Preliminary study about river-lake evolution effect due to Three Gorges Project
Gong ping
Senior engineer, Yangtze River Scientific Research Institute, People’s republic of China
Yang Wenjun
Lu Junyou
Wan Jianrong
1Introduction
The full length of river course between Yichang and Datong is about 1120km in midstream and downstream of ChangjiangRiver(Fig 1). The river course lengthbetween Yichang and Zhicheng is about 60km, its river course is basically straight and light curving, with tributary stream of Qingjiang river afflux at Yidu. The river section between Zhicheng and Chenglingji is called as Jingjiang river which is divided into Upper Jingjiang and Lower Jingjiang with Ouchikou as border line. Upper Jingjiang is about 170km long, belongs to curving fork river course and Lower Jingjiang is about 170km long, belongs to wriggling river course.Songzikou, Taipingkouand Ouchikou are three diversion passages on Jingjiang river (Tiaoxiankou has been shut off by anchor gate installing), by which the water in ChangjiangRiveris divided and flows into DongtingLakesubsequently. In addition, four rivers including Xiang, Zi, Yuan and Li merge intoDongtingLake. The water in all these rivers is regulated and stored byDongtingLake andflows into ChangjiangRiver again at Chenglingji, forming into complex river-lake relationship. The river section between Chenglingji and Wuhan is about 230km long, belongs to fork river course. Hanjiang, the biggest tributary stream of ChangjiangRiver, flows into ChangjiangRiver at Wuhan city. The river section between Wuhan and Datong is about 500km long, belongs to typicallyfork river course, with incoming water from PoyangLake merging into ChangjiangRiver.
Fig.1
Schematic drawing of main stream river section between Yichang and Datong in ChangjiangRiver
2Current situation of river-lake relationship
According to river bed scouring-siltation of three diversion passages in Jingjiang river and sediment deposition of Dongting Lake, in the condition of sediment decreasing in upstream water basin of Changjiang River (1996~2002), incoming sediment from these three diversion passages and four rivers including Xiang, Zi, Yuan and Li (called as “Four Waters” for short in the following text), which enters into Dongting Lake subsequently, decreases near 50%than its normal value of accumlated year(1956~1995). For DongtingLake, sediment out of the lake zone decreases near 50% accordingly, sedimentdeposition in the lake zone decreases distinctly, but silt deposit ratio hardly changes. Sediment deposition is still the main element in river bed scouring-siltation for three diversion passages of Jingjiang river, even in the condition of decreasing waterdiversion and silt diversion distinctly. The most serious sediment deposition happens in Ouchikou, its sediment deposition amount is more than that of Songzikou ,but its silt diversion amount is less than that of Songzikou.
The statisticalchanges of water diversion and silt diversion in different times for three diversion passages in Jingjiang river (called as “Three Passages” for short in the following text) is from 1956, when systematicsurvey data can be obtained. In nature condition, the ratio ofwater diversion and silt diversion tends to decrease. With systematic curve course correction of Lower jingjiang and the construction of Gezhouba hydro project, the decreasing process mentioned above accelerates. The main effect comes fromsystematic curve course correction of Lower jingjiang. Since Three Gorges Project is constructed, water diversion ratio decreases lightly, silt diversion ratio changes hardly, no sudden changes happen for now.
According to incoming water-slittime interval distribution, before Jingjiang river curve course is corrected (1956~1966), water inflow from “Three Passages” and “Four Rivers” is 43% and 49.5% of total water inflow into Dongting Lake respectively. After Jingjiang river curve course is corrected in 1967 and 1973 in succession, water inflow from “Three Passages” decreases to 28% of total water inflow into DongtingLake and water inflow from “Four Rivers” increases to 62.5% of total water inflow into DongtingLake. In the later period of 1990s, water inflow percentage mentioned above of “Three Passages” decreases further. After Three Gorges Reservoiris constructed, this trendstill continues. For the low flow year of 2006 in particular,annual average water inflow of 2003~2006 from “Three Passages” is 23.3% of total water inflow into Dongting Lake and water inflow percentage of “Four Rivers” continues to increase.
When water inflow changes, silt inflow changes accordingly. Before Jingjiang river curve course is corrected, silt inflow from “Three Passages” and “FourRivers” is 85.6% and 12.8% of total silt inflow into DongtingLake respectively. After Jingjiang river curve course is corrected, silt inflow percentage mentioned above is 76.0% and 21.3% respectively. In the later period of 1990s, silt inflow from “Three Passages” increases lightly to 81.1% of total silt inflow into DongtingLake. After Three Gorges Reservoiris constructed, with discharge silt from Three Gorges Reservoir decreasing, silt inflow from “Three Passages” decreases, total silt inflow into DongtingLake decreases accordingly. In the period of 2003~2006, silt inflow from “Three Passages” is only 58% of total silt inflow into DongtingLake.
When total silt inflow into DongtingLake decreases annually, silt outflow from DongtingLake decreases little by little too. Before Jingjiang river curve course is corrected, for Chenglingji station which is the exit of DongtingLake, annual average silt outflow from DongtingLake is 59.6 million tons, 26.1% of total silt inflow into the lake. After Jingjiang river curve course is corrected, annual average silt outflow is 33.1 million tons, 25.1% of total silt inflow into the lake. In the later period of 1990s, the percentage mentioned above is 28%, hardly changes. After Three Gorges Reservoiris constructed(2003~2006), the percentage mentioned above increases to 60.4% rapidly, that indicates the construction of Three Gorges Reservoir is favorable to sediment deposition decreasing in Dongting Lake zone. Sediment deposition trend becomes slow, resulting from changed makeup of silt suspensioninto the lake.
3River-lake relationship prediction after Three Gorges Reservoiris built
One-dimensionunsteady flow river network mathematical model is created to compute and analyze, adoptingone dimension Saint-Venantequation setas basic equations set and quatre implicit difference scheme to discrete, adopting three classes method to solve, analyzing the model’s consistency, astringency andstability. In order to find out mathematical model simulation accuracy of flood process, water-silt movement and quantification, the model is calibrated by flood processes of 1996 and 1998. Mathematical modelverification computation includes river bed scouring-siltation of Jingjiang river section, long series of united river-lake water-silt movement from 1993 to 2006. The verification computation data indicates that simulation inaccuracy of water level and flow discharge is in the range of rules, simulation inaccuracy of long period river bed scouring-siltation is less than 30%, the mathematical model is qualified for later computation and analysis.
Computation conditions: Computation river section includes 1123km long river section between Yichang and Datong. Computation terrain adopt river sections between Yichang and Chenglingji of September,2002 and between Chenglingji and Datong of May,1992~ November, 1993, Dongting Lake zone of 1995(part section using 1:5000 topographic map), Poyang Lake zone of 1998. Computation time sequence is from 1961 to 1970.
Entrance water-silt condition: water-silt out of Three Gorges Reservoir after regulation and storage.
Exit control condition: long-time average annual relationship curved line of water level and flow discharge at Datong hydrological station.
During the water storage beginning period of Three Gorges Project, large quantity sediment deposition appears inreservoirzone, silt outflow decreases distinctly, with silt grain becoming smaller. In order to adapt changed water-silt condition and fulfill its own siltentrainment capacity, water flow needs to scour the original downstream river bed. So unsaturated water flow out of Three Gorges Reservoir begins to absorb silt from downstream near dam river section, resulting in intense scour. Scour will make pebble river bedor pebble-slit river bed coarsening to form its own anti-scour protection layer, resulting in intense scour moving down. For sandiness river bed, intense scour changes its section hydraulic characteristics, resulting in water depth increasing, flow velocity decreasing, water level going down and hydraulic slope decreasing, which can suppress scour effect, so scour begins to move down. In this case, scour in Yichang-Datong river section tends to develop from upstream to downstream. With scour-siltation change happening in main stream river bed of ChangjiangRiver, water-silt quantity into DongtingLake and PoyangLake changes accordingly, resulting in changed scour-siltation of lake zone, changed water surface profile of river course, changed water-slit diversion in “Three Passages” of Jingjiang and changed water-silt outflow from DongtingLake, so the original river-lake relationship begins to change too.
3.1Scour-siltation change of different river sections
After water storage of Three Gorges Project, scour happens in upstream river course and siltation happens in downstream river course. By 30 years after water storage of Three Gorges Project, scour tends to balance in the upper reach river section of Songzhikou. For the pebble-sand river section between Yichang and Songzhikou which is 75.7km long, river bed becomescoarsening quickly, forming a anti-scour protection layer to suppress scour development, resulting in scour completed in several years. By 30 years after water storage of Three Gorges Project, considering river bed width of 1000m, average scour depth in the river section mentioned above is 1.0m. The river section between Songzhikou and Ouchikou belongs to curved river course, with bank protection project constructed on its concave bank, the lowest elevation of scour pit is lower than that of pebble layer upper plate, river bed becomescoarsening due to pebble moving down or its exposure to surface. The river section between Songzhikou and Ouchikou is 147km long, considering river bed width of 1200m,its average scour depth is 2.3m by 30 years after water storage of Three Gorges Project. Lower Jingjiang river section(between Ouchikou and Chenglingji) belongs to wriggling river course, is 170.2km long. In this river section, its sand layer is very thick. After water storage of Three Gorges Project, great scour-siltation changes happen in this river section. By 10 years after water storage of Three Gorges Project, intense scour happens in river bed of this section. By 30 years after water storage of Three Gorges Project, considering river bed width of 1400m, average scour depth in this river section is 4.2m. The river section between Chenglingji and Wuhan is 230.0km long. After water storage of Three Gorges Project, siltation and scour happens in this river section in succession. Due to intense scour in Jingjiang river section and greater sediment concentration of water flow, light siltation happens in early water storage period of Three Gorges Project. With Three Gorges Reservoir operation time extending, intense scour develops to downstream, scour is the main trend of this river section. By 30 years after water storage of Three Gorges Project, considering river bed width of 1900m, average scour depth in this river section is 1.23m. The river section between Wuhan and Datong is 500km long, belongs to fork river course, which is about 623km away from Yichang city. In early water storage period of Three Gorges Project, due to intense scour in lower reach of the dam, water flow can absorb silt on its way, resulting in decreased silt transportation capacity, siltation happens in the lower reach river bed of Wuhan city. By 30 years after water storage of Three Gorges Project, total accumulated sediment siltationquantity is 686 million tons(about 5.04×108m3), siltationquantity of river fork is 686 million tons.
After Three Gorges Reservoir construction, with main stream river bed of ChangjiangRiver scoured, the entrance water level of “Three Passages” falling, water-silt diversion quantity decreasing, scour-siltation change happens in river bed of “Three Passages”. In the early operation stage of Three Gorges Reservoir, scour appears in all river bed of Yichang-Ouchikou river section, bigger grain sediment is rush away into “Three Passages” diversion courses, making their river bed scour suppressed. By 10 years afterThree Gorges Reservoir construction, accumulated scour-siltationquantity of Songzhikou, Taipingkou and Ouchikou is negative 30 million tons, 33 million tons and 55 million tons repectively(negative means scour). During 30 years after Three Gorges Reservoir construction, with increased sediment concentration of outflow water and increased sediment grain, scour and siltation happens in Songzhikou river bed in succession, scour and siltation happens in different zone of Taipingkou river bed, siltation in Ouchikou river bed tends to be slow and irreversible.
DongtingLake zone includes “Three Passages” diversion courses, western lake zone, eastern lake zone and southern lake zone. Western lake zone includes Ping lake, Qili lake, Songligong course and Songhuhong course etc. After Three Gorges Reservoir construction, water diversion quantity of “Three Passages” decreases, its silt diversion quantity decreases greatly, water into lake zone has smaller sediment concentration. After regulation, sediment siltation is still main trend, but siltation speed slows down.
In the early operation stage of Three Gorges Reservoir, due to decreased water-silt diversion of “Three Passages”, sediment siltation quantity of DongtingLake zone decreases accordingly. By 10 years after Three Gorges Reservoir construction, average annual sediment siltation quantity of lake zone is about 50% of its long-time average annual value during1956~2006. By 30 years after Three Gorges Reservoir construction, average annual sediment siltation quantity is 38% of its long-time average annual value during1956~2006. So Three Gorges Reservoir construction is favorable to flood control of Jingjiang river, slows down sediment siltation in DongtingLake, enhances its regulation function and extends its life.
3.2Water-silt diversion in “Three Passages” of Jingjiangriver
After Three Gorges Reservoir construction, main stream river bed of ChangjiangRiver is scoured, the entrance water level of “Three Passages” falls, water-silt diversion quantity decreases too. Compared with long-time average annual value of 1956~2006, average annual runoff amount of “Three Passages” during 1~10 years after Three Gorges Reservoir construction decreases from 88.4×109m3 to 59.4×109m3, which is a one third reduction.Average annual silt transportation quantity of “Three Passages” decreases from 120 million tons to 40 million tons, which is a two third reduction.Compared with long-time average annual value of 1956~2006, average annual runoff amount of “Three Passages” during 21~30 years after Three Gorges Reservoir construction decreases 50% to 48.4×109m3, average annual silt transportation quantity decreases about 25% to 23.4 million tons.
Due to river bed sediment siltation, water-silt diversion quantity of “Three Passages” in Jingjiang river tends to decrease. For Songzhikou entrance, due to coarseningprotection layer forming in river bed quickly, entrance water level falls lightly, which has little effect on its water-silt diversion capacity. During 30 years after Three Gorges Reservoir construction,compared with long-time average annual value, water diversion quantity of Songzhikou decreases only 9%~18%, silt diversion quantity decreases only 51%~58%. During10 years after Three Gorges Reservoir construction, compared with long-time average annual value, water diversion quantity of Taipingkou decreases 43%, silt diversion quantity decreases 63%. During 21~30 years after Three Gorges Reservoir construction, water diversion quantity of Taipingkou decreases 70%, silt diversion quantity decreases 82%, compared with its long-time average annual value. The biggest water-silt diversion quantity loss happens in Ouchikou. During10 years after Three Gorges Reservoir construction, compared with long-time average annual value, water diversion quantity of Ouchikou decreases 64%, silt diversion quantity decreases 80%. During 21~30 years after Three Gorges Reservoir construction, water diversion quantity of Taipingkou decreases 96%, silt diversion quantity decreases 98%, compared with its long-time average annual value, there is almost no water in river bed.
4Conclusion
Three Gorges Reservoir will result in great effect on river courses in upstream and downstream dam, make river bed regulating automatically. In order to predict river bedscour-siltation changes due to this effect, many kinds of water-silt mathematical model, such as one-dimensionunsteady flow river network mathematical model and plane two-dimension silt mathematical model, are developed and created. In this text, computation achievements ofone-dimensionunsteady flow river network mathematical model is introduced with emphasis.
One-dimension water-silt mathematical model is created to compute river course scour-siltation changes, analyze river-lake relationship, during the beginning period of Three Gorges Reservoir operation(June,2003~31th, December, 2032). The computation achievements indicate,in early stage of Three Gorges Reservoir operation, scour will happen from upstream to downstream for main streams in the middle and lower reaches of Changjiang River, the depth of river beds will increase, specially for the river section upstream Zhicheng county，river course scour will complete in a short time and extend through to its lower reach river course. The most serious scour happens in Lower Jingjiang river section, average river bed scour depth is 4m. Due to main stream river bed scour effect, river bed scour-siltation of “Three Passages”in Jingjiang river changes accordingly, siltation will be the main trend.Siltsiltation quantity will decreases in DongtingLake zone. By 30 years after Three Gorges Reservoir construction, average annual silt siltation quantity of lake zone is 38% of that of last 50 years(1956~2006), silt siltation speed slows down distinctly. After Three Gorges Reservoir begins to operate, water-silt diversion of “Three Passages” decreases. Compared with long-time average annual value of 1956~2006,by 21~30 years after Three Gorges Reservoir construction, average annual runoff of“Three Passages” decreases near 50%, its average annual slit transportation capacity decreases to about 25% of long-time average annual value.