Around the Regions - SOUTH-EAST ASIA

This FRIEND project thus provides a framework in our region with the active participation of nearly all Member States, within which research is carried out to improve the understanding of hydrological sciences and water resources management in the region. The research can take advantage of the multi-continental scale coverage of the member countries and their diverse water resources management experiences and will focus on providing solutions to individual as well as common issues relevant to countries in the region covering areas such as:

• Better modelling of hydrological processes and application of systems approach to provide improved regional hydrological design and water resources management to meet the urgent needs for water and its control

• The impact on catchment hydrology and water resources of natural and human induced changes in landuse and management practices

• The impact of climate variability on water resource availability and management.

• The importance and effects of different spatial and temporal scales on hydrological analyses to address regional issues

• The basic conditions to make the water related technology transfer and exchange possible

1998 Floods in the Yangtze River, China

During the summer of 1998 the Yangtze River experienced the second largest flood since 1954. The reasons for this are due to both natural causes and human activities. Heavy storms in the river basin added to the already high water level in the main stream and adjacent lakes increased both the flood crests and the volume. Cutting down of forests upstream and land reclamation from lakes accelerated flooding. The 1998 floods covered the whole basin area. The high crests and large volume of water over a protracted period in Shashi and Hankou weakened parts of The Yangtze embankment. Due to a succession of high water levels, more than 10 million people, including 178,000 soldiers, manned the dykes for 24 hour periods for about three months. 1,320 people died during these floods. Economic losses, including loss of property, were much higher than during previous floods as economic development has vastly increased over the past 40 years.

Introduction

The Yangtze is the largest river in China and the third largest river in the World. It originates in the Tanggula mountain on the Tibetan plateau and flows from the west to the east sea. The Yangtze is 6,397 km long and covers an area of 1.8 million km2, a total of 18.8 % of the total land mass of China. It has 437 tributaries, which each cover an area of approximately 1,000 km2. The catchment areas of the main tributaries, such as the Yalong Jiang, the Min Jiang, the Jialing Jiang and the Han Jiang cover more than 100,000 km2.

The upper Yichang which is about 4,500 km long is considered to be the upper stream. According to previous records, flooding in the middle and lower streams was mainly caused by the upper stream. 50% of the flooding at the Datong hydrological station (catchment area 1,705,383 km2), the last feasible monitoring point not influenced by tidal change, is brought about by heavy discharges from the upper stream during the flooding season (May - October). In the upper streams, each tributary contributes to the upper stream flooding in a different way. The Jingsha Jiang has a stable flow forming a basic discharge, while the Min Jiang and Jialing Jiang, and the interzones of the main stream between Pingshan and Cuntan, Cuntan and Yichang are one of the main sources of historical floods.

The middle stream of The Yangtze River starts at Yichang and extends to the mouth of the Boyang Lake (Fig. 1). It is 955 km long. There are several tributaries and lakes connected to the main stream. The Zhicheng and Chenglingji section is widely known as the Jin Jiang, which is a key part of the Yangtze flood control system. From Yichang to Luoshan, the Qing Jiang and four other rivers flowing from the Dongting lake system join the main stream. They are also important causes of main stream flooding. Regarding flooding at the Luoshan station, two thirds emanate from the upper Yichang and one third comes from the Yichang and Luoshan section. About 30% of the flooding at Yichang flows into the Dongting lake through four diversion gates. These waters join the Xiang, Zi, Yuan and Li rivers which are regulated through the Dongting lake and return to the main stream at Chenglingji. As a result flooding at Luoshan is characterised by higher crests, a longer duration and slow increasing and decreasing periods. From Luoshan to Hankou, the Hong lake and the Han Jiang (Han river) join the Yangtze. The Han Jiang covers an area of 9,521 km2 and has many small tributaries. The water stage increases very quickly after the storm occurs, forming big flood crests. The lower reaches of the Han Jiang has a very limited safety release capacity. It cannot release floods from the upper stream and is, therefore, subject to frequent flooding disasters.

Historically, flooding of the Yangtze river is caused by heavy storms. The spatial and timely distribution of summer monsoons and sub-tropical high pressure weather systems causes storms to form and move. Storms in the largest area of the Yangtze river basin mainly occur from April to October. However, storms in the tributaries may occur at different periods. The storm period for Xiang Jiang and Gan Jiang is from April to June. Storms occur in other parts of the basin, such as the Wu Jiang tributary and the Dongting and Boyang lakes from May to July, upstream of the Yangtze, and downstream of the Jinsha Jiang and Han Jiang tributaries from June to August. In the Jialing Jiang tributary, upstream of the Han Jiang tributary and the delta of the Yangtze storms occur between July and September.

Mean precipitation in the Yangtze river varies between 800-1000 mm. Generally, precipitation in the southern part is larger than in the northern part, while a decreasing trend from east to west is also very clear. 70-90% of the annual rainfall is concentrated between May and October. Annual variation of rainfall is very small. Cv(Correlation of variation parameter) varies from 0.15-0.25. The division of maximum and minimum rainfalls located is 2-3.7.

Due to the extensive area which the tributaries with a combination of large regulation lakes cover, large floods involving the whole basin do not occur frequently. A concentration of storms in certain places, however, provoke regional floods, for example, the floods of 1860, 1870, 1935 and 1981. If heavy rainfall is widely distributed, and flood runoff from tributaries combine and flow into the main stream, flood duration is increased and the flooding becomes more widespread sometimes even involving the whole basin. The floods of 1931, 1954 and 1998 are examples in kind. In any case due to the limited storage and release capacities of the main channel, the plain in the middle stream of the Yangtze is subject to flooding.

The main characteristics of the Yangtze flooding include: (1) Floods in the upper stream are mainly the result of storms. When the storms move from west to east, the flood waters from the tributaries join the main stream. The upstream floods were high crested and of short duration. (2) The two sources of the middle and downstream floods are the upper stream and the Dongting and Boyang lakes. Usually, the floods are high crested, the flood duration is long and the volume is huge. (3) When storms concentrate in tributaries, regional floods may occur in other related tributaries.

The 1998 Yangtze Floods

In 1998, between 12 and 27 June, three heavy storms took place in the Yangtze river basin. Storms in the south part of the Yangtze were very frequent. Rainfall intensity in Jiangxi, Hunan and Anhui provinces was twice that of a normal year for the same period. A second series of storms of high intensity took place between 4 July and 25 July. Rainfall in the Three Gorges section, north and middle of the Jiangxi province, northwest of Hubei and other riparian areas was 50% to twice that of normal years. A third series of storms took place between the end of July and the end of August. Rainfall intensity in the upper reaches and the greater part of the middle stream was more than two to three times that of normal years.

Due to the high intensity of the storms which took place from June to August, the 1998 floods were the second largest for the Yangtze basin since 1954. Flooding took place in three phases. The first was from mid June to early July when flooding in the Dongting and Boyang lakes occurred. The storage capacity of the Boyang lake was almost completely filled. The water levels mid and downstream of the Yangtze rose quickly forming opposing trends to the flow from the upper stream. The water levels of the Jianli and Jiujiang stations exceeded previous records. The second phase took place during mid and late July. Storms and floods concentrated in the Dongting and Boyang lakes and the riparian banks, especially the two tributaries of the Dongting Lake. The Dongting Lake was filled to capacity. The water level of the lake exceeded any previous records. The water level both mid and downstream increased vastly. Water from upstream was unable to flow easily downstream. Interaction between upstream, mid and downstream caused further flooding in the Yangtze. The third phase covered end July to end August. The water level mid and downstream did not subside and six crests from upstream descended thereby increasing even further the water level mid and downstream. Water levels from Shashi to Luoshan, Wuxue to Jiujiang, Dongting and Boyang lakes exceeded previous records once again. Crest time can be clearly identified in Fig. 2.

Four points need to be emphasised for the 98 floods. (1) The floods covered the whole basin. Except in the upper stream, flooding took place several times and new water level records were attained in the mid and downstream reaches of the Yangtze. These are the most serious floods since 1954. (2) Flood waters from the main stream and its tributaries met mid and downstream and new crests appeared. When flooding occurred in the Dongting and Boyang lakes, the high water level remained stable for a considerably long period. (3) The water level remained high for a long time. From Yichang to the river mouth, the water level at each control point exceeded the warning level. The water levels at Shashi to Luoshan, Wuxue to Jiujiang (360km) exceeded anything previously recorded. The water level at Shashi reached 45.22m, 0.22 m above the flood diversion control level. (4) The volume of the flood waters approached or exceeded any previously recorded. Compared to the 1954 floods, the volume of flood water in 1998 (July-August 1998) was larger than it was in 1994 for the Yichang station. The maximum volume of the flood water for 30 days and 60 days was equal to or larger than those of 1954 (Table 1-Table 3). Probability is about one in 80-100 years. The volume of the flood waters in the middle stream in 1998 was slightly less than in 1954. (5) A series of crests appeared. A total of 8 crests was recorded. The interval between the first and second crests was relatively long, others lasted no more than a week. In early and mid August, there were three crests in 10 days. Due to a succession of crests, the water released in mid and downstream did not flow easily. Moreover, the tide level was high downstream. Due to both flood crests and the high tide, the water level in the Yangtze river was high over a long period.

(Units: Hmax: m, Qmax: m3/s, 30 and 60 day volume (D V.): 100Mm3)

Table 2. The Comparison of flood characteristics for 1954 and 1998

Note: The runoff for 1954 was not recalculated after flood diversion

Table 3. The Comparison of monthly runoff in 1954 and 1998 in 100 Mm3

In the summer of 1998 floods reached catastrophic levels and 1320 lives were lost. Direct economic loss in the Yangtze River basin was more than 166.6 billion-yuan (Table 4). The People's Liberation Army and the armed police forces dispatched some 178,000 soldiers to fight the floods and to participate in the rescue operation. This is the largest military deployment in the Yangtze river basin since 1949, and the soldiers played a decisive role in protecting people's lives and properties.

Table 4. Comparison of flood disasters of The Yangtze

Reasons for the 98 Floods in the Yangtze

There are many reasons for the 98 floods in the Yangtze, the main one being abnormal climatic conditions and related heavy storms, influenced by El Nino. Due to the impact of the southeast monsoon subtropical humid air and cold air from the north west met and stabilised above the Yangtze. The interaction between the two weather systems created a series of heavy rainfall. When the plum-rainfall over the Yangtze river stopped in June, the subtropical high-pressure system moved north. However, La Nina appeared, almost coincidentally, weakening the subtropical high-pressure system and pushing it back to the Yangtze River basin. Precipitation for a period of three months for parts of Jiangxi, Hunan and Hubei, was greater than that of normal years. Maximum rainfall measurements for one day, three successive days and three months (1 June to 1 September) at the Longshan station in Hunan province were 339mm, 655mm and 2067 mm respectively. On the other hand, tropical winds, storms and typhoons were also very abnormal. Usually, there are frequent tropical winds, storms and typhoons impacted over the Yangtze river basin. But this occurred only once in July 1998. The second occurrence did not take place until August and even later with the result that subtropical humid air could not move to the north as usual.

From the middle of June, rainfall intensity was high in most parts of China. Water levels in rivers and lakes, therefore, rose very quickly. In the Yangtze River basin, in the provinces of Jiangxi and Hunan rainfall was double that of normal years. According to statistical analysis, rainfall measurements at some stations from June-August 1998 were equal to or larger than the annual rainfall in a normal year. Higher previous water levels in the main stream and lakes is another reason for the 98 floods in the Yangtze.

A combination of flood waters from the main stream and its tributaries contributed to the 1998 floods. In the first phase, the Boyang and Dongting lakes were filled, causing the water level mid and downstream to increase. Flood waters reached levels superior to those recorded to date when the upstream runoff joined the main stream. The 6th crest was the worst as it attained its highest water level in the part of the river between Shashi and Hankou in 1998.

Apart from natural causes, the impact of human activity must also be taken into account as construction work increases sedimentation in the lakes reducing their storage capacity. For example, about 100 million tons of mud and sand has been deposited in the Dongting Lake reducing the surface area from 4350 km2 in 1954 to 2625 km2 in 1998, and reducing the flood storage capacity from 29.3 billion m3 to 17.4 billion 3. In the 1950s, the total surface area of the lakes connected to the Yangtze was about 17,200 km2, but by the 1980s it was only 6,600 km2. The storage capacity of the Dongting, Boyang lakes and other lakes in the Hanjiang basin has decreased by 30 billion m3.

As there have not been any large floods during the past 40 years, economic growth in the flood plain areas along the Yangtze has been rapid. 500,000 people are now living in these areas. However, insufficient flood prevention measures have been taken which means that during periods of intense flooding diversion of the flood waters is inadequate. Flood diversion, both planned and due to broken dikes was 102.3 billion m3 in 1954, however, flood diversion this year was only 10 billion m3. Flood waters collected mainly in the river channels.

The treatment of small rivers, increasing flood-control standards, and the cutting down of forests in the upper stream accelerated the flow from the tributaries to the main stream. In 1957, forest coverage upstream of the Yangtze was 22%. Water and soil erosion accounted for 20.2% of the total river basin area. But by1986, forest coverage was reduced by 50% (the total forest coverage was only 10% ), and water and soil erosion had increased accordingly. The soil erosion volume reached 2.24 billion tons.

The filling up of the riverbed, reclamation of low land near the banks and illegal construction along the river banks reduced the section across the river thereby decreasing flow capacity from up to downstream. The flood control system, including structural and non-structural measures, has not been completed. When above average floods occur serious flooding is inevitable.

Recommendations

It is essential to update flood control and awareness systems. Awareness of flood control measures has remained low in relation to the high-density of the population, fertilised, cultivated irrigation land and industrial outputs. Flood control technologies lag far behind economic growth. Flood control planning and further anti-flood strategies should include structural and non-structural measures. Storage, levees, excavation and diversion are integrated measures to counteract floods using structured measures. Early flood awareness systems are essential as they enable inhabitants to evacuate the area before the floods reach emergency proportions. There should be an increase in investment in water resources and flood control projects. Investment in water systems is, on the contrary, decreasing. Between 1991 and 1997, the State invested 115 billion yuan in water conservation, but during that period a great deal more funds were spent on other infrastructures, to the amount of 1400 billion yuan on energy, 880 billion yuan on communications, 475 billion yuan on mail and tele-communications and 440 billion yuan on urban construction. Protection and development should be balanced, otherwise disaster will influence economic growth.

The ecosystems of the upper stream should also be protected. Human activities, also contribute to flood disasters. As it is impossible to fully control the natural phenomena and related disasters, human activity should be reduced. Forest protection, water and soil conservation, river and lake reclamation are key measures to improve the natural ecosystem. Sustainable concepts should become one of the basic principles of future social/economic development.