- Email: [email protected]
Strategies for managing reservoir sedimentation
International Journal of Sediment Research 24 (2009) 369-384
Strategies for managing reservoir sedimentation Zhao-yin WANG1 and Chunhong HU2 Abstract Sediment deposition in reservoirs has caused the loss of 66% of the reservoir capacity in China. The main sedimentation control strategies are: 1) storing the clear water and releasing the turbid water; 2) releasing turbidity currents; 3) Draw-down flushing and empty flushing; and 4) dredging. The paper summarizes these strategies with examples. Sediment transport in many Chinese rivers occurs mostly during the 2-4 month flood season, that is, 80-90% of the annual sediment load is transported with 50-60% of the annual runoff. By storing the clear water after the flood season and releasing the turbid water during the flood season, less sediment deposits in the reservoir while the reservoir is still able to store enough water for power generation in the low flow season. The Three Gorges and Sanmenxia reservoirs apply this strategy and control sedimentation effectively. Turbidity currents have become the main sedimentation control strategy for the Xiaolangdi Reservoir. Empty flushing involves reservoir draw-down to temporarily establish riverine flow along the impound reach, flushing the eroded sediment through the outlets. Case studies with the Hengshan Reservoir and Zhuwo Reservoir are presented. Jet dredgers have been used to agitate the reservoir deposit so that the deposit is released from the reservoir with currents. The sediment releasing efficiency is 30-100% for storing the clear and releasing the turbid; 6-40 % for turbidity current; and 2,400-5,500% for empty flushing. Empty flushing causes high ecological stress on the ecosystem to the downstream reaches. Storing the clear and releasing the turbid is the best strategy to control reservoir sedimentation while achieving hydro-power benefit and maintain ecological stability. Key Words: Reservoir sedimentation, Storing clear and releasing turbid, Flushing, Turbidity current, Three Gorges Reservoir
1 Introduction According to the estimates of the International Commission on Large Dams (ICOLD), the leading dam-industry association, the world's rivers were obstructed by more than 40,000 large dams by the end of 20th century, all but 5,000 of them were built after 1950. A “large dam” is defined by ICOLD as one measuring 15 m or more from foundation to crest-, or with reservoir capacity greater than 1 million m3. According to ICOLD the total number of large dams in 2003 was 49,697 (Jia et al., 2004). There were only eight large dams in China in 1949. From 1950-1990 more than 19,000 large dams were constructed. In 2003, the country had 25,800 large dams, ranking first in the world. The U.S. is the country with the second highest number of large dams with 8,724, followed by the ex-USSR, Japan, and India. The U.S. is estimated to have around 96,000 small dams. If the proportion of small to large dams is similar in other countries, then at a rough estimate there are about 800,000 small dams in the world (McCully, 1999). Figure 1 shows the growth curves of the number of large dams in the world and in China. From the 1970s to 2000, most of the world's large dams were constructed in China.
1
Prof., State Key Lab of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China. Email: [email protected] 2 Prof., Secretary General of International Research and Training Center on Erosion and Sedimentation, Beijing 100048, China and Vice President of China Institute of Water Resources and Hydropower Research, No. A-1, Fuxing Rd, Beijing 100038, China. E-mail: [email protected] Note：The original manuscript of this paper was received in April 2009. The revised version was received in Sept. 2009. Discussion open until Dec. 2010. International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 369 -
41413 40000
36226 35157
Number of dams
31388 30000
24119 22039
1
18374 20000
18595
18820
14396 2
16570
17406
12300
10000 3
5268
1973 1977 1982 1950
1960
1970
1975 1980
1997 1986 1990
2000
Year
Fig. 1
Increasing number of large dams in the world and China: Curve-1 is the total number of large dams in the world; Curve-2 is the number of dams in the world except China; and Curve-3 is the number of dams in China (after Pan and He, 2000)
Sediment deposition in reservoirs causes not only the loss of storage capacity but also has an environmental impact. In China 8 billion m3 of storage capacity of 20 large reservoirs have been lost due to sedimentation, which is 66% of the total reservoir capacity of these reservoirs (IRTCES, 1985). The storage capacity of reservoirs is being lost at an annual rate of 2.3% in China and 1% in the world (White, 2001). Reservoir half-life is defined by Morris et al. (2008) as the time required to lose half the original capacity to sedimentation. Many large reservoirs in China have already passed their half-life. The Sanmenxia reservoir has a storage capacity of 5.96 billion m3 for the pool level of 330 m. The reservoir passed its half-life in just 4 years in 1964 and was rejuvenated by changing operation from storing water and sediment to flushing sediment. The accumulated sedimentation volume reached 3 billion m3 in 1990 (Wang et al., 2005), which implies that the half-life of the reservoir had been extended to 30 years after change of the operation mode. A wide range of sedimentation related problems occur upstream from dams as a result of sediment trapping. Because of storage loss the functions of the reservoir reduce for flood control, power generation, and water supply. Sediment can enter and obstruct intakes and greatly accelerate abrasion of hydraulic machinery, thereby decreasing its efficiency and increasing maintenance costs. Sediment deposition in the delta region of the reservoir may affect navigation and impact the ecology. Dam construction is the largest single factor influencing sediment delivery to the downstream reaches. The cutoff of sediment transport by dams can cause stream bed degradation, accelerate the rates of bank failure, and increase scouring at structures such as bridge piers. Stream morphology downstream of dams can be dramatically impacted by reduction in the supply of sediment. Clear water in the river channel, downstream of the dam tends to scour the stream bed causing it to coarsen, degrade, and become armored. Channel degradation can raise bank height, increase bank erosion rates，increase the severity of scouring at downstream bridges, lower water levels at intakes, reduce navigational depth in critical locations, and lower groundwater tables in riparian areas. All of these changes have adverse effects on both wetlands and agricultural areas. On the other hand, sediment trapping by reservoirs reduces the suspended solids concentration downstream，which may have many beneficial effects. The suspended solids leve1s of many rivers have been dramatically increased due to upstream deforestation and development. Sediment trapping in reservoirs is beneficial to aquatic ecosystems sensitive to e1evated suspended solid levels, including coastal marine ecosystems such as grass beds and coral reefs harmed by sediment discharged from rivers. - 370 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
Various strategies for reservoir sedimentation control have been studied and applied in reservoir management. Fan and Morris (1992 a and b) summarized the strategies with particular attention to turbidity currents. Garcia studied turbidity currents with poorly sorted sediment and sediment deposition in reservoirs (Garcia, 1994). Sediment bypassing is a method used to manage sediment by preventing it from entering the reservoir. An offstream reservoir can generate benefits in addition to a reduced rate of storage depletion (Morris et al., 2008). Empty flushing involves the opening of bottom outlets to completely empty the reservoir and allow stream flow to scour sediment deposits. The scouring of sediment by flood depends on the permeability, consolidation coefficient and the volume fraction of sand related to silt (Jacobs et al., 2007). Case studies were conducted by Morris and Fan (1998) and White (2001). Four strategies have been applied for reservoir sedimentation control in China: 1) storing the clear and releasing the turbid water; 2) releasing turbidity currents; 3) drawdown flushing or pressure flushing and empty flushing or free flow flushing; 4) dredging. This paper presents the management strategies with examples, with particular attention to the strategy of storing the clear and releasing the turbid water applied in the Three Gorges Reservoir and Sanmenxia Reservoir. 2 Management strategies 2.1 Storing the clear and releasing the turbid Sediment transport in many Chinese rivers occurs mainly in the 2 to 4 month flood season, that is, 80-90% of the annual sediment load is transported with 50-60% of the annual runoff. The sedimentation in reservoirs on such rivers can be controlled with the strategy of storing the clear water during low concentration periods and releasing the turbid water during the flood season. The Three Gorges Project on the Yangtze River is used for flood control, power generation, and inland navigation. For these purposes, it is important to maintain adequate storage in the reservoir. The total storage capacity of the reservoir is 39 billion m3, of which 22 billion m3 must be retained indefinitely and 17 billion m3 is allowed to be filled with sediment in a period of 120-150 years. The Three Gorges Dam is constructed at Yichang, allowing it to control floods from the upper reaches of the river. The main strategy to control sedimentation in the Three Gorges Reservoir is to draw-down the pool water level from 175 m to 145 m in the flood season from June to September when the sediment concentration is high; this allows the turbid water to wash downstream through the reservoir. The reservoir starts to store water in October when the inflowing water becomes clear (i.e. has a lower sediment concentration). The normal pool level of the reservoir is set at 175 m. Two other characteristic pool levels are the flood control level (145 m), to which the pool will be drawn-down at the beginning of the flood season, generally in June, and the low-flow season control level (155 m), to which the pool may be drawn down to satisfy the requirements of power generation and to provide adequate depths of flow, both up- and down-stream of the dam before the next flood season. Because the reservoir is a river-type reservoir, as shown in Fig. 2, the strategy of storing the clear and releasing the turbid water is effective for sedimentation control (Sedimentation Panel for TGP, 2008). The hydrological station closest to the dam is the Yichang Station (downstream from the dam). The long-term average (1950-1985) annual runoff at Yichang is 450 billion m3 and the annual sediment load is 532 million t, of which about 0.8 million t is gravel bed load. The median diameter of suspended load is 0.033 mm and the median diameter of bed load is 24 mm. More than 88% of the suspended load is finer than 0.1 mm. The long-term average discharge at Yichang is 14,300 m3/s. Figure 3 shows the typical variation process of sediment concentration at Yichang and the operation scheme of pool level for sedimentation control. By storing the clear water and releasing the turbid water, less sediment deposits in the reservoir, while the reservoir is still able to store enough water for power generation in the low flow season. The amount of the original flood regulating capacity that can be preserved with the management strategy of storing the clear and releasing the turbid depends upon, among other things, the morphology of the reservoir. The reservoir looks like a ribbon in the plan view, as shown in Fig.2. The 700-km long reservoir is quite uniform in width and is for the most part less than 1,000 m wide. Since the estimated width of the equilibrium channel in the reservoir is 1,300 m, little flood plain is expected to form in the reservoir. Thus, large percentages of both the flood control and low flow season regulation storage may be preserved indefinitely. International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 371 -
N
S113
S34 Yichang
S205
TGP Dam
0 2550km
Chonggqing
Fig. 2 River type Three Gorges Reservoir and locations of three cross sections
3
S (kg/m )
6 4 2
E (m)
0
Fig. 3
Sediment concentration at Yichang
175 170 165 160 155 150 145
Jan.
April
July
Oct.
Dec.
Reservoir pool level
Jan. April July Oct. Dec. Typical variation process of sediment concentration at the dam site of the Three Gorges Project and the operation scheme of pool level for sedimentation control
Permanently preserved storage
NPL FCL
Dam
Fig. 4
Fluctuating Sediment deposition backwater region Schematic diagram of the pool levels and deposition curve of the TGP reservoir: NPL=normal pool level (175 m); FCL = flood control level (145m)
Drawing the reservoir down to 145 m during the majority of the flood season may keep the upper limit of deposition below 145 m, as shown in Fig. 4. During the low-flow season, the river carries little sediment, but still contributes to 39% of the annual runoff. Water is then stored in the reservoir for power generation and navigation. The amount of water needed to fill up the reservoir to its normal pool level varies with the scheme adopted but is generally less than 22 billion m3, which is only a small portion of the runoff in the low flow season. Eventually, a new alluvial channel will form in the reservoir with a - 372 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
Sedimentation volume (109m3)
thalweg profile under the flood control pool level, as shown in Fig. 4. The sedimentation process of the reservoir has been simulated by using a one-dimensional numerical model. The computation was made for a selection of the reservoir operational levels schemes. The hydrological time series for the period 1961-1970 was used for the simulation with a 40-year flood inserted in the series. As the recurrence period of the 1954 flood is 40 years, the stream record of 1954 was inserted into the hydrological series in such a manner that it appears three times in 106 years. The time series, thus, consists of the hydrological records of the following sequence of years: 2 cycles of 1961-1970 plus 1954, 1955; 2 cycles of 1961-1970 plus 1954, 1955; 4 cycles of 1961-1970 plus 1954, 1955; 2 cycles of 1961-1970 According to numerical modeling, the reservoir sedimentation may be controlled by using the strategy of storing the clear water and releasing the turbid water. This method approaches equilibrium in a 100 year period. Figure 5 shows the accumulation of sediment in the reservoir from 0 to 100 years for various schemes calculated by the Yangtze River Conservancy Commission, in which 160-135 (Curve 2) represents the curve where the normal pool level is 160 m and the flood control pool level is 135 m. Curve 4 in the diagram represents the sedimentation volume over time with a normal pool level of 175 m and a flood control pool level 145 m (but in the first 10 years the two levels are 156 m and 135 m, respectively) (Sedimentation Panel for TGP, 2002). For the 175-145 configuration, the reservoir sedimentation will approach equilibrium in 100 years with a final capacity loss of about 16 billion m3. 20 18 16 14 12
(4) (3) (2) (1)
10 8 6 4 2 0 0
Fig. 5
(5)
NPL-FCL (1) 150-135 (2) 160-135 (3) 170-140 (4) 175-145 (5) 180-150
10
20
30
40
50
60
70
80
90
100
Years of operation Calculated sedimentation over a 100 year period of operation of the Three Gorges Reservoir for five operational level schemes (Sedimentation Panel for TGP, 2002)
In the reality, the Three Gorges Project was completed in 2009, but the reservoir began to impound in June of 2003. Sedimentation in the first years of operation has received great attention. In the first seven years of operation the pool level rose gradually following the completion of the dam in the ranges of 135-141, 145-156 and 145-175 m. Figure 6 shows the real pool level from Jan. 2007 to Jan. 2008，in which the data were collected from the daily reports of Ministry of Water Resources on the web (http://www.ctgpc.com.cn). In the non-flood season, the pool level was controlled at around 156 m. During the flood season, from June to Oct., the pool level was drawn to 145 m, so high sediment concentration flood water flowed through the reservoir and released to the downstream reaches. The Cuntan station is located near Chongqing, about 600 km upstream from the dam. The long term (1950-1985) average annual runoff, sediment load, and average concentration at Cuntan were 350 billion m3, 462 million t, and 1.32 kg/m3, respectively. Only about 0.3 million t of the sediment load was bed load. The median diameter of the suspended load was 0.037 mm and the median diameter of gravel bed load was 51 mm. In the past two decades, however, the sediment load has dramatically reduced due mainly to numerous dams in the upstream watershed, while the runoff remains unchanged. The average annual sediment load in the period from 2001-2007 has reduced to about 200 million t. Figure 7 shows the distributions of monthly runoff and sediment load at the Cuntan and Yichang stations in 2007, which represent water and load inflowing and out of the reservoir, respectively (MWR, 2008). The result shows that more than 30% of the incoming sediment load is discharged out of the reservoir during the flood season. International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 373 -
Pool level (m) 1
160 155 150 145 140 135 130 1
Fig. 6
2
3
4
5
6 7 8 9 10 11 12 13 Month Pool level of the Three Gorges Reservoir from January 2007 to January 2008
Trap efficiency, Te, is the proportion of the incoming sediment that is deposited or trapped in a reservoir and several estimation methods of the efficiency were studied (Jothiprakash and Garg, 2008). Similarly, we define sediment releasing efficiency as the amount of sediment exiting the reservoir as a percentage of inflowing sediment load, which in fact equals 100% - Te. According to the numerical models, the filling of the reservoir with sediment is very rapid in the first 20 years and slows down in the following years. The sediment releasing efficiency rises as the reservoir fills, reaching 100% in about 120 years. Figure 8 (a)-(c) shows the measured cross sections in the reservoir in 2003, 2005, 2006 and 2007 at 5.6 km, 160 km and 356 km from the dam, respectively (MWR, 2007, 2008). The location of the three cross sections is shown in Fig. 2. Sediment accumulated in the reservoir rapidly in the first two years (2003-2005) following impoundment, but from 2005 to 2007, the amount of sedimentation in the reservoir remained relatively constant. Comparing with the calculated sedimentation volume with a linear increasing trend in the first 10 years, as shown in Fig.5, the measured sedimentation volume in the first 4 years of operation was much less. The main reason was a dramatic sediment load reduction since the 1980s (Liu et al., 2008).
Monthly runoff
60
60
Monthly sediment load 40
40
20
20
0
0
3
(billion m )
1
Monthly runofff
Monthly sediment load (millionton)
80
Cuntan Station in 2007
2
3
4
5
6 7 Month
8
9
10
11
12
90
Yichang Station in 2007
24
60
Monthly runoff Monthly sediment load
16
30
8
Monthly sediment load (millionton)
3
(billion m )
Monthly runofff
80
0
0 6 7 8 9 10 11 12 Month Distributions of monthly runoff and sediment load at the Cuntan and Yichang stations, which represent water and sediment load flowing in and out of the reservoir, respectively (MWR, 2007) 1
Fig. 7
- 374 -
2
3
4
5
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
200
Cross sections No.34 (5.6 km upstream of the dam)
Elevation (m)
150 100
Mar 2003 Oct 2005
50
Oct 2006
0
Oct 2007
-50 -1000
-500
0 500 Distance from the left bank (m)
1000
1500
250 Cross sections No.113 (160.1 km upstream of the dam) Elevation (m)
200 Mar 2003 Oct 2006
150
Oct 2005 Oct 2007
100 50 0 0
500
1000
1500
2000
2500
Distance from the left bank (m) 240 Cross sections No.205 (356 km upstream of the dam)
Elevation (m)
220 200 180 160
Mar 2003
Oct 2005
Oct 2006
Oct 2007
140 120 100 -200
300 800 Distance from the left bank (m)
1300
Fig. 8 Measured cross sections at the Three Gorges Reservoir in 2003, 2005, 2006 and 2007 at 5.6 km (a), 160 km (b) and 356 km (c) from the dam, respectively (MWR, 2007)
The strategy of storing clear water and releasing turbid water has also been successfully used in the management of sedimentation behind the Sanmenxia Dam on the Yellow River after several failures of other sedimentation management strategies. The Sanmenxia Dam, 105-m high and 739-m long, was the first large dam on the Yellow River. The crest elevation of the dam is 353 m and the designed reservoir capacity is 35.4 billion m3 with a normal pool level of 350 m. The main purposes of the dam are flood control, ice jam flood control, sediment trapping, power generation, and irrigation. The reservoir controls a drainage area of 688,000 km2 and 89% of the total runoff from the Yellow River basin. The construction of the dam was initiated in 1957 and water impoundment commenced in Sept. 1960. The reservoir area extends upstream a distance of 246 km to Longmen. The Yellow River flows south from Longmen to Tongguan, then makes a 90° turn and goes east. The Wei River flows into the Yellow River at Tongguan. The reservoir area consists of three parts: 1) the Yellow River from the dam to Tongguan; 2) the Yellow River from Tongguan to Longmen; and 3) the Wei River from Tongguan to Xianyang, or the lower Wei River (Wang et al., 2007). The operation scheme of the Sanmenxia Reservoir has been substantially changed to achieve a balance between sediment inflow and outflow in the following three reservoir operation modes (Wang et al., International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 375 -
2005): (i) storage from Sept. 1960 to Mar. 1962, the reservoir was operated at a high storage level the whole year round; (ii) detaining flood water and sluicing sediment from Mar. 1962 to Oct. 1973, 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; and (iii) storing clear water and releasing turbid water from Nov. 1973 to the present, the reservoir has been operated at a high level (315–320 m) to store relatively clear water during non-flood seasons (Nov.–June) and at a low level (302–305 m) to release high sediment concentrations during flood seasons (July–Oct.), as shown in Fig. 9. Serious sediment accumulation occurred during the first operation mode period, which increased the risk of flooding in Xi-an City (the ancient capital of China). The operation pool level had to be changed to 303-318 m in 1962-1973, allowing flood water scoured sediment to flow through the reservoir. Sediment was sluiced out of the reservoir but at the cost of power generation. The strategy of storing the clear and releasing the turbid was finally applied with the pool level very low during high sediment concentration period and high pool level during low concentration period for power generation. 335 330
1960-1961
Elevation (m)kk
325 320 1974-2001 315 310 1963-1973
305 300 11
295 0
12 50
1
2 100
3
4 150
5
6
200
7 250
8 300
9
10 350
h
Time (day)
Fig. 9 Three operation modes of pool level of the Sanmenxia Reservoir in different time periods
Figure 10 shows the accumulated sediment deposition in the three parts of the reservoir and the storage capacity for pool levels of 330 m and 323 m. The sedimentation has been controlled and the accumulated sediment deposition volume in two parts of the reservoir, comprising Tongguan to the dam and the lower Wei River, remained unchanged since the operation mode changed to storing the clear and releasing the turbid in 1973. Only a slight increase of accumulated sediment occurred in the upper part of the reservoir extending from Longmen to Tongguan. The reservoir capacity has remained almost unchanged since 1973. 2.2 Turbidity currents Turbidity currents are sediment-laden density-driven currents that flow along the bottom of the reservoirs. These currents are created by the relative motion between two fluid layers that have different densities. The turbidity currents in reservoirs generally involve only slight differences in the densities of the upper and lower layers. Since the density difference is small, the reservoir water creates a large buoyancy effect within the inflowing liquid, so that the effective gravity of the flowing liquid is greatly reduced. Usually g′ is defined as effective gravity given by: g′ = g
Δρ
ρ
(1)
in which g is the gravitational acceleration, Δρ is the density difference between the upper and lower liquids, and ρ is the density of the reservoir water. Many formulae describing open-channel flow apply also to turbidity currents once g is replaced by g′. For example, the flow pattern in an open channel depends greatly on the Froude number of the flow; in a turbidity current, the Froude number remains the key parameter but its form is modified as follows (Qian et al., 1998): - 376 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
7
(a)
9
Tongguan to dam
3.0 2.5 2.0
Longmen to Tongguan
1.5 1.0 Lower Wei River
0.5
(b)
3
Reservoir storage capacity (10 m )
3.5
9
3
Accumulated deposition (10 m )
4.0
0.0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
6
Elevation < 330 m
5
Elevation < 323 m 4 3 2 1 0 1960
1965
1970
1975
1980
1985
1990
1995
Time (year)
Time (year)
Fig. 10 (a) Accumulated sediment deposition in Sanmenxia Reservoir; (b) Storage capacity of Sanmenxia Reservoir for pool elevations 330 m and 323 m
Fr ′ =
Uc g ′h′
(2)
in which Uc is the relative velocity between the two liquid layers and h′ is the thickness of the turbidity current. Because g′ is much smaller than g, the Froude number is larger than 1 for many turbidity currents, therefore, a hydraulic jump occurs very often in turbidity currents (Garcia, 1993) Data from both flume experiments and field observations show that the critical condition at the plunging point for the formation of turbidity current is (Fan, 1959): q2 (3) = 0.6 Δρ 3 gh ρ′ 0 in which q is the discharge per unit width and h0 is the depth at the immersion point. From the above equation, if the water level upstream of the dam remains constant, an increase of the inflow discharge would cause the plunging point to move downstream; and an increase in the density difference between the inflow and the reservoir water would cause the point to move upstream. Maintaining a turbidity flow needs a continuous supply of inflow sediment suspension and a force to overcome any resistance it encounters. If the inflow ceases to supply dense fluid so that it no longer forms turbidity current at the plunging point, the already formed turbidity current downstream would soon stop moving. Since the energy to maintain the flow comes from the density difference and the slope of the reservoir bed, a minimum density difference is required to produce and maintain the current flowing to the dam. According to field data from the Guanting Reservoir on the Yongding River and the Sanmenxia Reservoir on the Yellow River, the sediment concentration in a turbidity current must be more than 20 kg/m3 to allow the current to flow continuously to the dam (Qian et al., 1998). The Xiaolangdi Reservoir is located 125 km downstream of the Sanmenxia Dam and about 860 km upstream of the river mouth. The reservoir is the most downstream gorge-type reservoir on the Yellow River. The multi-purpose reservoir is mainly for flood control, power generation and sediment retention for reducing siltation in the lower Yellow River. The total capacity of the reservoir is 12.65 billion m3, of which more than 7.55 billion m3 will be used for trapping sediment. The reservoir began to impound in Oct. 1999. While the reservoir operates it traps coarse sediment and discharges fine sediment to the lower reaches. In the first 8 years, coarse sediment was trapped as clear water and a portion of fine sediment was released to the lower reaches mainly in the form of turbidity currents. Because a fraction of coarse sediment is trapped by the reservoir, the rate of sedimentation of the lower Yellow River channel is greatly reduced. Sediment finer than 0.02 mm was released to the lower reaches but it is mostly wash load and does not cause siltation of the lower reaches. Since impoundment turbidity currents occurred every year, especially during hyper-concentrated floods, the turbidity currents flowed to the dam and were discharged out of the reservoir when the bottom outlets International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 377 -
were open. Figure 11 shows the elevation of the interface between the turbidity currents and the upper clear water layer for five turbidity currents in 2001 (Hou and Jiao, 2003). The interface remained in a narrow range between 188-194 m in most parts of the reservoir, although the discharge of the turbidity currents varied in the range of 200-2,800 m3/s, the sediment concentration varied between 13 kg/m3 to 530 kg/m3, and the average velocity varied between 0.2 m/s to 1.2 m/s. The Froude number at the plunging point was measured at 0.5-0.61, which approximates the situation represented by Eq. (3) (Hou and Jiao, 2003). A continuous turbidity current flowing to the dam is critical for sediment release, which depends on the inflowing discharge, sediment concentration and the portion of sediment particles finer than 0.01mm. Figure 12 shows the sediment concentration, S, and discharge, Q, of turbidity currents in 2001-2004 (data from Wu et al., 2008). In the figure, the filled circles represent turbidity currents reaching the dam, in which the portion of sediment particles finer than 0.01 mm is between 25-75%; the black pyramids are turbidity currents moving to the dam but the portion of sediment particles finer than 0.01 mm is higher than 75%; the open triangles are turbidity currents not reaching the dam, although some of them consists of 90% fine sediment. The curve in the figure is given by: (4) SQ = 100000 (kg/s) For high sediment concentration and discharge, or the points above the curve, the turbidity currents flowed through the whole reservoir and arrived at the dam. For the points below the curve the turbidity currents stopped in the reservoir, failing to reach the dam.
Fig. 11 Elevation of the interface between the turbidity currents and the upper clear water layer (data are from Hou and Jiao, 2003)
500
3
S (kg/m )
400
P(D<0.01mm)>25% P(D<0.01mm)>75% Not to the dam
300 200 100 0 0
1000
2000
3000
4000
3
Q (m /s) Fig. 12 Sediment concentration, S, and discharge, Q, of turbidity currents reaching and not reaching the dam occurred in 2001-2004 (data from Wu et al., 2008)
In general, the sediment releasing efficiency of turbidity currents is about 20%. Nevertheless, the turbidity currents in the Xiaolangdi Reservoir had low sediment releasing efficiency because the reservoir bed had not silted up to the low sill of the bottom outlets. Arriving at the dam the turbidity currents filled the reservoir and formed a muddy water reservoir. Only after the surface of the muddy water reservoir - 378 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
rose to the elevation of the bottom outlets, were the turbidity currents released from the reservoir. The average sediment releasing efficiency, or the ratio of the released sediment to the inflowing sediment load, was only 6% in the period from 2000-2004 (Hou and Jiao, 2003). In 2006, turbidity currents occurred again in the Xiaolangdi Reservoir by using Sanmenxia Reservoir to create high sediment concentration flow (MWR, 2007). About 8.4 million tons of fine sediment, or 36% of inflowing sediment, was released during the turbidity currents because the sedimentation reservoir bed approached to the bottom outlets. The average sediment releasing efficiency of turbidity currents was about 20%. 2.3 Flushing Hydraulic flushing involves reservoir draw-down by opening low level outlets to temporarily establish riverine flow along the impound reach, eroding a channel through the deposits and flushing the eroded sediment through the outlets. Sediment entering the reservoir during flushing periods will also be routed through the impoundment and released. Fan (1985) has classified flushing into two general categories: (1) empty or free flow flushing, which involves emptying the reservoir to the level of the outlets with riverine flow through the reservoir, and (2) drawdown or pressure flushing, which requires less drawdown but is also less effective. The second method is not commonly used. While both strategies have been applied successfully, flood season flushing is generally more effective because it provides larger discharges with more erosive energy, and flood borne sediments can be routed through the reservoir. Flushing scours a single main channel through reservoir bed, while floodplain deposits on either side are unaffected. Figure 13 shows draw-down flushing and empty flushing. During pressure flushing, the reservoir is drawn down to a low pool level and the bottom outlets are opened allowing development of a conical scour hole in front of the outlets, as shown in Fig. 13 (a). Sediment from the upper portion of the reservoir is transported towards the dam, but only material in the scour hole in front of the outlets can be flushed out. Draw-down flushing or pressure flushing was used to flush sandy bed material in the Tarbela Reservoir in Pakistan (Lowe and Fox, 1995). The Tarbela Dam is a 137 m-high embankment dam on the Indus River mainly used for hydropower (3,750 MW installed capacity) and irrigation. Its total capacity was 14.3 billion m3 at closure in 1974 but it had decreased 17.4% due to sedimentation by 1992. The original project was designed for a 50-year life with no previsions for the eventual management of the inflowing sediment load of about 208 million tons per year. The inflowing sediment consisted of 59% fine sand, 34% silt, and 7% clay. Approximately 99% of the inflowing load was trapped and accumulates primarily in the form of a delta deposit which advanced toward the dam. Delta top-set beds have a slope of about s= 0.0006 to 0.0008. Most sediment deposited on the top-set bed as the reservoir filled and water levels rose during the wet season. However, when the reservoir was draw down for irrigation deliveries, the river reworks and transported these deposits downstream, extending the delta toward the dam. Most sediment is transported to the face of the delta at the onset of the wet season when the pool level is still low but discharge increases from 1,500 to 4,500 m3/s. Erosion
Drawdown Deposition
Empty Erosion
Fig. 13 (a) Pressure flushing causes erosion in the upper part of the reservoir and redeposition near the dam, with pressure flow through the bottom outlets; (b) Empty flushing results in erosion in the whole reservoir, with free flow through bottom outlets
Empty flushing has been applied in middle and small reservoirs, in which these reservoirs were emptied before the flood season and the sediment deposit in the reservoir was flushed out during the first part of the flood season. Because of the high sediment concentration that can be released during the flushing period, the downstream channel must be able to transport high sediment concentration flow. Seasonal emptying is also feasible when water demand is seasonal. Empty flushing has been used in the Sefid-Rud International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 379 -
Reservoir in Iran (Khosronejad, 2009). The sediment concentration increased from about 2 kg/m3 to 40 kg/m3 due to scour by flood flow when the reservoir was emptied (Forood and Ghafouiri, 2007) The Hengshan Reservoir on the Tangyu River (a tributary of the Yongding River) in Shanxi Province, China applied the empty flushing strategy to control sedimentation and restore the reservoir capacity. The Hengshan Dam is 69 m high with a reservoir capacity of 1.33 million m3. From 1966-1974, 3.19 million m3 of sediment deposited in the reservoir with a thickness of 27 m near the dam. To restore the capacity of the reservoir the reservoir was emptied to flush sediment in 1974 and 1979. During the empty flushing period from July 28 to Sept. 4 in 1974, the inflowing sediment was 0.13 million t but 1.19 million t of sediment were flushed out of the reservoir (Guo et al., 1985). The reservoir was emptied and flushed again in 1979 from Aug. 9 to Sept. 30, the inflowing sediment was 0.2 million t but 1.55 million t of sediment was flushed out of the reservoir. A mud flow with a maximum concentration of about 1,370 kg/m3 was created and flowed out of the reservoir. Figure 14 shows the sediment concentration and discharge of mud flow during the empty flushing. The flow cut a narrow yet deep channel in the reservoir sediment deposit. Guo et al. (1985) analyzed the empty flushing and noted that the channel can be deepened by using either small or large flows, but widening this channel can only be achieved by using large flows. It was found that the flushing effect is maximized if the reservoir is emptied immediately prior to the arrival of a flood because the flood flow exerts erosive force on deposits which have not yet had the time to fully dewater and consolidate. The phrase used to describe this strategy is “Deepen by small flow, widen by flood flow.”
600 0.5
300
0.0
0 0
Fig. 14
900 3
1.0
1200
S (kg/m )
Q S
3
Q (m /s)
1.5
10
20 30 40 t (hr) Sediment concentration and discharge of mud flow during the empty flushing from the Hengshan Reservoir in 1979 (Data from Guo et al., 1985)
Fig. 15 Critical velocity for initial scouring of cohesive sediment from the Zhuwo Reservoir as a function of the weight of sediment per volume in the deposit
The Zhuwo Dam is on the Yongding River near Beijing, China, which is 33 m high with a reservoir capacity of 14.75 million m3. The reservoir began to store water in 1961 and lost 5.3 million m3 of reservoir capacity due to sedimentation after 25 years of operation. Most of the sediment deposit was cohesive sediment with a median diameter of 0.004 mm. The first author of this paper conducted a physical model experiment to study the efficiency of empty flushing of cohesive sediment from the reservoir (Wang and Zhang, 1989). Cohesive sediment may be carried downstream if the sediment is - 380 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
scoured from the reservoir. Figure 15 shows the critical velocity, Uc, for initial scouring of cohesive sediment from the reservoir as a function of the weight of sediment per unit volume in the deposit, γ b , which followed the following formula: 4 U c = 4.6γ b (5) Because the length and depth scale of the model was 1:50 then the velocity scale can be calculated from the Froude number to be 1: 7.07. The scale ratio for the critical velocity for scouring similarity is then 1: 1.63. The measured γ b in the reservoir varied almost lineally from 0.55 g/cm3 at the surface of the deposit to about 1.22 g/cm3 at a depth of 20 m in the deposit. The modeling sediment is the cohesive sediment from the Zhuwo Reservoir and the value of γ b was controlled, lineally distributed from 0.33 g/cm3 at the surface of the deposit to 0.75 g/cm3 at a depth of 40 cm in the model, which meet the requirement of scouring similarity. Figure 16 shows the sediment concentration and discharge of the mud flow stage of empty flushing during the model experiment. During the process a very small inflowing discharge, about 1 m3/s, flowed to the reservoir and scoured the sediment. The sediment concentration varied in the range of 200 kg/m3 to 700 kg/m3 during the mud flow process. After 8 hours the mud flow stopped. Then the inflowing discharge of clear water increased to 12 m3/s, which was controlled by the Guanting Reservoir. Retrogressive erosion occurred as a consequence of the inflowing discharge. Figure 17 shows the sediment concentration during the retrogressive erosion stage of the empty flushing. Because the sediment was very cohesive, the retrogressive erosion lasted only about 40 hours. Almost no more sediment could be scoured except if the inflowing discharge dramatically increase, which was not allowed because of the safety of water resources utilization for Beijing. The retrogressive erosion created many gullies on the reservoir bed, which formed a palm shape channel network. About 0.19 million m3 of sediment deposit was discharged out of the reservoir during the empty flushing, of which 37% was released from the reservoir in the form of mud flow. Q S
3
600 3
Q (m /s)
12
800
S (kg/m )
16
8
400
4
200
0
0 0
2
4 6 8 t (hr) Fig. 16 Sediment concentration and discharge of mud flow during empty flushing of the Zhuwo Reservoir
The sediment releasing efficiency, E, can be estimated for empty flushing by using the following formula: E= time of accumulation of sedimentation volume V / time to flush reservoir sediment with volume V For the Hengshan Reservoir the efficiency E was 24.7 during the empty flushing in 1974 and was 23.9 during the empty flushing in 1979. For the Zhuwo Reservoir, the efficiency was 55 during the empty flushing experiment. Empty flushing has the highest sediment releasing efficiency, but the flushing may cause high ecological stress on the downstream reaches. Suspended sediment adsorbs pollutants from flowing water in rivers and deposits in the reservoirs. The concentration of heavy metals (Cr, Cd, Hg, Cu, Fe, Zn, Pb and As) in water, sediment, and fish/invertebrate were investigated and the results showed that the concentrations of heavy metals were highest in the sediment and lowest in the water. Benthic invertebrates had higher concentrations of heavy metals in their tissues due to their proximity to contaminated sediments and fish had lower concentrations of heavy metals (Yi et al., 2008). Empty flushing might release the pollutants from the sediment and increase sharply the concentration of International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 381 -
pollutants in water. The ecological system downstream of the Zhuwo Reservoir consists of numerous species of aquatic plants, benthic invertebrates and fish. Empty flushing can seriously impair the ecology. The Beijing government decided to dredge the Zhuwo Reservoir rather than implement an empty flushing technique. This had a higher economic cost but a much lower ecological cost. 400
3
S (kg/m )
300 200 100 0 0
Fig. 17
10
20 30 40 t (hr) Sediment concentration of mud flow during empty flushing of the Zhuwo Reservoir experiment
2.4 Dredging Dredging has been used for a long time for sedimentation management in small reservoirs. Different dredging methods are applied in reservoir management, including mechanical dredging and dumping outside of the reservoir and agitating sediment with jets so that that sediment can be transported downstream of the reservoir by currents. Various dredgers have been used: dredge boat; agitating dredger; dipper dredger; hauling scraper; excavator and bulldozer; and trailer dredger. In general dredging is more expensive than other strategies of sedimentation control in reservoirs. Nevertheless, jets in combination with turbidity currents or flushing are more economically feasible and have been applied in large reservoirs. Jet dredging was conducted in a reach near Tongguan in the Sanmenxia Reservoir from 1996 to 2003 during the flood season. Figure 18 shows the sediment amount scoured from the reservoir bed by jets. The sediment is composed mainly of silt and fine sand with 20% finer than 0.025mm and about 40% finer than 0.05 mm. It is reported that the sediment finer than 0.05 mm may be transported by flow out of the reservoir but coarser sediment may settle again in the reservoir (Jiao et al., 2008).
Fig. 18 Sediment amount scoured from the wake area of Sanmenxia Reservoir by jet dredging
Jet dredging was also used to create artificial turbidity currents to release sediment from the Xiaolangdi Reservoir. Since the Xiaolangdi Reservoir began to store water in 1999, about 1.5 billion m3 of fine sediment had deposited in a 40 km reach upstream from the dam in the reservoir by April 2005. The sediment is fine with a median diameter of about 0.01 mm. The reservoir sediment was scoured by jet, which resulted in fine sediment suspension in the reservoir. Because the suspension has a slightly higher density than the surrounding water, it flows to the dam along the reservoir bed in the form of a turbidity current. Most of the fine sediment is released from the bottom outlet of the reservoir with turbidity currents. - 382 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
3 Conclusions Reservoir sedimentation may be controlled by applying the strategies of storing the clear water and releasing the turbid water, releasing turbidity currents, empty flushing and dredging. The sediment releasing efficiency is: 30-100% for storing the clear water and releasing the turbid water, 6-36 % for turbidity current, and 2,400-5,500% for empty flushing. Empty flushing causes high ecological stress on the ecosystem to the downstream reaches. Storing the clear and releasing the turbid is the best strategy to control reservoir sedimentation while achieving hydro-power benefits and maintain ecological stability. Acknowledgement The study was supported by the National Natural Science Foundation of China (50725930, 50721006) and Ministry of Water Resources of China (2007SHZ0901034). References Fan J. and Morris G. L. 1992a, Reservoir sedimentation I: Delta and density current deposits. Journal of Hydraulic Engineering, Vol. 118, No. 3, pp. 354–369. Fan J. and Morris G. L. 1992b, Reservoir sedimentation II: Reservoir desiltation and long-term storage capacity. Journal of Hydraulic Engineering, Vol. 118, No. 3, pp. 370–384. Fan Jiahua. 1959, Experimental study on density current. Chinese Journal of Hydraulic Engineering, Vol. 5, pp. 30–48 (in Chinese). Fan Jiahua. 1985, The reservoir sediment problem. Journal of Hydraulic Engineering, pp. 24–31 (in Chinese). Foroods. and Ghafouiri M. 2007, Assessment of causes and effects of disastrous erosion and sediment flows and mitigation measures in Caspian Sea Watersheds-Iran. Proceedings of UNESCO Expert Meeting on Erosion in Arid and Semi-Arid Areas, Chalooz, Iran. Garcia M. H. 1993, Hydraulic jumps in sediment-laden bottom currents. Journal of Hydraulic Engineering, Vol. 119, pp.1094–1117. Garcia M. H. 1994, Depositional turbidity currents laden with poorly sorted sediment. Journal of Hydraulic Engineering, Vol. 120, No. 11, pp. 1240–1263. Guo Zhigang, Zhou Bin, Ling Laiwen, and Li Degong. 1985, The hyperconcentrated flow and its related problems in operation at Hengshan Reservoir. Proceeding Internal Workshop on Flow at Hyperconcentrations of Sediment: 3–1. Hou Suzhen and Jiao Enze. 2003, Analysis on turbidity currents in the Xiaolangdi Reservoir. Water Resources and Hydropower Engineering, Vol. 34, No. 6, pp. 11–14 (in Chinese). IRTCES (International Research and Training Center on Erosion and Sedimentation). 1985, Lecture notes of the training course on reservoir sedimentation. Series of publication, Beijing, China. Jacobs W., Van Kesteren W. G. M., and Winterwerp J. C. 2007, Permeability and consolidation of sediment mixtures as function of sand content and clay mineralogy. International Journal of Sediment Research, Vol. 22, No. 3, pp. 180–187. Jia Jinsheng, Yuan Yulan, and Li Tiejie. 2004, China’s and world’s large dams in 2003. China Water Resources, No. 13, pp. 25–33. Jiao Enze, Miao Fengju, and Lin Xiuzhi. 2008, Water and Sediment Management with Reservoirs. Yellow River Press, pp. 1–230. Jothiprakash V. and Garg V. 2008, Re-look to conventional techniques for trapping efficiency estimation of a reservoir. International Journal of Sediment Research, Vol. 23, No. 1, pp. 76–84. Khosronejad Ali. 2009, optimization of the Sefid-Roud Dam desiltation process using a sophisticated one dimensional numerical model. International Journal of Sediment Research, Vol. 24, No. 2, pp. 189–200. Liu Cheng, Sui Jueyi, and Wang Zhaoyin. 2008, Sediment load reduction in Chinese rivers. International Journal of Sediment Research, Vol. 23, No. 1, pp. 44–55. Lowe J. and Fox I. 1995, Sediment management schemes for Tarbela Reservoir. USCOLD Annual Meeting 1995. McCully. 1999, Silenced Rivers: The Ecology and Politics of Large Dam. Zed Books, London. Morris G. L. and Fan J. 1998, Reservoir sedimentation handbook. McGraw Hill, New York. Morris G. L. Annandale G., and Hotchkiss R. 2008, Reservoir Sedimentation, in Garcia M.H. ed. Sedimentation Engineering, Published by American Society of Civil Engineering. MWR (Ministry of Water Resources of China). 2007, China Gazette of River Sedimentation 2006. China Water Resources Press (in Chinese). MWR (Ministry of Water Resources of China). 2008, China Gazette of River Sedimentation 2007. China Water Resources Press (in Chinese). Pan Jiazheng and He Jing. 2000, China Dam Construction in The Past 50 Years. Chinese Water and Hydro-power Press, Beijing, China (in Chinese). Qian Ning, Wan Zhaohui, and McNown, J. 1998, Mechanics of Sediment Movement. ASCE Press, United States. Sedimentation Panel for TGP. 2002, Comprehensive analysis on “9.5” sedimentation study results for the Three gorges Project on the Yangtze River. Intellectual Copyright Press (in Chinese). International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 383 -
Sedimentation Panel for TGP. 2008, Brief report of water and sediment measurement during the coffer dam impoundment (2003-2006) period. China Water Resources Press (in Chinese). Wang Guangqian, Wu Baosheng, and Wang Zhaoyin. 2005, Sedimentation problems and management strategies of Sanmenxia Reservoir, Yellow River, China. Water Resources Research, Vol. 41, W09417, Doi: 10.1029/ 2004WR003919. Wang Zhaoyin and Zhang Xinyu. 1985, Experimental study of empty flushing of cohesive sediment from a reservoir with a physical model. Journal of Sediment Research, No. 2, pp. 62–69 (in Chinese). Wang Zhaoyin, Wu Baosheng, and Wang Guangqian. 2007, Fluvial processes and morphological response in the Yellow and Wei Rivers to closure and operation of Sanmenxia Dam. Geomorphology , Vol. 91, Issues 1–2, pp. 65–79. White R. 2001, Evacuation of Sediments from Reservoirs. Thomas Telford Press, London. Wu Lianchun, Zheng Minsheng, and Zheng Hui. 2009, Rising the sediment releasing efficiency and extending the life of the Xiaolangdi Reservoir, Proceedings of the Special Session for Comprehensive Utilization of Water Resources in the Middle and Lower Reaches of the Yellow River. 10th China Association for Science and Technology Annual Meeting. Zhenzhou, September 17–19, 2008 (in Chinese). Yi Yujun, Wang Zhaoyin, Zhang Kang, Yu Guoan, and Duan Xuehua. 2008, Sediment pollution and its effect on fish through food chain in the Yangtze River. International Journal of Sediment Research, Vol. 23, No. 4, pp. 338–347.
- 384 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
Strategies for managing reservoir sedimentation Zhao-yin WANG1 and Chunhong HU2 Abstract Sediment deposition in reservoirs has caused the loss of 66% of the reservoir capacity in China. The main sedimentation control strategies are: 1) storing the clear water and releasing the turbid water; 2) releasing turbidity currents; 3) Draw-down flushing and empty flushing; and 4) dredging. The paper summarizes these strategies with examples. Sediment transport in many Chinese rivers occurs mostly during the 2-4 month flood season, that is, 80-90% of the annual sediment load is transported with 50-60% of the annual runoff. By storing the clear water after the flood season and releasing the turbid water during the flood season, less sediment deposits in the reservoir while the reservoir is still able to store enough water for power generation in the low flow season. The Three Gorges and Sanmenxia reservoirs apply this strategy and control sedimentation effectively. Turbidity currents have become the main sedimentation control strategy for the Xiaolangdi Reservoir. Empty flushing involves reservoir draw-down to temporarily establish riverine flow along the impound reach, flushing the eroded sediment through the outlets. Case studies with the Hengshan Reservoir and Zhuwo Reservoir are presented. Jet dredgers have been used to agitate the reservoir deposit so that the deposit is released from the reservoir with currents. The sediment releasing efficiency is 30-100% for storing the clear and releasing the turbid; 6-40 % for turbidity current; and 2,400-5,500% for empty flushing. Empty flushing causes high ecological stress on the ecosystem to the downstream reaches. Storing the clear and releasing the turbid is the best strategy to control reservoir sedimentation while achieving hydro-power benefit and maintain ecological stability. Key Words: Reservoir sedimentation, Storing clear and releasing turbid, Flushing, Turbidity current, Three Gorges Reservoir
1 Introduction According to the estimates of the International Commission on Large Dams (ICOLD), the leading dam-industry association, the world's rivers were obstructed by more than 40,000 large dams by the end of 20th century, all but 5,000 of them were built after 1950. A “large dam” is defined by ICOLD as one measuring 15 m or more from foundation to crest-, or with reservoir capacity greater than 1 million m3. According to ICOLD the total number of large dams in 2003 was 49,697 (Jia et al., 2004). There were only eight large dams in China in 1949. From 1950-1990 more than 19,000 large dams were constructed. In 2003, the country had 25,800 large dams, ranking first in the world. The U.S. is the country with the second highest number of large dams with 8,724, followed by the ex-USSR, Japan, and India. The U.S. is estimated to have around 96,000 small dams. If the proportion of small to large dams is similar in other countries, then at a rough estimate there are about 800,000 small dams in the world (McCully, 1999). Figure 1 shows the growth curves of the number of large dams in the world and in China. From the 1970s to 2000, most of the world's large dams were constructed in China.
1
Prof., State Key Lab of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China. Email: [email protected] 2 Prof., Secretary General of International Research and Training Center on Erosion and Sedimentation, Beijing 100048, China and Vice President of China Institute of Water Resources and Hydropower Research, No. A-1, Fuxing Rd, Beijing 100038, China. E-mail: [email protected] Note：The original manuscript of this paper was received in April 2009. The revised version was received in Sept. 2009. Discussion open until Dec. 2010. International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 369 -
41413 40000
36226 35157
Number of dams
31388 30000
24119 22039
1
18374 20000
18595
18820
14396 2
16570
17406
12300
10000 3
5268
1973 1977 1982 1950
1960
1970
1975 1980
1997 1986 1990
2000
Year
Fig. 1
Increasing number of large dams in the world and China: Curve-1 is the total number of large dams in the world; Curve-2 is the number of dams in the world except China; and Curve-3 is the number of dams in China (after Pan and He, 2000)
Sediment deposition in reservoirs causes not only the loss of storage capacity but also has an environmental impact. In China 8 billion m3 of storage capacity of 20 large reservoirs have been lost due to sedimentation, which is 66% of the total reservoir capacity of these reservoirs (IRTCES, 1985). The storage capacity of reservoirs is being lost at an annual rate of 2.3% in China and 1% in the world (White, 2001). Reservoir half-life is defined by Morris et al. (2008) as the time required to lose half the original capacity to sedimentation. Many large reservoirs in China have already passed their half-life. The Sanmenxia reservoir has a storage capacity of 5.96 billion m3 for the pool level of 330 m. The reservoir passed its half-life in just 4 years in 1964 and was rejuvenated by changing operation from storing water and sediment to flushing sediment. The accumulated sedimentation volume reached 3 billion m3 in 1990 (Wang et al., 2005), which implies that the half-life of the reservoir had been extended to 30 years after change of the operation mode. A wide range of sedimentation related problems occur upstream from dams as a result of sediment trapping. Because of storage loss the functions of the reservoir reduce for flood control, power generation, and water supply. Sediment can enter and obstruct intakes and greatly accelerate abrasion of hydraulic machinery, thereby decreasing its efficiency and increasing maintenance costs. Sediment deposition in the delta region of the reservoir may affect navigation and impact the ecology. Dam construction is the largest single factor influencing sediment delivery to the downstream reaches. The cutoff of sediment transport by dams can cause stream bed degradation, accelerate the rates of bank failure, and increase scouring at structures such as bridge piers. Stream morphology downstream of dams can be dramatically impacted by reduction in the supply of sediment. Clear water in the river channel, downstream of the dam tends to scour the stream bed causing it to coarsen, degrade, and become armored. Channel degradation can raise bank height, increase bank erosion rates，increase the severity of scouring at downstream bridges, lower water levels at intakes, reduce navigational depth in critical locations, and lower groundwater tables in riparian areas. All of these changes have adverse effects on both wetlands and agricultural areas. On the other hand, sediment trapping by reservoirs reduces the suspended solids concentration downstream，which may have many beneficial effects. The suspended solids leve1s of many rivers have been dramatically increased due to upstream deforestation and development. Sediment trapping in reservoirs is beneficial to aquatic ecosystems sensitive to e1evated suspended solid levels, including coastal marine ecosystems such as grass beds and coral reefs harmed by sediment discharged from rivers. - 370 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
Various strategies for reservoir sedimentation control have been studied and applied in reservoir management. Fan and Morris (1992 a and b) summarized the strategies with particular attention to turbidity currents. Garcia studied turbidity currents with poorly sorted sediment and sediment deposition in reservoirs (Garcia, 1994). Sediment bypassing is a method used to manage sediment by preventing it from entering the reservoir. An offstream reservoir can generate benefits in addition to a reduced rate of storage depletion (Morris et al., 2008). Empty flushing involves the opening of bottom outlets to completely empty the reservoir and allow stream flow to scour sediment deposits. The scouring of sediment by flood depends on the permeability, consolidation coefficient and the volume fraction of sand related to silt (Jacobs et al., 2007). Case studies were conducted by Morris and Fan (1998) and White (2001). Four strategies have been applied for reservoir sedimentation control in China: 1) storing the clear and releasing the turbid water; 2) releasing turbidity currents; 3) drawdown flushing or pressure flushing and empty flushing or free flow flushing; 4) dredging. This paper presents the management strategies with examples, with particular attention to the strategy of storing the clear and releasing the turbid water applied in the Three Gorges Reservoir and Sanmenxia Reservoir. 2 Management strategies 2.1 Storing the clear and releasing the turbid Sediment transport in many Chinese rivers occurs mainly in the 2 to 4 month flood season, that is, 80-90% of the annual sediment load is transported with 50-60% of the annual runoff. The sedimentation in reservoirs on such rivers can be controlled with the strategy of storing the clear water during low concentration periods and releasing the turbid water during the flood season. The Three Gorges Project on the Yangtze River is used for flood control, power generation, and inland navigation. For these purposes, it is important to maintain adequate storage in the reservoir. The total storage capacity of the reservoir is 39 billion m3, of which 22 billion m3 must be retained indefinitely and 17 billion m3 is allowed to be filled with sediment in a period of 120-150 years. The Three Gorges Dam is constructed at Yichang, allowing it to control floods from the upper reaches of the river. The main strategy to control sedimentation in the Three Gorges Reservoir is to draw-down the pool water level from 175 m to 145 m in the flood season from June to September when the sediment concentration is high; this allows the turbid water to wash downstream through the reservoir. The reservoir starts to store water in October when the inflowing water becomes clear (i.e. has a lower sediment concentration). The normal pool level of the reservoir is set at 175 m. Two other characteristic pool levels are the flood control level (145 m), to which the pool will be drawn-down at the beginning of the flood season, generally in June, and the low-flow season control level (155 m), to which the pool may be drawn down to satisfy the requirements of power generation and to provide adequate depths of flow, both up- and down-stream of the dam before the next flood season. Because the reservoir is a river-type reservoir, as shown in Fig. 2, the strategy of storing the clear and releasing the turbid water is effective for sedimentation control (Sedimentation Panel for TGP, 2008). The hydrological station closest to the dam is the Yichang Station (downstream from the dam). The long-term average (1950-1985) annual runoff at Yichang is 450 billion m3 and the annual sediment load is 532 million t, of which about 0.8 million t is gravel bed load. The median diameter of suspended load is 0.033 mm and the median diameter of bed load is 24 mm. More than 88% of the suspended load is finer than 0.1 mm. The long-term average discharge at Yichang is 14,300 m3/s. Figure 3 shows the typical variation process of sediment concentration at Yichang and the operation scheme of pool level for sedimentation control. By storing the clear water and releasing the turbid water, less sediment deposits in the reservoir, while the reservoir is still able to store enough water for power generation in the low flow season. The amount of the original flood regulating capacity that can be preserved with the management strategy of storing the clear and releasing the turbid depends upon, among other things, the morphology of the reservoir. The reservoir looks like a ribbon in the plan view, as shown in Fig.2. The 700-km long reservoir is quite uniform in width and is for the most part less than 1,000 m wide. Since the estimated width of the equilibrium channel in the reservoir is 1,300 m, little flood plain is expected to form in the reservoir. Thus, large percentages of both the flood control and low flow season regulation storage may be preserved indefinitely. International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 371 -
N
S113
S34 Yichang
S205
TGP Dam
0 2550km
Chonggqing
Fig. 2 River type Three Gorges Reservoir and locations of three cross sections
3
S (kg/m )
6 4 2
E (m)
0
Fig. 3
Sediment concentration at Yichang
175 170 165 160 155 150 145
Jan.
April
July
Oct.
Dec.
Reservoir pool level
Jan. April July Oct. Dec. Typical variation process of sediment concentration at the dam site of the Three Gorges Project and the operation scheme of pool level for sedimentation control
Permanently preserved storage
NPL FCL
Dam
Fig. 4
Fluctuating Sediment deposition backwater region Schematic diagram of the pool levels and deposition curve of the TGP reservoir: NPL=normal pool level (175 m); FCL = flood control level (145m)
Drawing the reservoir down to 145 m during the majority of the flood season may keep the upper limit of deposition below 145 m, as shown in Fig. 4. During the low-flow season, the river carries little sediment, but still contributes to 39% of the annual runoff. Water is then stored in the reservoir for power generation and navigation. The amount of water needed to fill up the reservoir to its normal pool level varies with the scheme adopted but is generally less than 22 billion m3, which is only a small portion of the runoff in the low flow season. Eventually, a new alluvial channel will form in the reservoir with a - 372 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
Sedimentation volume (109m3)
thalweg profile under the flood control pool level, as shown in Fig. 4. The sedimentation process of the reservoir has been simulated by using a one-dimensional numerical model. The computation was made for a selection of the reservoir operational levels schemes. The hydrological time series for the period 1961-1970 was used for the simulation with a 40-year flood inserted in the series. As the recurrence period of the 1954 flood is 40 years, the stream record of 1954 was inserted into the hydrological series in such a manner that it appears three times in 106 years. The time series, thus, consists of the hydrological records of the following sequence of years: 2 cycles of 1961-1970 plus 1954, 1955; 2 cycles of 1961-1970 plus 1954, 1955; 4 cycles of 1961-1970 plus 1954, 1955; 2 cycles of 1961-1970 According to numerical modeling, the reservoir sedimentation may be controlled by using the strategy of storing the clear water and releasing the turbid water. This method approaches equilibrium in a 100 year period. Figure 5 shows the accumulation of sediment in the reservoir from 0 to 100 years for various schemes calculated by the Yangtze River Conservancy Commission, in which 160-135 (Curve 2) represents the curve where the normal pool level is 160 m and the flood control pool level is 135 m. Curve 4 in the diagram represents the sedimentation volume over time with a normal pool level of 175 m and a flood control pool level 145 m (but in the first 10 years the two levels are 156 m and 135 m, respectively) (Sedimentation Panel for TGP, 2002). For the 175-145 configuration, the reservoir sedimentation will approach equilibrium in 100 years with a final capacity loss of about 16 billion m3. 20 18 16 14 12
(4) (3) (2) (1)
10 8 6 4 2 0 0
Fig. 5
(5)
NPL-FCL (1) 150-135 (2) 160-135 (3) 170-140 (4) 175-145 (5) 180-150
10
20
30
40
50
60
70
80
90
100
Years of operation Calculated sedimentation over a 100 year period of operation of the Three Gorges Reservoir for five operational level schemes (Sedimentation Panel for TGP, 2002)
In the reality, the Three Gorges Project was completed in 2009, but the reservoir began to impound in June of 2003. Sedimentation in the first years of operation has received great attention. In the first seven years of operation the pool level rose gradually following the completion of the dam in the ranges of 135-141, 145-156 and 145-175 m. Figure 6 shows the real pool level from Jan. 2007 to Jan. 2008，in which the data were collected from the daily reports of Ministry of Water Resources on the web (http://www.ctgpc.com.cn). In the non-flood season, the pool level was controlled at around 156 m. During the flood season, from June to Oct., the pool level was drawn to 145 m, so high sediment concentration flood water flowed through the reservoir and released to the downstream reaches. The Cuntan station is located near Chongqing, about 600 km upstream from the dam. The long term (1950-1985) average annual runoff, sediment load, and average concentration at Cuntan were 350 billion m3, 462 million t, and 1.32 kg/m3, respectively. Only about 0.3 million t of the sediment load was bed load. The median diameter of the suspended load was 0.037 mm and the median diameter of gravel bed load was 51 mm. In the past two decades, however, the sediment load has dramatically reduced due mainly to numerous dams in the upstream watershed, while the runoff remains unchanged. The average annual sediment load in the period from 2001-2007 has reduced to about 200 million t. Figure 7 shows the distributions of monthly runoff and sediment load at the Cuntan and Yichang stations in 2007, which represent water and load inflowing and out of the reservoir, respectively (MWR, 2008). The result shows that more than 30% of the incoming sediment load is discharged out of the reservoir during the flood season. International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 373 -
Pool level (m) 1
160 155 150 145 140 135 130 1
Fig. 6
2
3
4
5
6 7 8 9 10 11 12 13 Month Pool level of the Three Gorges Reservoir from January 2007 to January 2008
Trap efficiency, Te, is the proportion of the incoming sediment that is deposited or trapped in a reservoir and several estimation methods of the efficiency were studied (Jothiprakash and Garg, 2008). Similarly, we define sediment releasing efficiency as the amount of sediment exiting the reservoir as a percentage of inflowing sediment load, which in fact equals 100% - Te. According to the numerical models, the filling of the reservoir with sediment is very rapid in the first 20 years and slows down in the following years. The sediment releasing efficiency rises as the reservoir fills, reaching 100% in about 120 years. Figure 8 (a)-(c) shows the measured cross sections in the reservoir in 2003, 2005, 2006 and 2007 at 5.6 km, 160 km and 356 km from the dam, respectively (MWR, 2007, 2008). The location of the three cross sections is shown in Fig. 2. Sediment accumulated in the reservoir rapidly in the first two years (2003-2005) following impoundment, but from 2005 to 2007, the amount of sedimentation in the reservoir remained relatively constant. Comparing with the calculated sedimentation volume with a linear increasing trend in the first 10 years, as shown in Fig.5, the measured sedimentation volume in the first 4 years of operation was much less. The main reason was a dramatic sediment load reduction since the 1980s (Liu et al., 2008).
Monthly runoff
60
60
Monthly sediment load 40
40
20
20
0
0
3
(billion m )
1
Monthly runofff
Monthly sediment load (millionton)
80
Cuntan Station in 2007
2
3
4
5
6 7 Month
8
9
10
11
12
90
Yichang Station in 2007
24
60
Monthly runoff Monthly sediment load
16
30
8
Monthly sediment load (millionton)
3
(billion m )
Monthly runofff
80
0
0 6 7 8 9 10 11 12 Month Distributions of monthly runoff and sediment load at the Cuntan and Yichang stations, which represent water and sediment load flowing in and out of the reservoir, respectively (MWR, 2007) 1
Fig. 7
- 374 -
2
3
4
5
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
200
Cross sections No.34 (5.6 km upstream of the dam)
Elevation (m)
150 100
Mar 2003 Oct 2005
50
Oct 2006
0
Oct 2007
-50 -1000
-500
0 500 Distance from the left bank (m)
1000
1500
250 Cross sections No.113 (160.1 km upstream of the dam) Elevation (m)
200 Mar 2003 Oct 2006
150
Oct 2005 Oct 2007
100 50 0 0
500
1000
1500
2000
2500
Distance from the left bank (m) 240 Cross sections No.205 (356 km upstream of the dam)
Elevation (m)
220 200 180 160
Mar 2003
Oct 2005
Oct 2006
Oct 2007
140 120 100 -200
300 800 Distance from the left bank (m)
1300
Fig. 8 Measured cross sections at the Three Gorges Reservoir in 2003, 2005, 2006 and 2007 at 5.6 km (a), 160 km (b) and 356 km (c) from the dam, respectively (MWR, 2007)
The strategy of storing clear water and releasing turbid water has also been successfully used in the management of sedimentation behind the Sanmenxia Dam on the Yellow River after several failures of other sedimentation management strategies. The Sanmenxia Dam, 105-m high and 739-m long, was the first large dam on the Yellow River. The crest elevation of the dam is 353 m and the designed reservoir capacity is 35.4 billion m3 with a normal pool level of 350 m. The main purposes of the dam are flood control, ice jam flood control, sediment trapping, power generation, and irrigation. The reservoir controls a drainage area of 688,000 km2 and 89% of the total runoff from the Yellow River basin. The construction of the dam was initiated in 1957 and water impoundment commenced in Sept. 1960. The reservoir area extends upstream a distance of 246 km to Longmen. The Yellow River flows south from Longmen to Tongguan, then makes a 90° turn and goes east. The Wei River flows into the Yellow River at Tongguan. The reservoir area consists of three parts: 1) the Yellow River from the dam to Tongguan; 2) the Yellow River from Tongguan to Longmen; and 3) the Wei River from Tongguan to Xianyang, or the lower Wei River (Wang et al., 2007). The operation scheme of the Sanmenxia Reservoir has been substantially changed to achieve a balance between sediment inflow and outflow in the following three reservoir operation modes (Wang et al., International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 375 -
2005): (i) storage from Sept. 1960 to Mar. 1962, the reservoir was operated at a high storage level the whole year round; (ii) detaining flood water and sluicing sediment from Mar. 1962 to Oct. 1973, 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; and (iii) storing clear water and releasing turbid water from Nov. 1973 to the present, the reservoir has been operated at a high level (315–320 m) to store relatively clear water during non-flood seasons (Nov.–June) and at a low level (302–305 m) to release high sediment concentrations during flood seasons (July–Oct.), as shown in Fig. 9. Serious sediment accumulation occurred during the first operation mode period, which increased the risk of flooding in Xi-an City (the ancient capital of China). The operation pool level had to be changed to 303-318 m in 1962-1973, allowing flood water scoured sediment to flow through the reservoir. Sediment was sluiced out of the reservoir but at the cost of power generation. The strategy of storing the clear and releasing the turbid was finally applied with the pool level very low during high sediment concentration period and high pool level during low concentration period for power generation. 335 330
1960-1961
Elevation (m)kk
325 320 1974-2001 315 310 1963-1973
305 300 11
295 0
12 50
1
2 100
3
4 150
5
6
200
7 250
8 300
9
10 350
h
Time (day)
Fig. 9 Three operation modes of pool level of the Sanmenxia Reservoir in different time periods
Figure 10 shows the accumulated sediment deposition in the three parts of the reservoir and the storage capacity for pool levels of 330 m and 323 m. The sedimentation has been controlled and the accumulated sediment deposition volume in two parts of the reservoir, comprising Tongguan to the dam and the lower Wei River, remained unchanged since the operation mode changed to storing the clear and releasing the turbid in 1973. Only a slight increase of accumulated sediment occurred in the upper part of the reservoir extending from Longmen to Tongguan. The reservoir capacity has remained almost unchanged since 1973. 2.2 Turbidity currents Turbidity currents are sediment-laden density-driven currents that flow along the bottom of the reservoirs. These currents are created by the relative motion between two fluid layers that have different densities. The turbidity currents in reservoirs generally involve only slight differences in the densities of the upper and lower layers. Since the density difference is small, the reservoir water creates a large buoyancy effect within the inflowing liquid, so that the effective gravity of the flowing liquid is greatly reduced. Usually g′ is defined as effective gravity given by: g′ = g
Δρ
ρ
(1)
in which g is the gravitational acceleration, Δρ is the density difference between the upper and lower liquids, and ρ is the density of the reservoir water. Many formulae describing open-channel flow apply also to turbidity currents once g is replaced by g′. For example, the flow pattern in an open channel depends greatly on the Froude number of the flow; in a turbidity current, the Froude number remains the key parameter but its form is modified as follows (Qian et al., 1998): - 376 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
7
(a)
9
Tongguan to dam
3.0 2.5 2.0
Longmen to Tongguan
1.5 1.0 Lower Wei River
0.5
(b)
3
Reservoir storage capacity (10 m )
3.5
9
3
Accumulated deposition (10 m )
4.0
0.0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
6
Elevation < 330 m
5
Elevation < 323 m 4 3 2 1 0 1960
1965
1970
1975
1980
1985
1990
1995
Time (year)
Time (year)
Fig. 10 (a) Accumulated sediment deposition in Sanmenxia Reservoir; (b) Storage capacity of Sanmenxia Reservoir for pool elevations 330 m and 323 m
Fr ′ =
Uc g ′h′
(2)
in which Uc is the relative velocity between the two liquid layers and h′ is the thickness of the turbidity current. Because g′ is much smaller than g, the Froude number is larger than 1 for many turbidity currents, therefore, a hydraulic jump occurs very often in turbidity currents (Garcia, 1993) Data from both flume experiments and field observations show that the critical condition at the plunging point for the formation of turbidity current is (Fan, 1959): q2 (3) = 0.6 Δρ 3 gh ρ′ 0 in which q is the discharge per unit width and h0 is the depth at the immersion point. From the above equation, if the water level upstream of the dam remains constant, an increase of the inflow discharge would cause the plunging point to move downstream; and an increase in the density difference between the inflow and the reservoir water would cause the point to move upstream. Maintaining a turbidity flow needs a continuous supply of inflow sediment suspension and a force to overcome any resistance it encounters. If the inflow ceases to supply dense fluid so that it no longer forms turbidity current at the plunging point, the already formed turbidity current downstream would soon stop moving. Since the energy to maintain the flow comes from the density difference and the slope of the reservoir bed, a minimum density difference is required to produce and maintain the current flowing to the dam. According to field data from the Guanting Reservoir on the Yongding River and the Sanmenxia Reservoir on the Yellow River, the sediment concentration in a turbidity current must be more than 20 kg/m3 to allow the current to flow continuously to the dam (Qian et al., 1998). The Xiaolangdi Reservoir is located 125 km downstream of the Sanmenxia Dam and about 860 km upstream of the river mouth. The reservoir is the most downstream gorge-type reservoir on the Yellow River. The multi-purpose reservoir is mainly for flood control, power generation and sediment retention for reducing siltation in the lower Yellow River. The total capacity of the reservoir is 12.65 billion m3, of which more than 7.55 billion m3 will be used for trapping sediment. The reservoir began to impound in Oct. 1999. While the reservoir operates it traps coarse sediment and discharges fine sediment to the lower reaches. In the first 8 years, coarse sediment was trapped as clear water and a portion of fine sediment was released to the lower reaches mainly in the form of turbidity currents. Because a fraction of coarse sediment is trapped by the reservoir, the rate of sedimentation of the lower Yellow River channel is greatly reduced. Sediment finer than 0.02 mm was released to the lower reaches but it is mostly wash load and does not cause siltation of the lower reaches. Since impoundment turbidity currents occurred every year, especially during hyper-concentrated floods, the turbidity currents flowed to the dam and were discharged out of the reservoir when the bottom outlets International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 377 -
were open. Figure 11 shows the elevation of the interface between the turbidity currents and the upper clear water layer for five turbidity currents in 2001 (Hou and Jiao, 2003). The interface remained in a narrow range between 188-194 m in most parts of the reservoir, although the discharge of the turbidity currents varied in the range of 200-2,800 m3/s, the sediment concentration varied between 13 kg/m3 to 530 kg/m3, and the average velocity varied between 0.2 m/s to 1.2 m/s. The Froude number at the plunging point was measured at 0.5-0.61, which approximates the situation represented by Eq. (3) (Hou and Jiao, 2003). A continuous turbidity current flowing to the dam is critical for sediment release, which depends on the inflowing discharge, sediment concentration and the portion of sediment particles finer than 0.01mm. Figure 12 shows the sediment concentration, S, and discharge, Q, of turbidity currents in 2001-2004 (data from Wu et al., 2008). In the figure, the filled circles represent turbidity currents reaching the dam, in which the portion of sediment particles finer than 0.01 mm is between 25-75%; the black pyramids are turbidity currents moving to the dam but the portion of sediment particles finer than 0.01 mm is higher than 75%; the open triangles are turbidity currents not reaching the dam, although some of them consists of 90% fine sediment. The curve in the figure is given by: (4) SQ = 100000 (kg/s) For high sediment concentration and discharge, or the points above the curve, the turbidity currents flowed through the whole reservoir and arrived at the dam. For the points below the curve the turbidity currents stopped in the reservoir, failing to reach the dam.
Fig. 11 Elevation of the interface between the turbidity currents and the upper clear water layer (data are from Hou and Jiao, 2003)
500
3
S (kg/m )
400
P(D<0.01mm)>25% P(D<0.01mm)>75% Not to the dam
300 200 100 0 0
1000
2000
3000
4000
3
Q (m /s) Fig. 12 Sediment concentration, S, and discharge, Q, of turbidity currents reaching and not reaching the dam occurred in 2001-2004 (data from Wu et al., 2008)
In general, the sediment releasing efficiency of turbidity currents is about 20%. Nevertheless, the turbidity currents in the Xiaolangdi Reservoir had low sediment releasing efficiency because the reservoir bed had not silted up to the low sill of the bottom outlets. Arriving at the dam the turbidity currents filled the reservoir and formed a muddy water reservoir. Only after the surface of the muddy water reservoir - 378 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
rose to the elevation of the bottom outlets, were the turbidity currents released from the reservoir. The average sediment releasing efficiency, or the ratio of the released sediment to the inflowing sediment load, was only 6% in the period from 2000-2004 (Hou and Jiao, 2003). In 2006, turbidity currents occurred again in the Xiaolangdi Reservoir by using Sanmenxia Reservoir to create high sediment concentration flow (MWR, 2007). About 8.4 million tons of fine sediment, or 36% of inflowing sediment, was released during the turbidity currents because the sedimentation reservoir bed approached to the bottom outlets. The average sediment releasing efficiency of turbidity currents was about 20%. 2.3 Flushing Hydraulic flushing involves reservoir draw-down by opening low level outlets to temporarily establish riverine flow along the impound reach, eroding a channel through the deposits and flushing the eroded sediment through the outlets. Sediment entering the reservoir during flushing periods will also be routed through the impoundment and released. Fan (1985) has classified flushing into two general categories: (1) empty or free flow flushing, which involves emptying the reservoir to the level of the outlets with riverine flow through the reservoir, and (2) drawdown or pressure flushing, which requires less drawdown but is also less effective. The second method is not commonly used. While both strategies have been applied successfully, flood season flushing is generally more effective because it provides larger discharges with more erosive energy, and flood borne sediments can be routed through the reservoir. Flushing scours a single main channel through reservoir bed, while floodplain deposits on either side are unaffected. Figure 13 shows draw-down flushing and empty flushing. During pressure flushing, the reservoir is drawn down to a low pool level and the bottom outlets are opened allowing development of a conical scour hole in front of the outlets, as shown in Fig. 13 (a). Sediment from the upper portion of the reservoir is transported towards the dam, but only material in the scour hole in front of the outlets can be flushed out. Draw-down flushing or pressure flushing was used to flush sandy bed material in the Tarbela Reservoir in Pakistan (Lowe and Fox, 1995). The Tarbela Dam is a 137 m-high embankment dam on the Indus River mainly used for hydropower (3,750 MW installed capacity) and irrigation. Its total capacity was 14.3 billion m3 at closure in 1974 but it had decreased 17.4% due to sedimentation by 1992. The original project was designed for a 50-year life with no previsions for the eventual management of the inflowing sediment load of about 208 million tons per year. The inflowing sediment consisted of 59% fine sand, 34% silt, and 7% clay. Approximately 99% of the inflowing load was trapped and accumulates primarily in the form of a delta deposit which advanced toward the dam. Delta top-set beds have a slope of about s= 0.0006 to 0.0008. Most sediment deposited on the top-set bed as the reservoir filled and water levels rose during the wet season. However, when the reservoir was draw down for irrigation deliveries, the river reworks and transported these deposits downstream, extending the delta toward the dam. Most sediment is transported to the face of the delta at the onset of the wet season when the pool level is still low but discharge increases from 1,500 to 4,500 m3/s. Erosion
Drawdown Deposition
Empty Erosion
Fig. 13 (a) Pressure flushing causes erosion in the upper part of the reservoir and redeposition near the dam, with pressure flow through the bottom outlets; (b) Empty flushing results in erosion in the whole reservoir, with free flow through bottom outlets
Empty flushing has been applied in middle and small reservoirs, in which these reservoirs were emptied before the flood season and the sediment deposit in the reservoir was flushed out during the first part of the flood season. Because of the high sediment concentration that can be released during the flushing period, the downstream channel must be able to transport high sediment concentration flow. Seasonal emptying is also feasible when water demand is seasonal. Empty flushing has been used in the Sefid-Rud International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 379 -
Reservoir in Iran (Khosronejad, 2009). The sediment concentration increased from about 2 kg/m3 to 40 kg/m3 due to scour by flood flow when the reservoir was emptied (Forood and Ghafouiri, 2007) The Hengshan Reservoir on the Tangyu River (a tributary of the Yongding River) in Shanxi Province, China applied the empty flushing strategy to control sedimentation and restore the reservoir capacity. The Hengshan Dam is 69 m high with a reservoir capacity of 1.33 million m3. From 1966-1974, 3.19 million m3 of sediment deposited in the reservoir with a thickness of 27 m near the dam. To restore the capacity of the reservoir the reservoir was emptied to flush sediment in 1974 and 1979. During the empty flushing period from July 28 to Sept. 4 in 1974, the inflowing sediment was 0.13 million t but 1.19 million t of sediment were flushed out of the reservoir (Guo et al., 1985). The reservoir was emptied and flushed again in 1979 from Aug. 9 to Sept. 30, the inflowing sediment was 0.2 million t but 1.55 million t of sediment was flushed out of the reservoir. A mud flow with a maximum concentration of about 1,370 kg/m3 was created and flowed out of the reservoir. Figure 14 shows the sediment concentration and discharge of mud flow during the empty flushing. The flow cut a narrow yet deep channel in the reservoir sediment deposit. Guo et al. (1985) analyzed the empty flushing and noted that the channel can be deepened by using either small or large flows, but widening this channel can only be achieved by using large flows. It was found that the flushing effect is maximized if the reservoir is emptied immediately prior to the arrival of a flood because the flood flow exerts erosive force on deposits which have not yet had the time to fully dewater and consolidate. The phrase used to describe this strategy is “Deepen by small flow, widen by flood flow.”
600 0.5
300
0.0
0 0
Fig. 14
900 3
1.0
1200
S (kg/m )
Q S
3
Q (m /s)
1.5
10
20 30 40 t (hr) Sediment concentration and discharge of mud flow during the empty flushing from the Hengshan Reservoir in 1979 (Data from Guo et al., 1985)
Fig. 15 Critical velocity for initial scouring of cohesive sediment from the Zhuwo Reservoir as a function of the weight of sediment per volume in the deposit
The Zhuwo Dam is on the Yongding River near Beijing, China, which is 33 m high with a reservoir capacity of 14.75 million m3. The reservoir began to store water in 1961 and lost 5.3 million m3 of reservoir capacity due to sedimentation after 25 years of operation. Most of the sediment deposit was cohesive sediment with a median diameter of 0.004 mm. The first author of this paper conducted a physical model experiment to study the efficiency of empty flushing of cohesive sediment from the reservoir (Wang and Zhang, 1989). Cohesive sediment may be carried downstream if the sediment is - 380 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
scoured from the reservoir. Figure 15 shows the critical velocity, Uc, for initial scouring of cohesive sediment from the reservoir as a function of the weight of sediment per unit volume in the deposit, γ b , which followed the following formula: 4 U c = 4.6γ b (5) Because the length and depth scale of the model was 1:50 then the velocity scale can be calculated from the Froude number to be 1: 7.07. The scale ratio for the critical velocity for scouring similarity is then 1: 1.63. The measured γ b in the reservoir varied almost lineally from 0.55 g/cm3 at the surface of the deposit to about 1.22 g/cm3 at a depth of 20 m in the deposit. The modeling sediment is the cohesive sediment from the Zhuwo Reservoir and the value of γ b was controlled, lineally distributed from 0.33 g/cm3 at the surface of the deposit to 0.75 g/cm3 at a depth of 40 cm in the model, which meet the requirement of scouring similarity. Figure 16 shows the sediment concentration and discharge of the mud flow stage of empty flushing during the model experiment. During the process a very small inflowing discharge, about 1 m3/s, flowed to the reservoir and scoured the sediment. The sediment concentration varied in the range of 200 kg/m3 to 700 kg/m3 during the mud flow process. After 8 hours the mud flow stopped. Then the inflowing discharge of clear water increased to 12 m3/s, which was controlled by the Guanting Reservoir. Retrogressive erosion occurred as a consequence of the inflowing discharge. Figure 17 shows the sediment concentration during the retrogressive erosion stage of the empty flushing. Because the sediment was very cohesive, the retrogressive erosion lasted only about 40 hours. Almost no more sediment could be scoured except if the inflowing discharge dramatically increase, which was not allowed because of the safety of water resources utilization for Beijing. The retrogressive erosion created many gullies on the reservoir bed, which formed a palm shape channel network. About 0.19 million m3 of sediment deposit was discharged out of the reservoir during the empty flushing, of which 37% was released from the reservoir in the form of mud flow. Q S
3
600 3
Q (m /s)
12
800
S (kg/m )
16
8
400
4
200
0
0 0
2
4 6 8 t (hr) Fig. 16 Sediment concentration and discharge of mud flow during empty flushing of the Zhuwo Reservoir
The sediment releasing efficiency, E, can be estimated for empty flushing by using the following formula: E= time of accumulation of sedimentation volume V / time to flush reservoir sediment with volume V For the Hengshan Reservoir the efficiency E was 24.7 during the empty flushing in 1974 and was 23.9 during the empty flushing in 1979. For the Zhuwo Reservoir, the efficiency was 55 during the empty flushing experiment. Empty flushing has the highest sediment releasing efficiency, but the flushing may cause high ecological stress on the downstream reaches. Suspended sediment adsorbs pollutants from flowing water in rivers and deposits in the reservoirs. The concentration of heavy metals (Cr, Cd, Hg, Cu, Fe, Zn, Pb and As) in water, sediment, and fish/invertebrate were investigated and the results showed that the concentrations of heavy metals were highest in the sediment and lowest in the water. Benthic invertebrates had higher concentrations of heavy metals in their tissues due to their proximity to contaminated sediments and fish had lower concentrations of heavy metals (Yi et al., 2008). Empty flushing might release the pollutants from the sediment and increase sharply the concentration of International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 381 -
pollutants in water. The ecological system downstream of the Zhuwo Reservoir consists of numerous species of aquatic plants, benthic invertebrates and fish. Empty flushing can seriously impair the ecology. The Beijing government decided to dredge the Zhuwo Reservoir rather than implement an empty flushing technique. This had a higher economic cost but a much lower ecological cost. 400
3
S (kg/m )
300 200 100 0 0
Fig. 17
10
20 30 40 t (hr) Sediment concentration of mud flow during empty flushing of the Zhuwo Reservoir experiment
2.4 Dredging Dredging has been used for a long time for sedimentation management in small reservoirs. Different dredging methods are applied in reservoir management, including mechanical dredging and dumping outside of the reservoir and agitating sediment with jets so that that sediment can be transported downstream of the reservoir by currents. Various dredgers have been used: dredge boat; agitating dredger; dipper dredger; hauling scraper; excavator and bulldozer; and trailer dredger. In general dredging is more expensive than other strategies of sedimentation control in reservoirs. Nevertheless, jets in combination with turbidity currents or flushing are more economically feasible and have been applied in large reservoirs. Jet dredging was conducted in a reach near Tongguan in the Sanmenxia Reservoir from 1996 to 2003 during the flood season. Figure 18 shows the sediment amount scoured from the reservoir bed by jets. The sediment is composed mainly of silt and fine sand with 20% finer than 0.025mm and about 40% finer than 0.05 mm. It is reported that the sediment finer than 0.05 mm may be transported by flow out of the reservoir but coarser sediment may settle again in the reservoir (Jiao et al., 2008).
Fig. 18 Sediment amount scoured from the wake area of Sanmenxia Reservoir by jet dredging
Jet dredging was also used to create artificial turbidity currents to release sediment from the Xiaolangdi Reservoir. Since the Xiaolangdi Reservoir began to store water in 1999, about 1.5 billion m3 of fine sediment had deposited in a 40 km reach upstream from the dam in the reservoir by April 2005. The sediment is fine with a median diameter of about 0.01 mm. The reservoir sediment was scoured by jet, which resulted in fine sediment suspension in the reservoir. Because the suspension has a slightly higher density than the surrounding water, it flows to the dam along the reservoir bed in the form of a turbidity current. Most of the fine sediment is released from the bottom outlet of the reservoir with turbidity currents. - 382 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
3 Conclusions Reservoir sedimentation may be controlled by applying the strategies of storing the clear water and releasing the turbid water, releasing turbidity currents, empty flushing and dredging. The sediment releasing efficiency is: 30-100% for storing the clear water and releasing the turbid water, 6-36 % for turbidity current, and 2,400-5,500% for empty flushing. Empty flushing causes high ecological stress on the ecosystem to the downstream reaches. Storing the clear and releasing the turbid is the best strategy to control reservoir sedimentation while achieving hydro-power benefits and maintain ecological stability. Acknowledgement The study was supported by the National Natural Science Foundation of China (50725930, 50721006) and Ministry of Water Resources of China (2007SHZ0901034). References Fan J. and Morris G. L. 1992a, Reservoir sedimentation I: Delta and density current deposits. Journal of Hydraulic Engineering, Vol. 118, No. 3, pp. 354–369. Fan J. and Morris G. L. 1992b, Reservoir sedimentation II: Reservoir desiltation and long-term storage capacity. Journal of Hydraulic Engineering, Vol. 118, No. 3, pp. 370–384. Fan Jiahua. 1959, Experimental study on density current. Chinese Journal of Hydraulic Engineering, Vol. 5, pp. 30–48 (in Chinese). Fan Jiahua. 1985, The reservoir sediment problem. Journal of Hydraulic Engineering, pp. 24–31 (in Chinese). Foroods. and Ghafouiri M. 2007, Assessment of causes and effects of disastrous erosion and sediment flows and mitigation measures in Caspian Sea Watersheds-Iran. Proceedings of UNESCO Expert Meeting on Erosion in Arid and Semi-Arid Areas, Chalooz, Iran. Garcia M. H. 1993, Hydraulic jumps in sediment-laden bottom currents. Journal of Hydraulic Engineering, Vol. 119, pp.1094–1117. Garcia M. H. 1994, Depositional turbidity currents laden with poorly sorted sediment. Journal of Hydraulic Engineering, Vol. 120, No. 11, pp. 1240–1263. Guo Zhigang, Zhou Bin, Ling Laiwen, and Li Degong. 1985, The hyperconcentrated flow and its related problems in operation at Hengshan Reservoir. Proceeding Internal Workshop on Flow at Hyperconcentrations of Sediment: 3–1. Hou Suzhen and Jiao Enze. 2003, Analysis on turbidity currents in the Xiaolangdi Reservoir. Water Resources and Hydropower Engineering, Vol. 34, No. 6, pp. 11–14 (in Chinese). IRTCES (International Research and Training Center on Erosion and Sedimentation). 1985, Lecture notes of the training course on reservoir sedimentation. Series of publication, Beijing, China. Jacobs W., Van Kesteren W. G. M., and Winterwerp J. C. 2007, Permeability and consolidation of sediment mixtures as function of sand content and clay mineralogy. International Journal of Sediment Research, Vol. 22, No. 3, pp. 180–187. Jia Jinsheng, Yuan Yulan, and Li Tiejie. 2004, China’s and world’s large dams in 2003. China Water Resources, No. 13, pp. 25–33. Jiao Enze, Miao Fengju, and Lin Xiuzhi. 2008, Water and Sediment Management with Reservoirs. Yellow River Press, pp. 1–230. Jothiprakash V. and Garg V. 2008, Re-look to conventional techniques for trapping efficiency estimation of a reservoir. International Journal of Sediment Research, Vol. 23, No. 1, pp. 76–84. Khosronejad Ali. 2009, optimization of the Sefid-Roud Dam desiltation process using a sophisticated one dimensional numerical model. International Journal of Sediment Research, Vol. 24, No. 2, pp. 189–200. Liu Cheng, Sui Jueyi, and Wang Zhaoyin. 2008, Sediment load reduction in Chinese rivers. International Journal of Sediment Research, Vol. 23, No. 1, pp. 44–55. Lowe J. and Fox I. 1995, Sediment management schemes for Tarbela Reservoir. USCOLD Annual Meeting 1995. McCully. 1999, Silenced Rivers: The Ecology and Politics of Large Dam. Zed Books, London. Morris G. L. and Fan J. 1998, Reservoir sedimentation handbook. McGraw Hill, New York. Morris G. L. Annandale G., and Hotchkiss R. 2008, Reservoir Sedimentation, in Garcia M.H. ed. Sedimentation Engineering, Published by American Society of Civil Engineering. MWR (Ministry of Water Resources of China). 2007, China Gazette of River Sedimentation 2006. China Water Resources Press (in Chinese). MWR (Ministry of Water Resources of China). 2008, China Gazette of River Sedimentation 2007. China Water Resources Press (in Chinese). Pan Jiazheng and He Jing. 2000, China Dam Construction in The Past 50 Years. Chinese Water and Hydro-power Press, Beijing, China (in Chinese). Qian Ning, Wan Zhaohui, and McNown, J. 1998, Mechanics of Sediment Movement. ASCE Press, United States. Sedimentation Panel for TGP. 2002, Comprehensive analysis on “9.5” sedimentation study results for the Three gorges Project on the Yangtze River. Intellectual Copyright Press (in Chinese). International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384
- 383 -
Sedimentation Panel for TGP. 2008, Brief report of water and sediment measurement during the coffer dam impoundment (2003-2006) period. China Water Resources Press (in Chinese). Wang Guangqian, Wu Baosheng, and Wang Zhaoyin. 2005, Sedimentation problems and management strategies of Sanmenxia Reservoir, Yellow River, China. Water Resources Research, Vol. 41, W09417, Doi: 10.1029/ 2004WR003919. Wang Zhaoyin and Zhang Xinyu. 1985, Experimental study of empty flushing of cohesive sediment from a reservoir with a physical model. Journal of Sediment Research, No. 2, pp. 62–69 (in Chinese). Wang Zhaoyin, Wu Baosheng, and Wang Guangqian. 2007, Fluvial processes and morphological response in the Yellow and Wei Rivers to closure and operation of Sanmenxia Dam. Geomorphology , Vol. 91, Issues 1–2, pp. 65–79. White R. 2001, Evacuation of Sediments from Reservoirs. Thomas Telford Press, London. Wu Lianchun, Zheng Minsheng, and Zheng Hui. 2009, Rising the sediment releasing efficiency and extending the life of the Xiaolangdi Reservoir, Proceedings of the Special Session for Comprehensive Utilization of Water Resources in the Middle and Lower Reaches of the Yellow River. 10th China Association for Science and Technology Annual Meeting. Zhenzhou, September 17–19, 2008 (in Chinese). Yi Yujun, Wang Zhaoyin, Zhang Kang, Yu Guoan, and Duan Xuehua. 2008, Sediment pollution and its effect on fish through food chain in the Yangtze River. International Journal of Sediment Research, Vol. 23, No. 4, pp. 338–347.
- 384 -
International Journal of Sediment Research, Vol. 24, No. 4, 2009, pp. 369–384