Field Investigation of Temperature and Do of the Ertan Reservoir D Uring Operation Period

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Journal of Hydrodynamics, Ser. B ,3 (2002) ,54 - 59 hina Ocean Press, Beijing - Printed in China

FIELD INVESTIGATION OF TEMPERATURE AND DO OF THE ERTAN RESERVOIR D URING OPERATION PERIOD

Jiang Hong State Key Hydraulics Laboratory of High Speed Flows , Sichuan University , Chengdu 610065 , China Xie Guang2wu , Lu Hong2wei Chengdu Hydropower Institute of Design and Investigation , Chengdu 610072 , China Luo Lin State Key Hydraulics Laboratory of High Speed Flows , Sichuan University , Chengdu 610065 , China

(Received Apr. 15 , 2001)

ABSTRACT : The Ertan hydropower station at the Yalong Riv2 key issue to impact the aquatic organism. It is almost er of Southwest Sichuan Province. It is one of the biggest in2 impossible to investigate the DO problem in an indoor stalled capacity hydropower stations in China. The main stream experiment. And mathematical simulation , if any, reservoir length reaches 145km and the branch Ganyu River must be supported by a solid data set. The DO distri2 reaches 40km under normal storage level. The reservoir surface bution data represented here can be used to help for covers 101km2 area , and has 5.8 billonm3 capacity. It began wa2 understanding of similar projects of DO issue , and as a ter storage since May 1998. The first generator was in operation in August 1998. The project was finished at 2000. The Ertan fundamental calibration data set. reservoir is a typical riverway style one. In order to understand The Ertan reservoir is a typical riverway style 3 the thermal effect , Dissolved Oxygen (DO) distribution and out2 one. Its total storage capacity is 5. 8 billion m , and let flow temperature under reservoir storage condition , three field regulating storage 3. 37 billion m3 , Mountains are measurements were carried out in January, April , and July of around the reservoir that distance from the water sur2 2000 , respectively. The measured results are analyzed in the pa2 face about 1500m. Average width of main stream of per. The results will provide reserchers a valuable data set for cal2 the reservoir is about 400m , and up to 1000m width at ibration of mathematical model and realistic understanding for some reach of the branch Ganyu river. The backwater stratified flow issue and DO profiles in a large scale deep reser2 zone of the main stream Yalong River is 145km, and voir. the branch Ganyu River 40km , at the highest storage KEY WORDS : reservoir , temperature , DO , field measure2 level - 1200m, respectively. The reservoir surface 2 ment area is 101km under this level.

1. INTROD UCTION[ 1～4] 2. TEMPERATURE AND DO OVERVIEW BE2 Stratifed flow occurs usually in deep reservoir. FORE THE RESERVOIR BUILT[ 5] This issue plays a very important role in aquatic ecosys2 2. 1 Temperature tem of the water body. Investigation for thisphenome2 According to statistics of the measurement data na is an interesting topic for both environmental and acquired from the Xiaodeshi hydrological station , hydroelectric engineers. A few theoretical case studies which is 12km downstream from the Ertan dam site , were carriedout , such as 2D , or even 3D , mathemati2 during 1960～1962 and 1988～1997 , the multi2year2 cal simulation for the thermal stratification. Physical averaged month2average temperature was 10. 5°C ; the model investigation for this kind of flow is too expen2 highest month2averaged temperature is 19. 5°C , which sive , and the experiment results is not good enough for appeared in August , and it was almost the same as that calibration of the mathematical model due to its size in June andJuly; the lowest month2averaged tempera2 limit. A full size field investigation of a typical large ture appeared in January , is 8.4°C. scale deep reservoir will provide researchers a conve2 2. 2 DO nient approach to calibrate their mathematical models , The monitoring periods at the Xiaodeshi hydro2 and realistic understanding for the stratification in the logical station for DO were 1973～1979 , 1981 , 1989 reservoir. and 1990. The Multi2year2averaged DO value is 7. DO profiles in large scale deep reservoir is another 8mg/L. The highest DO value of the measured data

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55 appeared in 1981 , was 8.8mg/L. The lowest value of tween surface and bottom of the reservoir in winter DO appeared in 1973 and 1977, was 7. 3mg/L. The was 3. 7～4. 9°C. variation, under which is unstable , occurred in Jan2 uary , April , August and December of each year due to combination effect of hydrologic , climatic and temper2 ature factors. In remained period DO was stable , and changed small.

3. FIELD MEASUREMENT OF TEMPERATURE AND DO 3. 1 Set up of sampling point To ensure the accuracy of the sampling cross sec2 tions, each monitor section is chosen from the existed sediment cross section and field investigation. Eleven monitor sections were chosen in the main stream.

Their positions are shown in Table. 1 (a) Longitudinal temperature profiles in winter 3. 2 Measurement f requency The measuring time were September 24 ～ 26 , 1999 , January 13 ～ 17, 2000, April 14 ～ 16 , 2000 and J uly 24～25 , 2000 , respectively.

4. MEASUREMENT RESULTS AND ANALYSES 4. 1 Data assembly Acquired data were inputted into computer imme2 diately after each field measurement , and were re2 viewed by assigned person. The obtained data were processed by meansof Excel for statistics , data gather2 ing , tabulation , and plotting. 4. 2 Results and analyses 4.2.1 Temperature

The measurement results are as follows: 1 (b) Vertical temperature profiles in winter Fig. 1 Winter temperature profiles (1) Winter results The longitudinal and vertical temperature profiles in winter are shown in Fig.1. From the figures it can (2) Spring results be seen that : The longitudinal and vertical temperature profiles The temperature near the dam was obviously in spring are shown in Fig.2. higher than expected. According to the measured From the figures it can be seen that : reservoir end temperature and the increasing rate of Estimating from the measured reservoir end tem2 ° temperature in winter , 0. 0125°C/ km, the tempera2 perature , 14.3 C, and the increasing rate of tempera2 ° ture near the dam should be 9.0°C. But the measured ture in spring, 0. 011 C/ km, the temperature at the temperature near the dam was 9.9～ dam site before the project construction should be 15. ° 13. 5°C, higher than estimated; 7 C. The measured temperature near the built dam 9. ° ～ ° Due to the low entrance rate at the reservoir end 7 C 17. 8 C , and stratified flow was observed. in winter ( measured rate was 549m3/ s) , the inflow Since the reservoir end temperature change con2 had small contribution to the reservoir temperature dis2 tributed to the temperature structure increased , even ( 3 tribution structure , which was limited within 35km the entrance flow rate was still small 516m / s at mea2 range from the end; sured time) , the disturbed range expanded to 70km The measured data represented that stratified flow from the end; occured even in winter. The thermocline was in the Stratification happened obviously in spring too , ～ range of 1171. 7m～1121. 7m, or 20m～70m below and the thermocline was in 1181. 5m 1111. 5m the surface. The temperature gradient of the thermo2 range , or 70m below and to the surface. The temera2 ° cline is 0. 1°C/ m; and the temperature difference be2 ture increasing rate was 0. 12 C/ m; and the tempera2 中国科技论文在线 http://www.paper.edu.cn

ture difference between the surface and the bottom ranged from 1.3°C～8. 6°C.

3 (b) Vertical temperature profiles in summer 2 (a) Longitudinal temperature profiles in spring Fig. 3 Summer temperature profiles perature in summer , 20.1°C, and the increasing rate , 0. 004°C/ km , the dam site before the project construc2 tion should be 20. 6°C. After the dam is built, the temperature near the dam was 16. 0～25. 0°C , ther2 mocline occured ; The entrance flow rate was large ( measured rate was 2279m3/ s) . The temperature at the reservoir end contributed the temperature structure of the reservoir arlgely, up to 90km from the end and 10m below the surface ; The thermocline ranged from 1182.6～1172. 6 m , or the surface to the 10m below the surface. The temperature gradient was 0. 38°C/ m～0. 60°C/ m. The 2 (b) Vertical temperature profiles in spring Fig. 2 Spring temperature profiles layer below the 10m depth still had some gradient ; The temperature difference between the surface and the bottom was in 6.1°C～9. 7°C range. (3) Summer results 4. 2. 2 Measured results of the dam downstream The longitudinal and vertical temperature profiles The Xiaodeshi hydrological station is located at in sunmer are shown in Fig.3. 12km downstream from the dam. Its temperature change reflects the change before and after the dam built. The long term temperature recordsof the station before and after the reservoir storage are collected. There were 13 years temperature records at the station before the reservoir storage , including 1960～1962 and 1988～1997. The temperatures are shown in Table 2. Since the reservoir storage , May 1998 , the tempera2 ture records are June 1998 and whole year of 1999. They are also shown in Table 2. From Table 2 , it can be seen that: (1) From October to January next year after the temperature of the dam downstream grew obviously than that before the storage. Because the water retain2 ing time in the reservoir in winter and fall seasons is

3 (a) Longitudinal temperature profiles in summer longer than that in natural condition , the water body From the figures it can be seen that : in the above period can obtain more energy , the temp2 Estimating from the measured reservoir end tem2 中国科技论文在线 http://www.paper.edu.cn 75

Table 1 Monitor sections in the main stream unit :km

Golden Dam River Item Front No.7 No.10 No.13 No.15 No.17 No.20 No.24 No.30 No.34 Bridge (DF) ( GRB)

Dist . from 0.7 10.5 18.4 29.1 40.4 49.3 60.7 80.0 95.7 111.6 120 the dam

Dist . bet . 9.8 8.9 10.7 11.6 8.9 11.4 19.3 15.7 20.3 8.4 sections

Table 2 Temperature comparison before and after the reservoir storage at the Xiaodeshi hydrological station °C

Item Jam。 Feb. March Apr. May June July Aug. Sept. Oct. Nov. Dec. Av.

Average 8.4 10.3 13.1 16.6 18.4 19.4 19.4 19.5 18.0 16.1 12.4 9.1 15.0

Month2Av.Max 9.4 11.1 14.4 18.9 19.4 20.6 20.8 21.6 19.9 17.6 13.8 10.2 15.8

Month2Av.Min 7.4 9.1 12.6 15.6 16.2 17.7 18.1 18.1 16.4 15.1 10.8 8.0 14.3

1997 8.2 9.7 13.8 16.5 18.6 18.6 18.8 20.4 18.6 15.8 12.6 9.7 15.1

1998 8.8 10.9 14.2 16.8 19.7 21.1 19.6 19.1 17.7 18.3 14.9 13.1

1999 11.4 10.9 13.0 15.4 17.1 20.1 19.7 20.0 18.9 17.0 12.6

erature is apparently higher than that in natural condi2 dam. Then 70km form the above range can be divided tion. into two zones, apparent stratification in the upper ( 2) The temperature in April 1999 is relative 20m layer , and irregular distribution below the upper low, and it is at least 0.2°C lower than the multi2year layer due to effect of entrance flow and bottom of the month2average value. It is resultedfrom that the water reservoir ; body in the reservoir is huge, and the effect to the Within 50km range from the dam, the distribu2 reservoir water temperature due to climate temperature tion profiles at six monitor sections were very similar recovery in spring is less than that in natural conditon. (see Fig. 4) . The lowest DO values appeared at 15m～ (3) From the temperature profile in 1999 , it is 30m layer below the surface , and most of them at 25m seen that the distribution trend in the reservoir condi2 level. It is believed that was resulted from mass algae tion has big difference with one in natural condition. growth. The DO increased with the water depth after The sine curve in natural condition is no longer clear. the lowes value occurred , decreased again near the The lowest temperature month delayed , occurred in reservoir bottom. The reason for DO increase can be February; the highest temperature month is a little explained by the lower temperature , which provides earlier than natural condition , occurred in June. higher saturated DO concentration , and higher pres2 4. 3 DO measurement results sure , which enhances the dissolvability of oxygen; 4. 3. 1 Winter results During 70km range of the reservoir end , the mea2 The measured results are shown in Fig. 4. The sured DO distribution had similar trend (see Fig. 4) . results can be summarized as follows: There were two peaks and two valleys along the water It has clear DO stratification within 50km of the depth direction at the distribution profiles. The first 中国科技论文在线 http://www.paper.edu.cn

4 (a) Longitudinal DO distribution in winter 5 (a) Longitudinal DO distribution in spring

4 (b) Vertical DO distribution in winter 5 (b) Vertical DO distribution in spring Fig. 4 Winter DO distributions Fig. 5 Spring DO distributions peak appeared at 5m～10m below the surface , and the 15m～ 25m below the surface , and most of them at second one at 15m～ 40m level. The first valley oc2 20m level. It is believed that this is resulted from cured at 15m～30m below the surface , and the second strong algae growth. The value of DO appeared at 3m one around 40m. The lowest DO value happened at the below the surface. In some of the measured sections , reservoir bottom. It is analyzed that the lowest value the peak DO value happened at bottom of the reser2 at the bottom was induced by probing into the sedi2 voir. ment . The measured values ranged from 8.1mg/L～10. When the lowest values at the bottom are not 5mg/L , most of them from 8.5mg/L～9. 9mg/ L . counted , the measured DO range is 7.8mg/L～ 4. 3. 3 Summer results 12.1mg/L , and most of the values is in the range of The measured results are shown in Fig. 6. The 9. 1mg/ L～10. 0mg/ L . results can be summarized as follows: 4. 3. 2 Spring results DO values in the whole reservoir area have no The measured results are shown in Fig. 5. The clearly statified feature. Within 80km near the reser2 results can be summarized as follows: voir end the DO distribution appeared with similar The DO distribution in the whole reservoir has no trend. noticeable stratification. Similar distribution zones in2 Besides the two sections in front of the dam and clude 1111. 5m～1061.5m, or 70m～120m below the the reservoir end , the vertical DO distributions at oth2 surface and 1156. 5m～1166.5m,or 15m～25m below er 9 measured sections are very similar ( see Fig. 6) . the surface ; The lowest DO value in front of the dam occurred at Within 80km range to the dam, the vertical dis2 10m below the surface. The peak DO values at most of tribution of DO at 8 measured sections is almost the the sections appeared at the reservoir bed. ( ) same see Fig. 5 . The lowest DO values appeared at The DO values ranged from 6.1mg/L～10. 2mg/ 中国科技论文在线 http://www.paper.edu.cn

in which DO concentration in deeper level is more than in lower one. The phenomena should be associated with temperature distribution. Lower temperature in deeper level means larger saturated DO , this should be the main reason for the measured DO behavior.

REFERENCES

1. Luo Lin, Deng Yun, Zhao Wenqian and Li Jia, 2001: 3D Computation of Temperature Field Near the Dam of a Deep Reservoir , 29th IAHR Congress Proc. , Theme B , Beijing , China ,352～357. 2. Deng Yun, Zhao Wenqian,LiJia andLuo Lin, 2001: Sim2 6 (a) Longitudinal DO distribution in summer ulation on Thermal Stratification of The Huge2Cubage and Deep Reservoir , 29th IAHR Congress Proc. , Theme B , Beijing , China , 584～592. 3. Chen Xiaohong, Liu Meinan, Lin Yanshan, 1997: Study on Two2Dimensional Water Quality Distribution in Reser2 voirs , Journal of Hydraulic Engineering , (4) , 9～16. (in Chinese) 4. Li Ran, Zhao Wenqian, LiJia andLi Kefeng, 2001:Study on 32D Reaeration Model for Natural Streams, 29th IAHR Congress Proc. , Theme B , Beijing, China , 593～599. 5. Chengdu Hydroelectic Investigation and Design Institute , 2000 : The On2Site Temperature Measurement of Ertan Reservoir Research Report. (in Chinese)

6 (b) Vertical DO distribution in summer Fig. 6 Summer DO distributions L , and most of themfrom 8.2mg/L～9mg/ L .

5. CONCLUSION (1) A full size field investigation for stratified flow and DO profiles in a large scale deep reservoir was carried out. The results shown here provide re2 searchers a convenient approach to calibrate their mathematical model , and realistic understanding for temperature field and DO distribution in a large scale deep reservoir. It is hopefully that the information rep2 resented here is helpful for plan and design of similar hydropower projects. (2) In the Ertan hydropower station, which is investigated in this paper , the stratification happened all the year. The outflow temperature in reservoir con2 dition is lower than that in its original river condition. This could be main concern for temperature sensitive aquatic creatures. The real impact risk of stratified flow for aquatic organisms should be further investigat2 ed. Adjustment of operation condition may be helpful to reduce the risk , which will be studied in the future. (3) DO profiles behave unusual in deep reservoir ,