Flood Risk Reduction in - Example of the Changjiang ()

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 " 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 Province

Nanjing

e Three Gorges tz g g Dam an

n e i z Y l gt a n i a

J Y Wuhan

ui sh Minjiang Li Chongqing Lake Lake Dongting Poyiang

Sichuan Province Province Yuanjiang Xiangjiang Zishui 200 km 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 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 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 (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. For flood protection the level of the lake is lowered to an altitude of 145 m during the summer flood season (June to September). During this period, about 84 % of the total sediment load and 61 % of total discharge flow into the lake. Thanks to the large bottom outlets a large part of the sediment load can pass the dam site. After the flood season, when the sediment concentration decreases, the level of the lake is allowed to rise to 175 m again, to enable navigation and power production. Investigations showed that sediments would not create too many problems during the first 30 years of operation. However, negative effects are to be expected in future decades during dry periods at low stages. Local excavation and additional flushing are envisaged as countermeasures, as in the case of other river dams.

Figure 5: Bottom outlet in operation. The colour of the water is a sign of high sediment load. (MFB-Geoconsulting / DigitalGlobe, 2003)

Environmental issues

The assessment of environmental issues has to consider fully the characteristics of water storage and of operation. The storage capacity of 39.3 billion cubic metres represents only 8.7 % of the average yearly discharge. This means that the total volume of water is exchanged within about one month, and during the summer the period is 5 to 7 days. Therefore, water management is seasonal and the possibilities for discharge control are limited. Given its length of 600 km and an average width of the water surface of only 1.1 km (as a consequence of topography) the situation resembles a river dam rather than a storage lake. Compared with current conditions, the water surface will be increased by 632 km2 to 1084 km2 at the maximum stage. This corresponds to about twice the area of Lake Constance. After the initial filling of the storage volume, the yearly and even monthly discharge will remain almost unchanged and will fluctuate within the current range between dry and wet years, except for the month of October (rise in lake level after the flood sea- son). It is envisaged that during dry periods the minimum discharge downstream of the dam site will increase from 3,000 to 5,000 m3/s. This will have positive effects on water quality and navigation. The main negative effects of the installation are on landscape and cultural heritage. Along the reach within the gorges the water level will rise by 40 to 110 metres. How- ever, the overall aspect will not fundamentally change with the depth of the gorges ranging from 800 to 1,100 metres. One also has to consider new access to lovely landscapes. As regards cultural assets, the damming of the river will affect 44 valu- able archaeological sites and cultural monuments. The most famous cultural assets in the affected area, for instance Fengdu Ghost City, the city of Baiicheng in Fengjie and the Huanggling Temple at Yichang will not be flooded. Those cultural monu- ments that are directly affected will be relocated if feasible and economically justified, as will be the case for the Baihheliang low water tablets and the Zhanfei Temple.

Relocation

As a consequence of the damming of the River Yangtze about 1.1 million people will be relocated and 28,000 hectares of agricultural land will be submerged to protect 30 million people and their livelihoods - including 1.6 million hectares of agricultural land - directly from flooding by this river. To minimize the negative effects of the dam - in contrast to similar projects around the globe - solutions were searched to foster de- velopment of the region and to increase significantly the living standard of the popu- lation. Of the 360,000 rural inhabitants about 60 % will continue to work in the agricul- tural sector, but for the others new jobs have to be created in the industrial and tertiary sectors. Compensation for the submerged areas requires about 45 % of the total costs for the Three Gorges Project. This illustrates how important compensation is and how seri- ously it is taken.

Figure 6: View of one of the new towns, built to accommodate relocated people. Final remarks

The requirements of sustainable development are fulfilled if present needs are met without reducing the opportunities of future generations. Sustainability encompasses the ecological dimension (protection and conservation of natural resources), the social dimension (comparable living conditions for all parts of the population and for all parts of the country; protection from natural hazards and health risks) and the economic dimension. In the past we mainly heard about the negative aspects of the Three Gorges Project. However, an in-depth debate about the multi-purpose project for the Yangtze shows that the various dimensions of sustainability have been well assessed and carefully balanced against each other. Switzerland is supporting the development of a flood forecasting system for the Yangtze River basin, in order to fulfil the function of the Three Gorges storage lake as a flood retention basin. This is to include the installation of remote sensing systems to monitor the necessary hydro-meteorological parameters and providing the Chinese partners with good education and training about these systems. The reactivation of numerous former natural flood retention areas such as Lake Dong- ting and Lake Poyang is an important supplement to the flood protection measures. The provincial and local governments, together with the WWF and with support from financial institutions, have established a comprehensive action plan to revitalize these areas. Several thousand square kilometres of lakes, moors and swamps are being re- linked with the river. The internationally accepted terms for integrated watershed ma- nagement are to be implemented according to this comprehensive action plan.

Andreas Götz Dipl. Ing .ETH Vice Director Federal Office for the Environment CH-3003 Bern www.bafu.admin.ch www.planat.ch

Berne, July 24 2006