ANALYSIS ON THE ICE CONDITIONS CHANGES IN INNER MONGOLIA REACH OF YELLOW RIVER

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 2002 International Association of Hydraulic Engineering and Research ANALYSIS ON THE ICE CONDITIONS CHANGES IN INNER MONGOLIA REACH OF YELLOW RIVER Yang Xianghui 1, Wang Ling 1 and Zhang Xuecheng 1 ABSTRACT Inner Mongolia reach of Yellow River is the place where the most serious disaster taken place in the upstream reach of Yellow River, there the problem of the ice flood is as important as the problem of flood in summer and autumn. In this paper, the effect of main factors to ice conditions in Inner Mongolia reach is analyzed, and the effect of Longyangxia reservoir and Liujiaxia reservoir regulations in upstream reach of Yellow River to ice conditions changes in Inner Mongolia reach is analyzed. The results of analysis show that break-up ice jams in Inner Mongolia reach have become less since the reservoirs were regulated, the break-up form is mainly of thermal break-up, but the threat of disaster by ice-jam is more serious in the freeze-up period. FOREWORD Inner Mongolia reach sits at the northern end of the Yellow River basin in China. It s extreme cool in a long period in winter, the below 0 ºC time can last 4~5 months, the lowest temperature can reach 35 ºC. The river regime and channel characteristics are unusual in this reach, the freeze up is from down reach to up reach in winter and the break up is from up reach to down reach in spring. The ice melt flood increases from up reach to down reach, usually leads to ice blocking and ice jams, causes disasters easily. After the Liujiaxia reservoir in Yellow River upstream came into use in 1968, the flow discharge in channel and water temperature were adjusted in the ice frozen period, disasters were much less than before. But by the reasons of mutual influence of many factors and complexity of ice flood, the bigger disasters can still occur in some cases and the ice flood threat in upper Yellow River hasn t been eliminated yet. THE CHANNEL SHAPE CHARACTERISTICS Yellow River Inner Mongolia reach lies between 106º 10 E ~ 112º 50 E and 37º 35 N ~ 41º 50 N in the northern end of Yellow River basin. It comes from Shizuishan city in Ningxia province and Lasengmiao village of Yikezhaomeng county in Inner Mongolia to Jiucheng village of Hequ county in Shann xi Province and Mazha village of Yikezhaomeng county in Inner Mongolia, it is 820 km long and a U shape bend reach, its slope is large in upper reach and small in lower reach. The channel varies 1 Yellow River hydrology and water resources institute, No.12 east Chengbei road, Zhengzhou, Henan province, , P.R.China.

2 gradually from narrow and deep to wide and shallow. The biggest surface width of the Yellow River is 0.4~1.2 km at the Bayangaole hydraulic station, the smallest width is 0.2~0 km at the Zhaojunfen hydraulic station. The reach below Bayangaole hydraulic station is more tortuous, especially in Zhaojunfen and Toudaoguai reaches, the channel curvatures are 1.58 and 1.75 respectively. There are about 69 large bend reaches from Bayangaole to Toudaoguai station, the maximal curvature is The flowing ice used to be jammed at the bend or narrow reaches in break up period. There are about 5 main ice blocking reaches above Sanhuhekou station, 27 ice blocking reaches below Sanhuhekou station. Table 1 shows the characteristics of Inner Mongolia Reach Shanhuhekou Zhaojunfe Bayangaol Toudaoguai Shizuisha Hequ Longyangxia Figure 1: Map of the upper Yellow River, including the Inner Mongolia reach. AIR TEMPERATURE AND WATER TEMPERATURE Air temperature One of the reasons for ice conditions changes is the climate change. The upper reach of Inner Mongolia reach has lower latitude and warmer weather than that of lower reach. The annual average air temperature in Inner Mongolia reach varies between 2~6 ºC, the air temperature range is big. From Table 2 we can see that the maximal cumulative monthly average air temperature is more than twice times of the minimum one, the air temperature decreases gradually along the reach, and the lowest is at Toudaoguai Station. Table 3 offers the average air temperature in ten days periods in Inner Mongolia Reach (1954~1994). From this we can see that the multiyear average air temperature is below 0 ºC from the last ten day of November at the Shizuishan station, from the middle ten day of November below Bayangaole station. The air temperature is the lowest in January, begins to rise in February and gets to be positive in March.

3 Table 1: The characteristics of Inner Mongolia Reach. Station Shizuishan Bayangaole Sanhuhekou Zhaojunfen Baotou Toudaoguai Hequ length(km) Channel sloop( ) Channel curvature Table 2: Air temperature characteristics from Dec. Feb. in Inner Mongolia Reach (ºC). Cumulate air temperature from Dec. to Feb. Shizuishan Bayangaole Sanhuhekou Baotou Toudaoguai Multiyear average value Lowest value in past years Highest value in past years Table 3: A ten day average air temperature characteristics from Nov. to Mar. in Inner Mongolia Reach (ºC). Station Nov. Dec. Jun. Feb. Mar. Shizuishan Bayangaole Sanhuhekou zhaojunfen Toudaoguai Water temperature After the operation of Liujiaxia reservoir in 1968, the discharged water temperature got higher. The ice regime was affected in Inner Mongolia reach. The higher air temperature and water temperature that make the freeze-up date later than before. From Table 4 we can see the water temperature variation in November before and after the operation of Liujiaxia reservoir. Table 4: Comparative water temperature of Nov. before and after 1968 in Inner Mongolia Reach (ºC). Period Before 1968 After 1968 Station First Middle Last ten days Month First Middle Last ten days Shizuishan Bayangaole Sanhuhekou Zhaojunfen Toudaoguai Month

4 DISCHARGE Discharge is the dominant dynamic factor affecting ice regimes such as freeze-up, break-up, ice block, ice jam, etc. Table 5 shows that the freeze-up discharge has increased in varying degrees after the operation of Liujiaxia reservoir in Table 6 shows that the break-up discharge has decreased in varying degrees after the operation of Liujiaxia reservoir, especially after the operation of Longyangxia reservoir in upstream in 1986, but 5 days average discharge before break-up increased. It indicates that the channel storage discharge has been released gradually before break-up by the reason of the operation of reservoirs, releasing time is longer than before, so makes the break-up discharge decreased. ICE CONDITIONS CHANGES In Inner Mongolia reach, ice run usually begins in November of a year, freeze-up at the first or the second ten-day of December, and break-up at the second or last ten-day of March of the following year. With the operations of Liujiaxia and Longyangxia reservoirs in the upstream of Yellow River, the hydraulic factors and channel conditions have been changed significantly, and ice conditions have been changed accordingly. Table 7 shows multiyear average date of ice run, freeze-up and break-up. Table 5: The multiyear average freeze-up discharge and 5 days average discharge before freeze-up in Inner Mongolia Reach (m 3 s -1 ). Station Item Before ~ ~1995 Shizuishan Bayangaole Sanhuhekou Zhaojunfen Toudaoguai Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge In natural, the multiyear average ice-flowing date is about on Nov.18~19 below Sanhuhekou station, on Nov 20~24 above Bayangaole station, there are about 3~5 days difference. The shortest average ice-flowing period is 19 days at Zhaojunfen station, the longest is 33 days at Shizuishan station. The earliest average freeze-up date is on Dec. 2~3 at Zhaojunfen and Sanhuhekou station, the latest is on Dec. 26 at Shizuishan station, about 24 days later than the date at Zhaojunfen. The shortest freeze-up period is 72 days at Shizuishan station and the longest is 111 days at Zhaojunfen station. The earliest average break-up date is on Mar. 7 at Shizuishan station, the latest is on Mar. 22 at Zhaojunfen and Toudaoguai station.

5 Table 6: The multiyear average break-up discharge and 5 days average discharge before break-up in Inner Mongolia Reach (m 3 s -1 ) Station Item Before ~ ~1995 Shizuishan Bayangaole Sanhuhekou Zhaojunfen Toudaoguai Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge Average freeze-up discharge days average discharge Table 7: Multiyear average date of initial ice-run, freeze-up and break-up statistics of stations in Inner Mongolia reach Item Station Ice-run date(mon,date) Freeze-up date(mon,date) Break-up date(mon,date) Shizuishan Bayangaole Sanhuhekou Zhaojunfen Toudaoguai Because the changes in hydraulic conditions, thermal conditions and channel conditions have been changed after the Liujiaxia and Longyangxia reservoirs operation for ice flood control, the channel ice conditions also have been changed. Firstly, as water temperature increased, the ice period has been decreased. There was no freeze-up in the 154 km above Shizuishan station. In some years, such as 1989~1990, 1990~1991 etc., freeze-up didn t occur even at Shizuishan and Toudaoguai station. Secondly, the average freeze-up date of Shizuishan station in the period of 1968~1985 (the period between Liujiaxia reservoir operation and Longyangxia reservoir operation) is 10 days later than the date in the period of 1950~1967 (before Liujiaxia reservoir operation), and Bayangaole station 6 days later, Sanhuhekou and Zhaojunfen 2 days later, Toudasoguai 13 days earlier contrarily. The average freeze-up date of Shizuishan station in the period of 1986~1996 (after Longyangxia reservoir operation) is 2 days later than the date in the period of 1968~1985, and Bayangaole station 10 days later, Sanhuhekou 7 days later, Zhaojunfen 2 days later, and Toudasoguai 5 days earlier contrarily (see Table 7). These showed that the effect of flow discharge and water temperature regulation by upstream reservoirs can work on to Zhaojunfen station. Thirdly, after Liujiaxia reservoir operation, the average break-up dates of all stations have been delayed, except for Shizuishan station, which became 1 day earlier. The reason for this is the decrease of

6 hydraulic effect caused by reservoirs controlling discharge. However, break-up date has got earlier for every station. This is because of warm weather since 1986 and higher air temperature in spring. Because of controlling discharge properly in the break-up period and reducing channel discharge since the reservoirs operation, the effect of hydraulic factors has got small and that of thermal factors has got bigger. The break-up regime is mainly of thermal break-up, the number of ice jams has got less. According to the record, there were 236 ice jams in the 18 years before Liujiaxia reservoir was built, averaging 13 ice jams per year. Mature break-up, premature break-up, partially mature break-up three break-up regimes made up of one third each. There were 84 ice jams among 22 years after Liujiaxia reservoir was built, averaging 4 ice jams per year, 70 percent of the years were mature break-up, 30 percent years were partially mature break-up, and no premature break-up. However, some new problems appeared. Because of the heavier deposition and curve, the capacity of channel transporting ice and water got smaller, ice jam and ice block got severe, ice jam disaster increased in the freeze-up period. According to the statistics, there were only 2 ice jam disaster occurred in the 18 years between 1950~1967 before Liujiaxia reservoir operation in the upstream reach, probability is 11 %, but there were 11 ice jam disaster occurred in the 28 years between 1968~1995 after Liujiaxia and Longyangxia reservoirs operation, the probability is 39 %, and that ice jam disaster mainly occurred in Bayangaole reach since CONCLUSION From the above analysis, we can conclude that the number of ice jams has decreased and ice regime has alleviated since Liujiaxia and Longyangxia reservoirs operation in break-up period in Inner Mongolia reach, the break-up regime is mainly of mature break-up, but ice jam disaster has got heavier in freeze-up period.