ICE FLOOD CHARACTERISTIC AND REGULATION PLAN DURING INITIAL FREEZE-UP PERIOD OF WANJIAZHAI RESERVOIR

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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 ICE FLOOD CHARACTERISTIC AND REGULATION PLAN DURING INITIAL FREEZE-UP PERIOD OF WANJIAZHAI RESERVOIR Ke Sujuan 1,WangMin 1 and Pan Qimin 1 ABSTRACT Wanjiazhai Reservoir has been in operation since October 1, In this paper, according to the observed data and regulated condition of the reservoir in ice flood period, the ice flood character, the reasons of surface ice jam and frazil jam (hanging dams) formation and breakup are analyzed. The regulation plan during initial freeze up period is developed using hydraulics principle. INTRODUCTION Wanjiazhai Reservoir lies on the Yellow River between Shanxi province and Inner Mongolia Autonomous Region. The reservoir area was a gorge reach, the width of the river valley was m. The hydrologic station at the entrance to the reservoir is Toudaoguai station. The main objectives of the reservoir are to supply water, generate electricity, and ice flood prevention and flood prevention. The total storage is m 3, the height of the dam from base to top is 982 m, the maximum water level is 980m, the designed water level is 977 m, the minimum water level for power generation is 952 m. THE CHANNEL CHARACTERISTICS OF THE RESERVOIR The length of the whole reservoir region reach (from the dam to Toudaoguai hydrologic station) is 114 km. For the pre-project condition, the slope of the river reach is generally over 1 g. In the winter, frazil ice run is the main ice condition in the reach from Lamawan Bridge to the dam. Since the reservoir stored water, stable ice cover forms in the whole reach from Toudaoguai to the dam every winter. Because the velocity in the backwater end of the reservoir reduces suddenly, it has the function of resisting running ice, so during freezeup period, it is easy to form frazil ice jam and during break-up period easy to form surface ice jam. The abovementioned condition is the normal ice flood characteristic of Wanjiazhai Reservoir region. Because the hydraulic condition and thermodynamic condition of every winter is different, the thickness of ice cover, progression speed of accumulating ice leading edge position, the scale and standing time of ice jams, etc. were different every year. For example, was a warm winter, in ice period the temperature was 1 Hydrology Bureau, YRCC, East No.12, Chengbei Road, Zhengzhou, , CHINA

2 relatively high. Because of the high temperature, during initial freeze up period, serious frazil jam was formed, and during break up period, serious surface jam was formed. But in , because of the relatively low temperature, the Toudaoguai reach freeze up early, and there was no serious ice flood during ice flood period. The daily average temperature of and in ice flood period are shown in figure 1. Temperature " " " " " " " " " " " " Date (month.day) Figure 1: The average day temperature, comparing figures of and during ice flood period. ANALYSIS OF ICE JAM FORMATION CHARACTERISTICS To analyzethe condition offormingicejams in and , it is observed that it is can not avoid to form surface and frazil ice jams in the end of the reservoir backwater zone. Because the difference in hydraulic condition and thermodynamic condition, the degree of blockage is different in every winter. The second characteristic observed is that in Niulongwan Reach, which sites km upstream of the Dam, is easy to form ice jams. This is because in this reach, the river condition is bad, having alternated wide and narrow sections. The widest section is 250 m, the narrowest section is 83 m only. Also, the steep slope and gentle slope sections are alternated, the biggest river slope is 1.77 g, the smallest river slope is 0.43 g. In addition, the Huishuihe River enters Yellow River in the reach, and Fengzhun Yellow River Railway Bridge sites in the reach, the ice run is resisted by the mouth and the bridge pier. The river channel has an S-curve, which is difficult for running ice to pass. STUDY OF THE REGULATING PLAN DURING INITIAL FREEZE-UP PERIOD OF THE WANJIAZHAI RESERVOIR Wanjiazhai Reservoir has the action of generating electricity, irrigation, ice flood prevention and flood prevention, during ice flood period, the pre-requisite of operation principle during ice period is to avoid ice floods, then generate electricity as much as possible. After the reservoir stored water, it is easy to form ice jam in the backwater zone. To make regulating plan of initial freeze up period, the hydraulic condition, thermodynamic condition and ice condition must be considered. The rising water level caused by ice jam must be calculated.

3 The principle of calculating rising water level caused by ice jam In this paper, the rising water level caused by ice jam is calculated using hydraulic method, according to the following hypothesis: Assuming the amount of incoming ice is enough, when the rising water level caused by ice jam is the highest, the water surface slope and hydraulic condition in every section of the ice jam reach is about balanced, and close to a stable value. The following formulas are used. General roughness coefficient n cp : b + ni n ncp = 2 During the stable period of ice jam, the section average velocity V: (1) V 0.35 = 0.71 Q (2) 0.36 B The section of Waijiazhai Reach is wide and shallow. In freeze up period, the hydraulic radius R: H R (3) According to Maning's formula V = R j calculate rising water surface slope j: n cp 2 2 V n cp 4/3 j = Ê H ˆ Á Ë 2 The recurrence formula of ice surface height of every section is: The formula for calculating ice volume is: ( ) G = G + D - D * J (5) i+ 1 i i+ 1 i i (4) W = W + ( k + k )*( B + B )*( D - D)/4 (6) i+ 1 i i i+ 1 i i+ 1 i+ 1 i in which U = discharge of ice jam stable period; H = average water depth; B = river width; n cp = general roughness coefficient; n b = river bed roughness coefficient; n i = roughness coefficient of ice cover; V = average velocity; R = hydraulic radius; J = water surface slope of ice jam; G = ice surface level; D = distance between section and the Dam; W = ice volume; K = ice jam thickness; subscript i is sequence number. Method of calculating rising water level caused by ice jam Generally, ice jam top sites the backwater end of reservoir, and the position of backwater

4 end varies with the storage water level of reservoir. The main index to control rising water level caused by ice jam in Wanjiazhai Reach is keeping the water level at Guaishang section, which is 72 km from the Dam, below 984 m. In calculating the general roughness coefficient, the river bed roughness coefficient after flood with balanced river bed erosion and deposition is used. The ice cover roughness coefficient at the top part, middle part and end part of ice jam is different, so different values are used. According to the formulas mentioned above, the steps for calculate rising water level caused by ice jam are as follows: (1) Simplify the ice jam body as a right-angle echelon, according to observed data to determine the proportion of ice jam top part and end part; (2) According to designed ice volume to determine thickness distribution of ice jam along river; (3) According to storage water level of the Reservoir to determine the position of ice jam top part and end part; (4) Using the section at ice jam top as upper boundary condition, the water level of that section is storage water level of the reservoir, according to water level, thickness of ice jam, and section data to calculate section area of flow water, width of water surface and water depth; (5) The average velocity is calculated by formula (2), the discharge is daily average discharge of Toudaoguai Station in stable ice jam period; (6) The general roughness of section is calculated by formula (1); (7) The rising water surface slope is calculated by formula (4); (8) According to the ice surface level, slope and the distance to next section of the first section, the ice surface level of next section can be calculated by formula (5); Repeat steps (1) (8), until the ice jam end is calculated, then the rising water level of every section can be calculated. (9) To count ice volume by formula (6) by comparing the counted ice volume and designed ice volume, whether the simplified proportion of ice jam thickness is right can be tested, if it is not right, it should be presumed again. Using above mentioned method, the two year s water level of and were calculated, the calculated result as: For the winter of , according to observed data, freeze up discharge was 500 m 3 /s, storage water level of the Reservoir was 960 m, ice volume was 28,000,000 m 3, the calculated rising water level caused by ice jam at Guaishang was m, the observed water level was m, the error only is 0.27 m. The simulated condition of calculated water level and observed water level of every section shown as figure 2. From figure 2, we can see, the precision of water level simulated is high. For the winter of , according to the observed data, during stable ice jam period, the discharge was 300 m 3 /s, storage water level of the Reservoir was 962 m, ice volume was 26,000,000 m 3, the calculated rising water level caused by ice jam at Guaishang was m, the observed water level was m, the error only is 0.03 m.

5 980 stage(m) Þ Distance to the Dam (km) Figure 2: The comparing figure of calculated value and observed value of rising stage caused by ice jam in To establish the regulating plan of initial freeze up period Storage water level of the Reservoir varies, so the position of backwater end is different. Because the initial ice jam is formed at the backwater end, and then progress upstream, the backwater end is more in upper reach, when the storage water level is higher. The position of ice jam body is more upstream, and the range of rising water level caused by ice jam is more in the upper reach, so it is more easy to cause disaster. To make regulating plan of initial freeze up period, according to regulating plan of Tianqiao Reservoir, determine the ice volume as three level: low level, m 3 (I level); middle level, m 3 (II level); high level, over m 3 (III level). Determine the storage water level of the Reservoir as five level: 950 m, 955 m, 960 m, 965 m, 970 m. Determine the freeze up discharge as three level: 500m 3 /s, 600 m 3 /s, 700 m 3 /s. To the I level ice volume: when the discharge is 500 m 3 /s, using the above mentioned method to count water level of five water level respectively, in the condition of Guaishang water level below 984 m (height of emigration), the best regulating plan during initial freeze up period is calculated, then to the discharge of 600 m 3 /s and 700 m 3 /s, using the same method to count the best regulating plan during initial freeze up period. Then to II level ice volume and III level ice volume, using the same method to count the best regulating plan during initial freeze up period. The counted result is shown as table 1 and figure 3. Table 1: The calculated best storage water level of the Reservoir for different ice volume and discharge Ice volume discharge Unit: ice volume ten thousand m 3 discharge, m 3 /s; water level, m

6 8000 ten thousand cubic meter ice volume cubi c met er per second 600cubi c met er per second 700cubi c met er per second storage stage (m) Figure 3: Regulating chart of different discharge and ice volume of Wanjiazhai Reservoir. From Table 1 and figure 3, we can see that when ice volume and discharge entering the Reservoir are known, the storage water level of the Reservoir can be determined. So to make the regulating plan during initial freeze up period, it is necessary to know the discharge entering the Reservoir of initial freeze up period and ice volume. The discharge entering the Reservoir can be forecasted by the method of discharge perform mathematical calculations, and the ice volume can be calculated by temperature forecast and discharge forecast. The formula is derived from the formula for Yanguoxia Reach, and the coefficient and the index are obtained from observed data of the Wanjiazhai Reach Wt = T12,1 Q12 (7) in which: T 12,1 = the average temperature of December and January; Q 12 = the average discharge of December; W t = ice volume. CONCLUSION After the Waijiazhai Reservoir stored water, the ice flood condition of the river in the Reservoir region has changed a lot, before, running ice was the main ice condition, now stable freeze up is the main ice condition, and it is easy to form ice jam and ice dam at the end of backwater, easy to cause ice disaster. Because the operation time of the Reservoir is short, lack of the regulation experience, and observed data is limited, so the regulating plan of initial freeze up period needs to be improved further.