confluence, the saturation of the two adjacent sections falls by 1.07%; while the saturation of Tributary II stands at 9.49%, and after the confluence, the saturation of the two adjacent sections sees a drop of 19.57%. In addition, Tributary I has a smaller water flow than Tributary II, from which it is found that, the lower DO content and larger water flow there are, the stronger dilution effect there is. 3 Mitigation Measures With the conclusion of this study, and with a view to reducing surplus water and mitigating downstream DO super-saturation, it's advisable to take such measures as using deep hole instead of spillway tunnel for discharge, and increasing flow available for power generation. Meanwhile, when it comes to regulating mode, it's recommendable to make full use of the joint operation of cascade power stations on upstream and downstream as well as on the main stream and tributaries, especially in the flood season, it's recommended, by utilizing adjustable reservoir capacity, to minimize the frequency and duration of high capacity discharge so as to mitigate downstream DO super-saturation. 饱和度 Saturation (%) (%) Dam 1 大坝 Dam 135,00 130,00 15,00 10,00 支 115,00 流 110, ,00 100,00 95,00 90,00 85,00 80,00 75,00 Tributary I Tributary II 支流 (KM) Figure 4 Distribution of DO Saturation along the Section in Downstream River Ways 4 Conclusions 6

7 1) High dam discharge leads to DO super-saturation in downstream water. The leading causes of DO super-saturation include discharge means and discharge flow of the dam. Specifically, the discharge means of spillway tunnel has the greatest impact, while the discharge of generating unit has no impact. ) The process of DO super-saturation reduction is lengthy, with DO saturation in the river water dropping around 6-8% for every 100km. 3) Confluent water catchment can provide dilution for DO super-saturation in the river, and the confluent water catchment with large flow and low DO content tends to have better dilution effect. 4) DO super-saturation in downstream water can be effectively mitigated by reduced surplus water flow and increased flow for power generation resulting from optimizing operation of the power station, changing discharge means of the dam, and making full use of capacity of the reservoir. References [1]Geldert D A, Gulliver J S, Wilhelms S C. Modeling dissolved gas super-saturation downstream of discharge plunge pools [J]. Journal of Hydraulic Engineering, 1998, 14(5): 513~ 51. []China Institute of Water Resources and Hydropower Research,Yangtze River Fisheries Research Institute,the Three Gorges Hydrological Bureau,Tsinghua University. The Three Gorges reservoir water dissolved gases super-saturation and its effects on fish and the protection measures[r] [3] Yangtze River Water Resources Protection Bureau. Investigation of discharge of Gezhouba Project and gas disease of fish fries [R]. Wuhan: Yangtze River Water Resources Protection Bureau [4] CHENG Xiang-ju, CHEN Yong-can, GAO Qian-hong, CHEN Yan. Analysis of super-saturation reoxygenation caused by discharge of dam in Three Gorges Reservoir [J]. Journal of Hydroelectric Engineering, 005, 4(6): 6~67. [5] TAN De-cai. Study of fish-killing effect of gas super-saturation associated with Three Gorges Project [D]. Chongqing: Master's Thesis of Southwest University [6] FENG Jing-jie, LI Ran, LI Ke-feng, etc. Study of release process of super-saturation TDG in downstream high dam [J]. Journal of Hydroelectric Engineering, 010, 9(1): 7~1. [7] PENG Qi-dong, LIAO Wen-gen, YU Xue-zhong, etc. Study of effects of dynamic operation limit during flood season of Three Gorges Reservoir on mitigation of gas super-saturation. [J]. Journal of Hydroelectric Engineering, 01, 31(4): 99~103. 7