EFFECTS OF ICE ON THE HYDRAULICS OF INNER MONGOLIA REACH OF THE 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 EFFECTS OF ICE ON THE HYDRAULICS OF INNER MONGOLIA REACH OF THE YELLOW RIVER Jueyi Sui 1 and Bryan W. Karney 1 ABSTRACT Using long-term observations at four gauging stations along the Inner Mongolia Reach of the Yellow River in China, this paper explores both the ice regimen in general and the specific effects of the ice on the hydraulics of this river reach. The hydraulics of the reach and water levels are compared and contrasted for four different conditions: under open channel flow, during the ice flowing period, during the ice covered period, and during break-up. Associated rating curves, which are well behaved under open channel situations, are sometimes poorly defined and extremely variable under ice conditions. The results show that the water level is usually insensitive to flowing ice prior to freezeup. However, significant, but hardly surprising, variations are observed during icecovered conditions. The rating curves for both ice covered condition and river breakup period are developed and some related hydraulic issues are explored. INTRODUCTION In winter or late fall, after the formation of ice, the boundary conditions of the channel flow become quite different from those under pure open channel conditions (classifying the ice flowing period as a special case of open channel flow). This leads to difference in the hydraulic features, such as water level and flow velocity, to name only two. Given the important effect ice has on the hydraulics of northern rivers, further investigation is needed and justified. On many northern rivers, a major consequence of ice cover formation is the increase in water level that occurs during both ice-covered and ice-jammed periods. In the past two decades, many scientists have conducted research into the variation in water level under ice covered conditions (Beltaos, 1983, 1995, 1996; Hicks, et al., 1995; Prowse, 1986; Prowse et al., 1998; Shen and Ho, 1986; Shen and Wang, 1995; Sun et al., 1986 and 1990; Tuthill et al., 1996, to mention only a few). Ice jamming is one of the most conspicuous and momentous river ice phenomena. Ice jams form when ice being transported by the current is arrested by an obstacle such as stationary ice cover, or by congestion resulting from a local reduction in the stream s ice-transporting capacity. Due to a large aggregate ice thickness and a high hydraulic resistance relative to sheet 1 Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, Ontario, Canada, M5A 2E2

2 ice, ice jams tend to disturb the riverbed and can cause high water stages, as well as other impacts. Under open channel conditions, the variation in stream flow is generally determined through a simple water level measurement that is then related to a discharge value obtained from an established rating curve at an observed site, such as gauging station. The rating curve is usually developed by conducting simultaneous water level and discharge measurements over a wide range of stream flow, and is a well-researched issue under open channel flow conditions. However, the rating curve is poorly defined, and the water level extremely variable, with flow under ice-covered conditions and during the river breakup period. With this situation in mind, the present paper investigates and explores the effects of ice on the water level at four gauging stations along Inner Mongolia Reach of the Yellow River. GEOGRAPHIC LOCATION OF THE INNER MONGOLIA REACH Generally speaking, the Inner Mongolia Reach (Fig. 1) of the Yellow River is located north of 40 N latitude and flows from west to east (excluding station 1 which is located in the gorge reach with mainly pebble riverbed). The elevation of the riverbed generally exceeds 1000 m above sea level. The riverbed of the Inner Mongolia Reach is predominately alluvial, broad and shallow. The bed material is easily transported, being mainly fine sand (with pebbles present in some stretches). The channel slope between stations 2 and 4 is less than %. The river reach between stations 2 and 4 flows first from southwest Figure 1: The Inner Mongolia Reach of the Yellow River. to northeast and then to the southeast, contributing to a climatic difference. Together with the smaller channel slope and shallow broad meandering river regimen, river ice jams often form, particularly between sections 3 and 4 (Baotou Reach), and thus possibly leading to ice flooding. The flooding, of which the most recent was in March/April 2002, can cause heavy economic loss or even loss of life. Although the thermal processes controlling the ablation of ice jam is important, and can play a significant role in the river breakup (Prowse and Marsh, 1989), especially with respect to this river reach with a clearly climatic difference, the present study has not focused directly on this issue in view of the shortage of the related data. By contrast, direct data from along this river reach is taken from four gauging stations run by the Yellow River Commission. For the current study, the following annual data series have specifically been used: the minimum and maximum water level during ice

3 flowing period; minimum and maximum water level during river frozen-up; the maximum water level during river breakup and the water level without flowing ice immediately after breakup (used as the open channel condition, although the riverbed should be significantly deformed during river breakup period). The data series for station 1 is between 1960 and 1980, station 2 between 1967 and 1980, station 3 between 1964 and 1980 and station 4 between 1961 and EFFECTS OF ICE ON THE WATER LEVELS The Inner Mongolia Reach is located in the cold region, with a severe winter usually lasting 4 to 5 months. Ice flowing starts mid November and the freeze-up usually occurs in early December. In usual, the maximum thickness of ice cover during the frozen period is less than 0.8 m, and less than 0.35 m during river breakup, as shown in the Table 1. Ice flowing period Ice flowing starts usually mid November at gauging stations 2, 3 and 4. Because of climatic differences, ice flowing starts in late November or early December at gauging station 1. According to field observations, the minimal water level occurs at the beginning of ice flowing period (less ice flowing period), and the maximal water level occurs before freeze up if the incoming discharge doesn t increase dramatically. Generally speaking, the water level during the ice flowing period (at least, with low ice concentrations) doesn t change significantly. As shown in Fig. 2, the dependence of water level on the flow rate during ice flowing period is significant at gauging 3 and 4, no matter what kind of water level during ice flowing period (water level under open channel condition, minimal water level and maximal water level during ice flowing period). For stations 1 and 2, however, the maximal water levels the during ice flowing period lie above the rating curve formed by the open channel water level and minimal water level during ice flowing period. This may be the effect of backwater caused by the downstream freeze-up. As shown in Fig. 1, the Yellow River along stations 1 and 2 flows from southwest to northeast. In view of the climatic difference, the river will be likely freeze from downstream to upstream, as shown in table 1. This leads to in the backwater influence caused by downstream ice cover. And therefore, the maximal water level during ice flowing period at stations 1 and 2 present a downstream-ice-cover affected water level; not surprisingly, this line lies above the rating curve. Table 1: Average parameter related to ice regimen at the Inner Mongolia Reach Gauging station Data periods Ice flowing date Freezingup date Breakup date Maximal ice cover thickness (m) Ice cover thickness during breakup (m) Nov. 25 Dec. 31 March Nov. 17 Dec. 11 March Nov. 18 Dec. 2 March Nov. 18 Dec. 17 March Ice covered period After formation of the ice cover, the water level increases comparing to the same flow rate under open channel condition due to the resistance of the ice cover acting on the flow. From field observations, it is found that the maximal ice cover thickness during ice-covered condition is less than 0.8 m, although it may occasionally exceed 1.0 m (for

4 example, a value of 1.17 m was recorded at station 3 on 22 February 1967). Figure 2: Rating curve during ice flowing period at these 4 gauging station. As shown in Fig. 3, the effect on ice cover on water level is significant at station 1. The minimal water level during ice-covered condition at station 1 is at least 1.0 m higher than that under open channel condition. In one extreme case, the water level on 24 February 1976 is about 2.5 m higher than that under open channel condition with the same discharge (835 m 3 /s). The relationship between water level and flow rate during ice-covered condition, namely ice covered period rating curve, is clearly a prominent issue. At station 2, the increase in water level due to ice effect is also significant. Comparing to the water level under open channel condition, the increase in water level caused by ice cover is about 1.0m. However, the water level on 9 March 1976 has an about 2.2 m increase comparing to that under open channel condition. The ice covered period rating curve is also prominent. At station 3, the ice effect on water level is not so significant as those at station 1 and 3. Generally, comparing to the water level under open channel condition, the increase in water level caused by ice cover is about 0.8 m, although it may sometimes exceed one meter. The ice covered period rating curve is prominent, and is parallel to that under open channel condition.

5 Figure 3: Ice-covered rating curve at these 4 gauging station. The ice effect on water level at station 4 is significant when the incoming flow rate is larger than 600 m 3 /s. The larger the incoming flow rate is, the more the increase in water level during ice covered condition comparing to that under open channel condition. The ice covered period rating curve is also prominent. Overall, the ice covered period rating curve is prominent, although significant deviation of some data groups from the curve exists. However, in view of the movable riverbed and the ice effects, these rating curves may be practical applied for the water level forecast during ice-covered period at this river reach. River breakup period During river breakup period, the extra storage of water caused by the ice cover will be usually released in short order. River breakup is most often the period when ice flooding occurs, giving practical significance to the study of water level during river breakup. However, in view of the extreme shortage of field data and our still limited understanding of breakup, most attempts at forecasting its occurrence have been largely empirical or extremely simplified (Prowse, 1995). In general, there is no significant relation between water level and the associated incoming flow rate during river breakup period. During river breakup, water level depends not only on the incoming flow rate, including the surge caused by the upstream breakup, but also on the hydro-thermal properties of ice cover in addition to the river geomorphology and the ice regimen of the downstream reach. As shown in Fig. 4, the breakup water level at stations 3 and 4 is independent of the flow rate. Low flow rate

6 may cause tremendous increase in water level during river breakup. For example, the water level at station 3 on 25/26 March 1967 had an about 3-m increase in water level comparing to that under open channel condition. For stations 3 and 4, it is extremely difficult to forecast the water level during river breakup period. Figure 4: River breakup rating curve at these 4 gauging station. The relationship between water level and the incoming breakup flow rate is prominent at station 1 and 2, as shown in Fig. 4. Water level during river breakup period at stations 1 and 2 can be thus forecasted based on the formed breakup rating curve, although there is deviation with some data groups. However, as a basic approach to forecast the breakup water level at these 2 stations, it is at least applicable. As shown in Figs. 2, 3 and 4, the rating curve under open channel condition at station 1 dominates. However, the rating curves at stations 2, 3 and 4 are less prominent, and some data groups deviate significantly from the curves. The main reasons for this are believed to be the effects of riverbed features. As described earlier, pebbles are the dominant riverbed material at station 1, leading to a relatively stable riverbed at this station. However, the broad and shallow alluvial channel at station 2, 3 and 4 is easily deformable, especially during river breakup period when huge quantities of stored water (due to ice effects) are quickly released. Conceptually at least, this release often leads to dramatically riverbed deformation, mostly though various scouring processes. For the same reason, both the ice covered rating curve and the river break-up rating curve are expected to be influenced by an easily deformable riverbed. And thus, these rating curves are not so pronounced.

7 CONCLUSIONS Using long term observations at four gauging stations along the Inner Mongolia Reach of the Yellow River in China, this paper explores the ice effects on the rating curves during ice periods includes ice flowing period, ice-covered condition as well as river breakup period, comparing to that under open channel flow. It is found that water level is usually not significantly affected by ice during flowing ice period, and the rating curve during ice flowing period should be coincident with that under open channel condition, although it may be affected by ice if the water surface is highly occupied by flowing ice before freeze-up. The significant variation in water level during ice-covered condition is not surprising. The ice covered rating curve for these four stations are developed; in that these conditions produced higher water levels at the same flow, they lie above the other curves, even though some data groups deviate dramatically from the curves. The curves do have some value for water level forecast during ice-covered period. During the breakup period, the water level is usually independent on the incoming flow rate, although the large flow rate should have usually a high water level. The developed breakup rating curve at stations 1 and 2 again show prominent effects of ice, although some data groups again deviate dramatically from the curves. However, the curves could be used for the water level forecast during river breakup. REFERENCES Beltaos, S., ed. River Ice Jams. Water Resources Publications, Littleton, Colorado. USA (1995). Beltaos, S. River ice jam: theory, case studies and application. Journal of Hydraulic Engineering, ASCE 109(10): (1983). Beltaos, S., Burrell, B. and Ismail, S.1991 ice jamming along the Saint John River: a case study. Canadian Journal of Civil Engineering 23: (1996). Hicks, F., Chen, X. and Andres, D. Effects of ice on the hydraulics of Mackenzie River at the outlet of Great Slave Lake, N.W.T.: a case study. Canadian Journal of Civil Engineering 22: (1995). Prowse, T. Ice jam characteristics, Liard-Mackenzie rivers confluence. Canadian Journal of Civil Engineering 13: (1986). Prowse, T. River Ice Processes (Chapter 2). In River Ice Jams, Spyros Beltaos ed., Water Resources Publications, LLC, USA (1995). Prowse, T. and Marsh, P. Thermal budget of river ice covers during breakup. Canadian Journal of Civil Engineering 16: (1989). Shen, H. T. and Ho, C.F., Two-dimensional simulation of ice cover formation in a large river. In Proceedings, IAHR Ice Symposium 2, Iowa City, Iowa, USA, (1986) Shen, H. T. and Wang, D. Under cover transport and accumulation of frazil granules. Journal of Hydraulic Engineering, ASCE 121(2): (1995). Sun, Z., Sui, J. and Ni, J. Experimental study on formation and evolution of frazil ice jams. Research report, Hefei University of Technology, China (1990). Sun, Z.C., Yang, S., Yao, K., Wang, Z., Ren, Z. and Zhou, M. Prototype observation and study of ice jam at Hequ Section of the Yellow River. In Proceedings, IAHR Ice Symposium 2, Iowa City, Iowa, USA (1986) Tuthill, A.M., Wuebben, J.L., Daly, S.F. and White, K.D. Probability distribution for peak stage on river affected by ice jams. Journal of Cold Regions Engineering, ASCE 10(1): (1996).