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Flood Risk Reduction in China: Example of the Changjiang (Yangtze)

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Flood Risk Reduction in China: Example of the Changjiang (Yangtze) Flood Risk Reduction in China - Example of the Changjiang (Yangtze) Andreas Götz Water means life. However, an abundance of water - indispensable for society and the economy - may easily change from a blessing to a curse. Too much water can destroy the fundamentals on which life is based and, in the worst cases, it may even be a threat to life. The main objective of the multi-purpose water management project for the middle reaches of the River Yangtze is to reduce the risks that floods present for life and property in the area downstream to a tolerable level. In addition to flood protection, the project is designed to produce energy, to improve the waterway for shipping purposes, and to increase discharge in the downstream reaches during peri- ods of drought. Much has been said about the negative implications of the "Three Gorges Project". However, a closer look at the dam and its engineering works re- veals that the various dimensions of sustainability were carefully assessed and bal- anced against each other. Hanjiang Hubei Province Nanjing e Three Gorges tz Shanghai g g Dam an n e i z Yichang Y l gt a n i a J Y Wuhan ui sh Minjiang Li Chongqing Lake Lake Dongting Poyiang Sichuan Province Hunan Province Yuanjiang Xiangjiang 200 km Zishui Figure 1: Map of the River Yangtze The Yangtze ("Long River") is the longest and most important river in China, with a total length of about 6,300 km. So it is the third longest river in the world, after the Amazon and the Nile. The Yangtze rises in the Eastern Highlands of Tibet and flows across China, separating the northern and southern parts of the country. It flows into the East China Sea near Shanghai, a megalopolis with 16 million inhabitants. The watershed of the Yangtze covers about one fifth of the area of China. The largest agricultural and industrial areas are located along the river and nearly one third of the Chinese population lives within this catchment basin. The navigable reach of the Yangtze main stem is about 2,800 km long and has been the most important trade route in China for more than 2,000 years. The city of Chongqing (15 million inhabitants) is located on the upper navigable reaches of the Yangtze. This city is the political and economic centre of Southwest China, and is one of the most important cities in China. A 200 km section down- stream of the city is one of the most scenic reaches of the river, which winds through deep gorges, between high mountain peaks, often with strong currents and danger- ous shallows. The city of Yichang (about 350,000 inhabitants) is located at the exit from the last of the three major gorges. Four hundred kilometres (straight distance) or 600 kilometres (river channel distance) downstream is the city of Wuhan (9 million inhabitants), which is the capital of Hubei Province. This communications junction is not only an important cultural and political centre, but also a lively centre of trade and industry, thanks to rich deposits of iron ore nearby. The progressive increase in the density of settlements and the steady rise in the value of property have increased the risk of flooding along the Yangtze. About 100 million people now live within an area of 140,000 km2 (about three times the area of Switzerland) along the middle and lower reaches of the Yangtze. Reducing flood risk to a tolerable level presents a big challenge because the protection of life and prop- erty are essential for sustainable development. In the past, flooding along the Yangtze has caused enormous damage. During the worst floods - in 1870, 1931, 1935, 1954, 1981, 1998, and 2002 - discharges were 66,000 to 110,000 m3/s. Depending on the location of the disaster, 0.5 to 4 million hectares of agricultural land (equivalent to the area of Switzerland) were flooded. The total death toll from these floods reached 550,000. The most recent flooding, in 2002, destroyed about 50,000 houses, and the economic damage was huge. Figure 2: Newly established riparian flood zones in Wuhan city following the 1998 flood event. Note: flood marks on a building. Even during small events, which statistically recur about every 10 years, the dikes along the river may fail and there may be catastrophic consequences. For many years it has been obvious that merely strengthening the existing dikes cannot safely discharge the extreme flood peaks. Countermeasures also have to be taken, such as flood retention in the upper reaches and the creation of designated flood areas. As early as 1921 it was proposed that flood peaks on the Yangtze be temporarily re- tained in the area of the Three Gorges, upstream of Yichang City. A retaining dam at the exit of the lowest gorge could protect millions of people from recurrent flooding and could mitigate the effects of extreme events. The planning period and subsequent discussions of the effects of such a structure lasted for about seventy years. The environmental effects of the construction and operation of the dam have been assessed since 1950. The investigations focussed on questions about flood protection, energy production, navigation, and relocation due to flooding in the water storage area. Aspects covering cultural heritage, changes to the landscape, effects on flora and fauna and sediment transport have been assessed in detail. Countless mathematical and physical models have been developed for the quantitative and qualitative investigations. In addition, the expertise of specialists from around the world and experience with similar projects have been considered. Dam site and technical structures The area of the watershed at the dam site is approximately 1,000,000 km2 (twice the size of Spain). The average annual discharge amounts to 451 billion m3 of water, and the sediment load is 526 million tons (for comparison: annual discharge of the Rhine at Basel: 33 billion m3, and at Lobith near the Dutch border: 70 billion m3). The technical structures of the so-called "Three Gorges Project" comprise one retain- ing dam, two hydroelectric power plants, a lift for boats and a multilevel system of locks for navigation. The design of the installations followed the results of compre- hensive investigations over several years. Although the earthquake intensity at the dam site is considered to be low, the structures are designed to withstand an earth- quake of magnitude 7 on the Richter scale. The dam is designed as a concrete gravity dam with a total length of 2,309 m. The crest level is at an altitude of 185 m, the maxium dam height is 181m. The spillway has a length of 483 m, a total of 23 bottom outlets (7x9 m, at an altitude of 90 m), and 22 overflow gates (8 m wide). The spillway has a capacity of 106,000 m3/s, which covers the probable maximum flood (PMF). The surface of the tailwater is at an altitude of 62 m. This provides a maximum visible dam height of 123 m (Grande Dixence 285 m). In spite of its huge volume of concrete (115 million m3) the Three Gorges Dam is not one of the top ten installations in the world with regard to its length, height or storage capacity. Figure 3: Overview of all installations During the summer months the level of the lake is lowered to an altitude of 145 m to create a storm water storage of 22.15 billion m3 (for comparison: the total volume of all Swiss dams is 3.4 billion m3). With this flood retention volume flood peaks are re- duced, so a 10-year flood will occur only every 100 years, considerably improving flood protection. Even during a 1,000-year flood in the upper reaches the effects in the downstream area will be mitigated through better forecasting, targeted flooding and the management of retention areas. Figure 4: Satellite image of the dam site. (MFB-Geoconsulting / DigitalGlobe, 2003) A system of locks allows commercial and passenger vessels up to 10,000 tons to pass the dam site. The 5-level sets of locks, one each for the upstream and downstream passage, have lock chambers 280 m in length and 34 m in width, with 5 m depth of water. In addition to the systems of locks, a vertical lift for vessels up to 3,000 tons can be used. It has a lock chamber 120 m in length and 22 m in width, with 3.5 m depth of water. Thanks to the dam and the lake behind, it is now possible to maintain two-way navigation in the previously dangerous gorge reach between Yichang and Chongqing. So environmentally-friendly water transport has been increased from 10 to 50 million tons per year, resulting in a reduction of costs of about 35 %. The problem of sediments Sediment transport in the river was one of the major problems that had to be ad- dressed. These aspects have been investigated in detail by Chinese and foreign ex- perts, since in 1950. According to the long-term analyses the river transports a total of 526 million tons of suspended sediment per year. The yearly bed load transport amounts to 8.6 million tons. Considering a total discharge of 451 billion m3, the aver- age suspended sediment concentration is about 1.2 kg/m3. The equivalent value for the Yellow River (Hwangho) is 37 kg/m3, which is very different from the Yangtze. Sediment concentrations in Swiss rivers are much lower: Alpenrhein (upstream from Lake Constance): 0.4 kg/m3, River Rhone (upstream from Lake Geneva): 0.3 kg/m3.
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