Sedimentation problems and management strategies of Sanmenxia Reservoir, Yellow River, China

Transcription

1 WATER RESOURCES RESEARCH, VOL. 41,, doi: /2004wr003919, 2005 Sedimentation problems and management strategies of Sanmenxia Reservoir, Yellow River, China Guangqian Wang, Baosheng Wu, and Zhao-Yin Wang Department of Hydraulic Engineering, Tsinghua University, Beijing, China Received 22 December 2004; revised 24 May 2005; accepted 6 June 2005; published 21 September [1] Reservoir sedimentation management is of critical importance for the sustainable development of surface water resources. Sanmenxia Dam, located on the middle reach of the Yellow River, in China, is notorious for its severe sedimentation problems. Because of the alarming rate of loss of reservoir storage capacity and the unacceptable negative impact induced by the rapid upstream extension of sediment deposited in the river s backwater region, the dam has been reconstructed to provide high sediment releasing capacity, and the dam operation has been changed in order to achieve a balance between sediment inflow and outflow. As a result, the dam is still providing the basin with flood control, irrigation, and hydropower generation, even though some benefits are lower than the original design. Complex sedimentation processes in response to the dam reconstruction and changes of dam operation are discussed in this paper. The engineering experiences and management practices of Sanmenxia Dam are valuable assets to the sustainable use of reservoirs built on sediment-laden rivers. Citation: Wang, G., B. Wu, and Z.-Y. Wang (2005), Sedimentation problems and management strategies of Sanmenxia Reservoir, Yellow River, China, Water Resour. Res., 41,, doi: /2004wr Introduction [2] The worldwide water demand is rising but reservoir storage capacities across the globe are dwindling. It is estimated that the worldwide average annual rate of storage loss due to reservoir sedimentation is between 0.5 and 1% of the total storage capacity [Mahmood, 1987; White, 2001]. On average, this loss in storage is approximately 45 km 3 yr 1. This amounts to having to replace approximately 300 large dams on an annual basis worldwide, with an estimated cost of $13 billion required to replace this storage capacity, even without taking into account the environmental and social costs associated with new dams [Palmieri et al., 2003]. [3] Sediment-related problems can occur upstream and downstream and vary widely from one site to another. A major effect of sedimentation is the loss of storage capacity, which can have a serious impact on water resources development by reducing water supply, hydropower production, the supply of irrigation water, and the effectiveness of flood control schemes. Growth of the delta deposits at the upstream end of the reservoir due to deposition may cause increased flooding or perhaps a rising groundwater level in the backwater upstream. The combination of sediment trapping and flow regulation also has a dramatic impact on the ecology, water transparency, sediment balance, nutrient budgets, and river morphology downstream of the reservoir. As such, sediment deposition in reservoirs is most probably the principal problem hindering the sustainable use of surface water resources that are already developed and will be developed in the future. Copyright 2005 by the American Geophysical Union /05/2004WR Integrated sediment management in reservoirs is therefore one of the highest concerns for governments and engineers across the globe. [4] Natural river reaches are usually in a state of morphological equilibrium where the sediment inflow on average balances the sediment outflow. Sediment deposition occurs as the flow enters the impounded reach of a reservoir due to a decrease in flow velocity and drop in transport capacity of the flow. Coarse sediment is deposited first in the upper part of the reservoir, while finer sediment is transported further into the reservoir. The impounded reach will accumulate sediment and lose storage capacity until a new balance with respect to sediment inflow and outflow is again achieved. [5] Reservoirs have traditionally been planned, designed, and operated on the assumption that they have a finite life, frequently as short as 100 years, which will eventually be terminated by sediment accumulation [Morris and Fan, 1997]. Little thought has been given to reservoir replacement when today s impoundments are lost to sedimentation, or to procedures to maintain reservoir services despite continued sediment inflow. Transforming reservoirs that are subject to sedimentation into sustainable resources that generate long-term benefits requires fundamental changes in the way they are designed and operated. It requires that the concept of a reservoir s life being limited by sedimentation be replaced by a concept of managing both water and sediment to sustain reservoir function. [6] The Sanmenxia Reservoir in China offers an excellent example of the consequences of reservoir sedimentation, sediment management strategies, and potential solutions to critical sedimentation problems. The Sanmenxia Dam is located on the lower part of the middle reach of the Yellow River in China. It is a large-scale multipurpose hydroproject 1of17

2 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR first constructed on the main stem of the Yellow River, as a main part of the Comprehensive Yellow River Basin Plan approved by the central government in the 1950s. The construction of the dam was initiated in 1957, and the impoundment of water commenced in September Because the reservoir storage capacity was decreasing at an alarming rate and because of the unacceptable negative impact of rapid upstream extension of backwater sediment deposition, the dam had to be reconstructed to provide high sediment-releasing capacity of outlet structures, and dam operation has been substantially changed in order to achieve a balance between sediment inflow and outflow, and eventually to minimize the adverse effect of reservoir sedimentation. This project not only demonstrated the perils of giving inadequate consideration to sediment management in the planning and design of a reservoir, but it also demonstrated that a sediment balance may be achieved across an impounded reach even in a large reservoir built on a river with an extremely high sediment load. [7] This paper provides a thorough review and analysis of the complex reservoir sedimentation processes in response to the dam reconstruction and management strategies applied. The unique engineering experience and management practices learned from the case study are particularly useful for sustainable development and management of reservoirs built on heavily sediment laden rivers. 2. Construction of Sanmenxia Dam [8] The Yellow River, with a drainage area of 752,000 km 2 and a length of 5464 km, is the second largest river in China. The river basin is mostly arid and semiarid, with a long-term average annual runoff depth of 77 mm and a total annual runoff volume of m 3. The water resource per capita in the basin is 593 m 3, only one quarter of the average in China. In the central part of the basin, the river cuts through a vast loess plateau with an area of 640,000 km 2, of which 454,000 km 2 suffers from severe soil erosion. The annual sediment yield of the gullied loess plateau area amounts to more than 10,000 t km 2. The long-term annual sediment load at Sanmenxia station is t, ranking first of all the world s rivers. The average sediment concentration is 35 kg m 3, and hyperconcentrated floods with sediment concentration over 100 kg m 3 occur quite often; a sediment concentration of 911 kg m 3 was measured on 7 September The majority of the sediment load consists of silt with a median diameter of about 0.03 mm. [9] Integrated basin management in the Yellow River is a complex and difficult task. Soil loss, sedimentation, and flooding in the basin have been a concern for thousands of years [Wu et al., 2004a; Wu et al., 2005]. Severe soil loss is not only a serious problem for land use and the ecologic environment in the middle basin, but also the primary reason for the sedimentation in the lower Yellow River. In the loess plateau area, soil and water conservation, involving terracing, afforestation, and silt-control dams, is critically important to reducing the amount of sediment entering into the lower Yellow River. However, because of the harsh climate and morphological and geological conditions, effective control of soil erosion in the loess plateau is extremely difficult. Therefore soil and water conservation still has a long way to go. [10] As the river flow delivers a huge amount of sediment through the course of the lower reaches, which has a relatively small slope and a wide cross section, about 25% of the sediment load is deposited on the river corridor and another 50% is deposited on the delta prior to being discharged to the Bohai Sea. Since 1855, because of flood control measures to contain the river within its banks, the accelerated sedimentation has caused the channel bed of the lower Yellow River to rise at a rate of 5 10 cm yr 1.Itis now more than 10 m higher than the surrounding land in some places, a condition known as a perched river. The perched river poses a great threat to the North China Plain with the possibility of levees being breached during flood events. This problem is exacerbated because the river has extremely active channels that often shift and migrate rapidly in a random fashion. The great amount of sedimentation and its composition are the main reasons for the channel instability and river evolution in the Yellow River. For example, the riverbed can be scoured down as much as 10 m in only 24 hours and the channel is extremely unstable [Wan and Wang, 1994]. [11] Throughout the country s history, the Yellow River has caused more flood disasters than any other Chinese river. Statistics show that during the period of 602 B.C. to 1938 A.D., the embankment of the lower Yellow River was breached 1590 times and the channel changed its course 26 times, of which six were major changes. Thus flooding is a constant threat to the safety of communities and farmlands on both sides along the lower reaches of the river, bringing about serious problems for river management [Li, 2003; Zeng, 2004]. In the lower reaches, from Tiexie to the sea, the river is confined to a levee-lined course with a length of 876 km as it flows northeasterly across the North China Plain. The levee system can hold a flood discharge of up to 22,300 m 3 s 1 at Huayuankou. The strategy adopted to control the lower Yellow River is to confine large flood flows in a wide floodplain between levees, while using an inner main channel strengthened by river training works to transport the high sediment load and to guide the flow away from the levees and to prevent the flow from scouring against the levee foundations [Li et al., 2002]. [12] Chinese people have long dreamed of a dam to control the floods. Early in 1935, some river engineers and experts proposed the construction of Sanmenxia Dam on the middle reach of the Yellow River to control flood flows entering the downstream reaches. A landmark report entitled Comprehensive river basin planning report, which officially proposed the construction of Sanmenxia Dam, was submitted to the National People s Congress in 1955 and was immediately approved. [13] The Sanmenxia Dam was constructed within the period of As a multipurpose project for flood control, hydropower, irrigation, navigation, and ice jam control, it is the first dam ever built on the Yellow River in China. The location of the Sanmenxia Dam and the plan view of the reservoir area of the dam are shown in Figure 1. The drainage area above the dam amounts to 688,400 km 2, which is 92% of the total drainage area of the Yellow River. It controls 89% of the runoff and 98% of the sediment coming into the lower Yellow River. The maximum historical floods records at the dam site were 22,000 m 3 s 1 in 2of17

3 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 1. Plan view of Sanmenxia Reservoir: (a) sketch of the Yellow River basin and (b) sketch of the Sanmenxia Reservoir and 36,000 m 3 s 1 in 1843 [Yang et al., 1995; Morris and Fan, 1997]. [14] The reservoir consists of a dam, outlet structures, and a power plant. In the original design, the maximum pool level in the reservoir was set at an elevation of 360 m, with a total storage capacity of m 3, and an installed capacity of 116 MW. Under such conditions, 3500 km 2 of the floodplains would be inundated, requiring the relocation of 870,000 people. To reduce this impact, the dam was built at an elevation of 353 m with a maximum dam height of 106 m and a maximum pool level of 340 m. The reservoir area extends upstream a distance of 246 km to Longmen. [15] Hydrological reservoir routing indicated that the reservoir needed a maximum water level of 335 m for floods with 1000-year recurrence, and a maximum water level of m for floods with 200-year recurrence. Therefore people living at altitude below the contour line of 335 m in the reservoir area of Sanmenxia had to be relocated before the flood season of Under such an elevation the reservoir had a total storage capacity of about m 3. Since then, the elevation of 335 m has been used as the maximum water level for flood control. In practical operation, a maximum water level of m was reached on 2 February [16] As shown in Figure 1, the Yellow River flows south from Longmen to Tongguan, then makes a 90 turn and goes east. The dam site is located at a gorge section km downstream of Tongguan, which is situated immediately downstream of the confluence of the Yellow and Wei rivers. The river is constricted from a width of more than 10 km to less than 1 km at Tongguan, forming a naturally constricted river reach which acts as a hydraulic control section that influences the river s stage, flow velocity, and thus the sediment transport capacity in the reaches of both the Yellow and Wei rivers upstream of Tongguan. During floods, the stage at Tongguan rises rapidly and creates backwater conditions extending tens 3of17

4 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR of kilometers upstream, thereby reducing the sediment transport capacity and promoting sediment deposition upstream of the constricted Tongguan reach. Because the bed elevation and stage at Tongguan exert hydraulic control on both the Yellow and Wei rivers upstream, the control of channel bed aggradation in the vicinity of Tongguan is the key to limiting sediment deposition and backwater effect further upstream. 3. Reconstruction of Sanmenxia Dam [17] Following completion of the dam, the reservoir began to function as a storage basin from September 1960 to March 1962, when the maximum pool level reached m. Severe sedimentation problems became evident immediately after impoundment. During the first 18 months after the dam s completion, 93% of the incoming sediment was trapped in the reservoir, representing a release efficiency of only 7%. The reservoir lost 17% of its storage capacity of m 3 below an elevation of 335 m and 26% of its storage capacity of m 3 below an elevation of 330 m due to sedimentation. [18] To mitigate the sedimentation problems, the operation scheme of the dam was changed to detain only floodwater in flood seasons. However, the flood-releasing capacity of the outlet structures was limited. Though the reservoir was operated at a low level during flood seasons with all the outlet structures fully opened, the reservoir stage was still high and serious sedimentation in the reservoir due to the detention of large amounts of flood water was still inevitable. By October 1964, the accumulated sediment deposition below the elevation of 335 m reached a volume of m 3, which was a 41.5% loss of the corresponding storage capacity; and the accumulated sediment deposition below the elevation of 330 m reached a volume of m 3, which was a 62.9% loss of the corresponding storage capacity. [19] During the impounding period from September 1960 to March 1962, the elevation of Tongguan, which indicates riverbed variations and is defined as the stage corresponding to a discharge of 1000 m 3 /s at Tongguan station, was raised 4.5 m [Long and Chien, 1986; Long, 1996], finally reaching m in March Backwater sediment deposition extended over Chishui in the lower Wei River, about 187 km upstream of the dam or 20 km upstream of Huaxian; and extended 152 km in the main stem of the Yellow River, reaching to cross section 49 [Yang et al., 1995; Chen et al., 1999]. [20] After the mode of dam operation was changed, the backwater sediment deposition was still rapidly extending upstream. This increased the flood risk due to the decrease in the discharge capacity and the rise of the flood stage, threatening the industrial and agricultural economic productivity, and more important, Xi an, the capital city of Shanxi province, in the lower reaches of the Wei River. In addition, it potentially would require the relocation of an additional 1 million people. There was much pressure to improve the situation because of the dense population and the scarcity of farmland in China. In order to alleviate the serious reservoir sedimentation problem and to achieve a balance between sediment inflow and outflow, a special meeting was held in Xi an in December 1964 to find a solution to the sedimentation problem in the reservoir. The late Premier Enlai Zhou presided over the meeting, showing the high demand for a resolution to this problem. A policy was established to ensure the safety of Xi an in the upstream as well as that of the lower Yellow River, and a decision was made on the reconstruction of outlet structures to increase the discharge capacity. [21] The reconstruction work was carried out in two stages. The work of the first stage was started in July 1966 and completed in August In the first stage, two tunnels at an elevation of 290 m were added on the left bank and four penstocks were remolded into outlets for the purpose of sluicing sediment, as shown in Figure 2. After the first stage work, the discharge capacity had been increased from 3180 m 3 s 1 to 6178 m 3 s 1 at a pool level of 315 m. The increased discharge capacity played a definite role in reducing sediment deposition in the reservoir area below Tongguan. However, the sills of the outlet structures were too high and the capability of the reservoir to release floodwater was inadequate. The ratio of outflow-inflow sediment reached 80%, and the amount of backwater deposition increased accordingly and the bed elevation at Tongguan continued to rise. [22] The work of the second stage commenced in December In this stage, eight bottom outlets at an elevation of 280 m previously used for diversions were reopened to sluice sediment at the lower elevation and to generate stronger headward erosion. In order to suit power generation at a low altitude by the river s current during the flood seasons, the intake of penstocks 1 5 were lowered from an elevation of 300 m to 287 m, and five generation units with a total installed capacity of 250 MW were installed. The first generating unit started to operate at the end of 1973, and the rest were put into operation by the end of After the second stage of reconstruction, the releasing capacity of all the outlets increased to 9319 m 3 s 1 at an elevation of 315 m (see Figure 3). With this capacity, no significant backwater could accumulate immediately behind the dam in medium or minor flood conditions, and the outflow and inflow ratio of sediment had reached 105%. In the period from the beginning of summer in 1970 to the end of the flood season in 1973, about m 3 of sediment was scoured away from the dam to Tongguan, and a part of the reservoir capacity was restored. The reservoir storage capacity below the elevation of 330 m was recovered from a volume of m 3 to a volume of m 3, resulting in a net increase of m 3. Correspondingly, the elevation of Tongguan dropped by 2 m. [23] After these two stages of reconstruction, the dam can discharge sediment-laden water and causes no significant detention of large amounts of flood water. However, because of surface abrasion and cavitation, the bottom sluices were severely damaged, and therefore they underwent repairs from 1984 to As a result, the total discharge capacity of bottom sluice openings 1 8 was reduced by about 471 m 3 s 1 due to the reduction in flow area caused by lining of the sluices [Chen et al., 1999]. To compensate for the reduction resulting from bottom sluice repairs, two more bottom sluices, 9 and 10, were opened in In an attempt to make the most of the dam by fully utilizing the potential for hydropower generation in the 4of17

5 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 2. General layout and outlet structures of Sanmenxia Dam: (a) plan view; (b) front view of original design; (c) front view after reconstruction. 5of17

6 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 3. Increased discharge capacity of the outlets of Sanmenxia Dam (excluding the discharge capacity of penstocks). nonflood season, penstocks 6 and 7 were converted back to power generation in 1994 and 1997, respectively. Considering the effectiveness of sediment flushing by discharge at low levels, the last two bottom sluices, 11 and 12, were also opened in 1999 and 2000, respectively. To date, there are 27 outlets, excluding penstocks, at Sanmenxia Dam with a total discharging capacity of 9701 m 3 s 1 at the elevation of 315 m for the release of flood flows, as summarized in Table Management of Reservoir Sedimentation [24] Sedimentation in the reservoir depends on the incoming water and sediment, the discharge capacity, and the operational level. Reconstruction of outlet structures has significantly increased the discharge capacity, providing the dam with the necessary facility for avoiding significant detention of floodwater, which is important for maintaining sediment balance across the impounded reach in the reservoir. On top of this, the dam operation has also been substantially changed in order to achieve a sediment balance, and it can be divided into three time periods or three different modes of operation. The annual inflows of water and sediment measured at Tongguan hydrological station, annual average pool level, accumulated volume of sediment deposition, and the elevation of Tongguan are given in Table 2. Average pool levels corresponding to each operation mode are shown in Figure 4, and the annual variation of characteristic pool levels is shown in Figure 5. [25] 1. The first mode is storage. This mode was employed during the initial period of reservoir impoundment, from September 1960 to March 1962, when the reservoir was operated at high storage level throughout the whole year, according to the original project design. [26] 2. The second mode is flood detention. This mode was applied from March 1962 to October 1973, during which the reservoir was used for flood detention and sediment sluicing, water being released without restrictions. The reservoir was operated at a low storage level throughout the year, detaining floods only during flood seasons and sluicing sediment with the largest possible discharges. [27] 3. The third mode is controlled release. This mode has been used ever since November 1973, to store relatively clear water in nonflood seasons (November June) and discharge turbid water in flood seasons (July October). In this period, the reservoir has been operated at a high Table 1. Outlet Structures in Sanmenxia Dam Outlet Dimensions Total Number Serial Number Invert Elevation, m Discharge Capacity at 335 m, m 3 s 1 Year of Initial Operation Deep holes width depth = 3 8 m each 1960 Bottom sluices width depth = 3 8 m each and 2000 Tunnels diameter = 11 m and 1968 Flushing pipe diameter = 7.5 m Penstocks a each 1994 and each a Penstocks 5 8 were converted to flushing pipes for discharging floodwater in Penstock 5 was converted back to power generation in Penstocks 6 and 7 were converted back to power generation in 1994 and 1997, respectively. 6of17

7 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Table 2. Inflows of Water and Sediment, Dam Operation Level, and Sediment Deposition Operation Conditions Year Water Runoff, a 10 9 m 3 Sediment Load, a 10 9 t Average Pool Level, m Accumulated Volume of Deposition, b 10 9 m 3 NFS d FS e Year NFS d FS e Year NFS d FS e Year Elevation of Tongguan, c m Natural state Storage Flood detention Controlled Release Before reconstruction First stage of reconstruction Second stage of reconstruction a Data measured at the Tongguan hydrological station. b Accumulated volume of sediment deposition downstream of Tongguan. c Elevation of Tongguan measured at the end of flood season of each water year. d Nonflood season, time period from November of the previous year to June. e Flood season, time period from July to October. operational level, with the maximum level not exceeding a value between 322 m and 326 m, to store water for irrigation and hydropower generation in nonflood seasons; and at a low storage level of 305 m during flood seasons, allowing low-head hydropower generation, but in times of flood discharge greater than 2500 m 3 s 1 all the outlets were to be opened to sluice the sediment as much as possible. [28] Sedimentation in the reservoir was different, corresponding to different operational modes and the outlet discharge capacities. Figure 6a shows the accumulated volume of sediment deposition in different reaches in the reservoir area, and Figure 6b shows the variation of reservoir storage capacity in the reservoir. Because the reservoir was operated at a low storage level and the discharge capacity was increased in the period of flood detention, the reservoir was changed from deposition to scouring. In the period of controlled release, deposition occurred during nonflood seasons while scouring occurred in flood seasons, maintaining a sediment balance in the reservoir within a water year or in several years. Within the range of operation levels below 323 m, the capacity of m 3 is available for controlling medium floods in the flood season whenever necessary. About m 3 of reservoir capacity below the level of 330 m and m 3 below the level of 335 m have been kept for use in the event of an extremely large flood. In the normal operation of the reservoir, the floodplain in the reservoir would be inundated only during extremely large floods; the major task of operation in flood seasons is the detention of such floods [Wu, 1992]. The detained sediment would be eroded in the 7of17

8 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 4. Average pool levels of Sanmenxia Dam in different time periods. next several years. However, once the direct backwater effect reaches the confluence area above Tongguan during a flood event and lasts for a certain period of time, sediment deposition on the floodplain would be inevitable and a part of the reservoir capacity would become unavailable for use in the latter periods. It is obvious that for preserving a usable capacity in the reservoir, one of the guiding principles of the reservoir s operation is to prevent the direct backwater effect from exceeding Tongguan as much as possible. [29] The longitudinal and transverse profiles in the reservoir have been varied with the changes of operational conditions as shown in Figures 7 and 8. The figures show that during the impoundment up to October 1961, i.e., the first year of operation, deposition was in the form of a delta with a topset slope of , nearly half the original river bed slope, and a foreset slope of The apex of the delta was near section 31, and the cross section was raised evenly in the transverse direction, making no distinction between the main channel and floodplain. During the wet year of 1964, the annual water and sediment inflows were m 3 and t, respectively. The reservoir was severely silted because the outlet capacity was too small and the sluice holes were located too high. About t of oncoming sediment was deposited in the reservoir during the flood season, which was 70% of the total oncoming sediment. This was the most serious year of siltation. Longitudinal deposition was in the form of a cone, and a channel-floodplain configuration was formed. This can be seen from the transverse section, in which the floodplain rose simultaneously with the main channel. In 1973, the deposition occurred with the main channel being eroded because the reservoir was used for flood detention only during the flood season and the outlet discharge was enlarged after reconstruction. The figure reveals that the main channel had been lowered by erosion with basically no changes in the floodplain. The longitudinal bed slope of the main channel was , and that of the floodplain was A high floodplain and deep main channel had been formed; about m 3 channel storage below Tongguan was recovered. [30] Since 1974 when the mode of controlled release has been used, the longitudinal channel bed profile has basically been composed of superimposed deltas in the nonflood season, as shown in Figure 9. When the reservoir stage was lowered in the flood season, the deposits in the main channel were eroded along with the drawdown of the reservoir. If the oncoming flow in the flood season was too small and/or the operational stage in the nonflood season was relatively too high, depositions that took place at the head reach may not have been entirely eroded and thus would not be carried out of the reservoir in the same year. The residue would continue to be eroded until a large flood came the next year. In this case, the equilibrium of the deposition and erosion in the reservoir could not then be maintained within an operational year. [31] As mentioned earlier, one of the main purposes of the Sanmenxia Reservoir is to control flood flows for the Figure 5. Annual variation of characteristic pool levels of Sanmenxia Dam. 8of17

9 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 6. Variations of accumulated deposition and reservoir storage capacity in Sanmenxia Reservoir: (a) accumulated deposition and (b) reservoir storage capacity. lower Yellow River. Since the adoption of the controlled release mode in 1974, the dam has kept a reservoir capacity of about m 3 for flood control. Therefore, despite the sedimentation problems, the Sanmenxia Reservoir has played an important role in flood control for the lower Yellow River. Since the construction of the dam, six big floods with peak discharges higher than 10,000 m 3 s 1 have occurred upstream of the dam. During these flood events, the reservoir s detention of flood flows greatly reduced the burden of flood control in the lower Yellow River and therefore reduced the flood damage caused by the inundation of over-floodplain flows. [32] The regulation of both water and sediment of Sanmenxia Reservoir has a pronounced effect on fluvial processes in the lower Yellow River located downstream of the dam. In the earlier period of dam operation from September 1959 to October 1964, a volume of m 3 of sediment (equivalent to t) was deposited in the reservoir area. During this time period, the release of clear water caused extensive channel scouring in the lower Yellow River, resulting in a volume of t net sediment erosion from the channel bed of the lower Yellow River (mostly above Aishan, 452 km downstream of Huayuankou). As indicated in Figure 6, after October 1964, continuous sediment deposition in the reservoir area basically stopped, and correspondingly sediment-laden flows were released from the dam and the condition of the lower Yellow River was again changed from scouring to deposition. Analysis [Yang et al., 1995] indicated that up to the flood season of 1971, the lower Yellow River was generally returned to its predam conditions. This is to say that the Sanmenxia Dam had stopped the sediment deposition and channel bed rising in the lower Yellow River for about 11 years, from 1960 to [33] Since 1974, channel scouring usually occurs in the nonflood season and channel deposition occurs in the flood season in the lower Yellow River. This pattern is clearly related to the pattern of sediment release from the dam determined by the mode of the controlled release for dam operation. Concentrated release of sediments from the dam during the flood season is basically beneficial to the transport of sediment in the lower reaches. As a matter of fact, how to reduce the sediment deposition in the downstream reaches of Sanmenxia Dam is an important factor being considered in the operation of the dam. A general Figure 7. Typical longitudinal profiles in Sanmenxia Reservoir. 9of17

10 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 8. section 31. Typical transverse profiles in Sanmenxia Reservoir: (a) cross section 22 and (b) cross principle is to avoid the release of sediments to the downstream in low flows because low flows have a relatively low sediment transport capacity and can easily cause severe sediment deposition in the downstream channel. 5. Sedimentation and Flood Hazards in the Lower Wei River [34] The most serious adverse effect caused by the impoundment of water at the Sanmenxia Reservoir is the sedimentation in the lower Wei River and consequently the flooding risk to Xi an, an ancient capital of China. The Wei River is the largest tributary of the Yellow River, and it empties into the Yellow River at Tongguan, as shown in Figure 1. The lower Wei River is in the backwater region of Sanmenxia Reservoir. The elevation of Tongguan, acting as the base level of the bed profile of the lower Wei River, has been increasing since the operation of Sanmenxia Dam. Figure 10 shows the elevation of Tongguan varying with time from 1960 to There are three increasing periods (periods 1, 3, and 5) and two decreasing periods (periods 2 and 4). The first increasing period was associated with high pool level in earlier periods of operation due to an unrealistic operation method and later limited outlet capacity. The second and third periods of increasing were mainly caused by reduction in inflow discharges, since in the controlled release period after 1973 the pool level was kept essentially the same within each operational year, as shown in Figure 5. On the other hand, the first decreasing period mainly resulted from the low pool level and the enlarged release Figure 9. Channel bed profile adjustments in Sanmenxia Reservoir below Tongguan within a water year in the time period of controlled release. 10 of 17

11 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 10. Variation of the elevation of Tongguan. capacity of the dam; and the second decreasing period mainly resulted from high flow discharges. [35] Generally speaking, sedimentation in the lower Wei River occurred during the periods of increased elevation of Tongguan and erosion occurred during the periods of decreased elevation of Tongguan. Figure 6a shows the accumulated volume of sedimentation in the lower Wei River, indicated by the line with solid squares. The total deposition volume up to the year 2001 was about m 3. A comparison of the two curves in Figure 6a and Figure 10 shows that sedimentation in the lower Wei River is closely related to the elevation of Tongguan. Accelerated sedimentation occurred as the elevation of Tongguan increased and erosion took place as the elevation of Tongguan decreased. The increased elevation of Tongguan prevented sediment transport from the tributary into the Yellow River, thus resulting in accumulated sedimentation in the lower Wei River. Figure 11 shows that sedimentation was occurring mainly in a 100-km-long reach upstream of Tongguan. The accumulated deposition volume per unit length was reduced to nearly zero at Xianyang, an ancient town near Xi an. [36] Figure 12 shows the transverse bed profiles of cross sections 2 and 7 in the lower Wei River, measured in 1960 and The floodplain had risen by 3 5 m due to sedimentation, and the main channel had shrunken and become unstable. The flood discharge capacity of the channel was thence reduced, and the flood stage at the same discharge was substantially increased. [37] Erosion and sedimentation have been propagating upstream in a series of retrogressive waves. Figures 13a, 13b, and 13c show the distribution of the deposition rate per unit river length in the periods of , , and In these figures, V s is the volume of sediment deposition per unit length per year in the given periods of time. [38] In the period of , the elevation of Tongguan rose abruptly from 323 m to m (see Figure 10). As a result, sedimentation occurred in the reach around Huaxian station at a rate of up to 2.5 Figure 11. River. Longitudinal distribution of accumulated deposition volume per unit length in the lower Wei 11 of 17

12 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 12. Channel bed aggravation near Huaxian Station from 1960 to 2001: (a) cross section 2, 21.2 km from Tongguan and (b) cross section 7, 59.9 km from Tongguan tkm 1 yr 1 (Figure 13a). The numeral 1 indicates that the sedimentation in this period corresponded to the first period in which the elevation of Tongguan increased. In the period of the sedimentation moved upward to the reach between Huaxian and Lintong, but the rate of sedimentation decreased to about tkm 1 yr 1 (Figure 11b). In the meantime the first erosion wave occurred at the river mouth, which corresponded to the first period in which the elevation of Tongguan decreased, indicated by numeral 2. In the first sedimentation wave had moved upstream to Lintong and the first erosion wave had moved to Huaxian station, and the peaks had obviously decreased too. During this time period the second sedimentation wave occurred in the reach between the river mouth and Huaxian station, indicated by numeral 3. Of course this was associated with the second period of the increased elevation of Tongguan. The result described above clearly illustrated that the increase and decrease of the elevation of Tongguan generated erosion and sedimentation waves which could propagate retrogressively toward the upstream in the lower Wei River, at a speed of about 10 km yr 1. [39] Severe aggradation caused the flood stage to rise, and consequently escalating flood hazards occurred in the lower Wei River. Figure 14 shows the stage rising at Huaxian station for flood discharges of 200 and 4000 m 3 s 1. A flow with a discharge of 200 m 3 s 1 is usually confined in the main channel, and therefore the corresponding stage rise reflects only the sedimentation and shrinking of the main channel. For a flood discharge of 4000 m 3 s 1 the stage rise is due to sedimentation both on the floodplain and in the main channel. Sedimentation during the periods of increased elevation of Tongguan made the low-flow stage sharply increase because sediment was first deposited in the main channel. Similarly, during periods when the elevation of Tongguan decreased, erosion occurred in the channel. Therefore the low flood stage rise varied in a range of 1 4 m. In the meantime the high flood stage rise for a discharge of 4000 m 3 s 1 has also been increasing in a similar fashion. The flood stage is now 6 m higher than before the operation of Sanmenxia Dam, which poses a higher flood risk to the lower reaches of the Wei River. After 1990s, both the bank-full discharge and the frequency of inundation of floodplain have dropped significantly due to the reduced flood discharges, and the latter resulted in an increase in roughness values at the floodplain that had more crops. Consequently, the high flood stage rose significantly. [40] On 1 September 2003 a flood with a peak discharge of 3570 m 3 s 1 occurred in the lower Wei River and the stage at Huaxian station reached m, 0.51 m higher than the highest stage record at the station, even though the flood discharge was 230 m 3 s 1 less than in the 1998 flood (3800 m 3 s 1 ). Several factors contributed to the extremely high flood stage: (1) continuous aggradation caused by high elevation of Tongguan in recent years; (2) channel shrinking and bank-full discharge decreasing due to a lack of high flow discharges which were necessary to maintain the channel; and (3) unrealistic development and vegetation on the floodplain which increased the roughness. Five levee breaches occurred around Huaxian city, and more than 20,000 people had to be evacuated. The record-breaking flood stage associated with a peak discharge of 3570 m 3 s 1, which is only a 2-year flood, warns us of the severity of the sedimentation problem and the flooding hazard in the lower Wei River. One of the effective countermeasures is to change the operation of Sanmenxia Dam once again by lowering the pool level, consequently lowering the elevation of Tongguan as much as possible. 6. Impact of Water Resources Development [41] Briefly speaking, the severe sedimentation problems that occurred in the Sanmenxia Reservoir were 12 of 17

13 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 13. Erosion (negative values) and sedimentation (positive values) per unit length per year in a series of retrogressive waves in the lower Wei River, as a result of increase and decrease of elevation of Tongguan: (a) , (b) , and (c) mainly due to the following factors: (1) The original reservoir planning and design were performed according to the standards for and experiences with clear water. The principle of streamflow regulation that was adopted was basically the same as that used in reservoirs on nonsediment-laden rivers. Of course, the deposition of sediment in the reservoir was unavoidable. (2) The difficulties involved in resettling the inhabitants of the reservoir area were underestimated. (3) In the original design, anticipating that the sediment inflow of the reservoir would be rapidly reduced was too optimistic and unrealistic. Sediment retention reservoirs were proposed to be built in the upland area, and soil conservation works were also planned, with a goal of 50% in less than 15 years. Through practice, it has become clear that the enormous amount of the sediment load could not be greatly reduced in a short period of time. A reservoir built solely for the storage of water and the retention of sediment on a river heavily laden with sediment is not the solution for water resources development. By the operational practice from 1974 on, it has been proven that the reconstruction of the outlets and the new operational mode are more suitable to the conditions of the Yellow River. The adopted operational mode presently in use has provided valuable experience for solving sediment problems of large-sized reservoirs built on sediment-laden rivers. As an example, the Xiaolangdi Project, which is 130 km downstream of Sanmenxia Reservoir on the Yellow River, was planned 13 of 17

14 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Figure 14. Flood stage rising at Huaxian station for discharges of 200 m 3 s 1 and 4000 m 3 s 1. and designed essentially based on the experience gained from the operation of Sanmenxia Reservoir. The Three Gorges Project, being built on the Yangtze River in China, is also designed based on the strategies and principles developed in Sanmenxia Reservoir to deal with its possible sedimentation problems. [42] The Yellow River is deficient in water and abundant in sediment. Because of the regulation of water in large reservoirs and the utilization of water resources for irrigation in recent years, the annual runoff and its temporal distribution within a year have been greatly modified [Li, 2003; Cai and Rosegrant, 2004; Wu et al., 2004a, 2004b], as indicated by Figure 15 and Table 3. The mean annual runoff was reduced from m 3 in to m 3 in , and further to m 3 in Inflow in the flood season was reduced from 55.9% in to 46.0% of the annual runoff in , and further to 42.8% in After regulation of reservoirs in the upstream, the peak discharges of the flow have been attenuated. If the rainfall intensity is moderate, all the soil conservation measures are also effective in the detention of flood flow. Therefore observations have shown that the peak discharge and the frequency of floodwaters entering the Sanmenxia Reservoir have been decreasing. Figure 15b shows the number of days that had daily mean discharges was greater Figure 15. Variation of flow at Tongguan station: (a) annual runoff and (b) number of days with a daily mean discharge greater than 3000 m 3 s of 17

15 WANG ET AL.: MANAGEMENT STRATEGIES OF SANMENXIA RESERVOIR Table 3. Statistics of Annual Runoff and Sediment Load at Tongguan Hydrological Station Time period Water Year Runoff, 10 9 m 3 Flood Season Ratio, % Sediment Load, 10 9 t Water Year Flood Season Ratio, % 11/ / / / / / / / / / / / than 3000 m 3 s 1 at Tongguan station. The results shown in the figure illustrate the decreasing frequency of flood flows entering the reservoir. [43] Variation of the inflow has pronounced effects on the reservoir sedimentation. Increased inflow during the nonflood season has the ability to scour the alluvial channel, bringing more sediment into the reservoir. As a result, more sediment would be deposited at the backwater area of the reservoir, and this sediment probably could not be eroded during subsequent floods. Decreased runoff in flood seasons and fewer occurrences of floods are unfavorable to the scouring or transport of the deposited sediment from previous nonflood seasons. As a result, the backwater deposition again has been extending upstream since 1986, and the channel bottom elevation and water surface elevation have been gradually rising, as shown in Figure 10. This has caused further sedimentation problems and exacerbated the flooding hazards in the lower reaches of the Wei River. [44] The variations of water and sediment inflows required that the operation level used from 1974 to 2002 be adjusted to properly regulate the water and sediment in order to maintain balanced deposition in the reservoir area. Moreover, the construction of Xiaolangdi Dam relaxes constraints for the adjustment of the operation; many functions borne by Sanmenxia Reservoir now can be undertaken at the Xiaolangdi Reservoir. For example, the functions of flood control and water storage for irrigation can be partially accomplished using the Xiaolangdi Reservoir. Debates on adjustments of the reservoir s operation level have continued for several years. Some experts argue that the upstream extension associated with backwater deposition in recent years was mainly caused by the reduction of runoff that resulted from excessive water resource development. This is supported by the good correlation between the elevation of Tongguan and the annual runoff indicated by Figure 16. Therefore lowering the pool level has no effect on the backwater deposition. In addition, the reservoir has established and maintained a new balance between the newly established social and ecological system within the reservoir area below Tongguan and the backwater generated by the pool level. Any change in the operation level may disturb the new balanced environment and should be avoided. First, lowering the pool level will reduce the output of hydropower generation. Second, it may create difficulty for water withdrawal for agricultural irrigation and municipal domestic and industrial uses because water intakes were 15 of 17 constructed according to the earlier operational level. Third, there is about 210 km 2 of wetland below Tongguan in the high floodplains formed by sediment deposition [Zhang and Zhang, 2004]. The water in this wetland is usually sustained when there are high pool levels in nonflood seasons, and it forms an ideal habitat for birds and other wildlife. Lowering the pool level will cause a reduction in the area of wetland and will upset the riparian ecosystem. [45] Some other experts believe that the operation level used in the controlled release period was determined based on earlier inflow conditions. However, it is no longer compatible with the changed inflow conditions, notably the reduced annual runoff after Lowering the pool level could compensate, at least partially, for the negative effect of reduced runoff and could also prevent the backwater extension. [46] An extreme plan proposed by a few agencies even suggests decommissioning the Sanmenxia Dam and allowing the flow to run freely without control. By doing so, it is estimated that the channel bed elevation at Tongguan station could be scoured down more than 2 m. A lower channel bed elevation at Tongguan would greatly slow down the sedimentation process in the lower reaches of the Wei River and consequently reduce the demand for flood control. However, dam decommissioning is not widely accepted because (1) it simply neglects the economic, social, and ecological values of the dam and therefore its rationality cannot be justified; and (2) the task of flood control function at Sanmenxia Reservoir can not be fully replaced by Xiaolangdi Reservoir, which was completed in 2000 and has a reservoir capacity of m 3 designated for flood control for long-term use. As a matter of fact, the Xiaolangdi Reservoir was designed according to the principle of joint operation with Sanmenxia. Considering the sedimentation problems, the Sanmenxia Reservoir will allow normal floods to pass through the reservoir without detention in order to minimize sediment deposition, but it will still need to store floodwater for floods with 100-year recurrence or higher. [47] Research is being conducted to find the best operation level for preventing backwater deposition, without substantially affecting the newly formed ecological system Figure 16. Relationship between the elevation of Tongguan measured at the end of each flood season and the annual runoff (data of ).