On '98 Extraordinary Floods in Yangtze River Valley of China

 

ZHIYONG DONG

 

Hydraulic Engineering Department of Nanjing Hydraulic Research Institute,

Guangzhou Road No.225, Nanjing 210029, Jiangsu Province, P. R. China

E-mail: zydong@njhri.edu.cn

 

 

ABSTRACT

As is known to all, the extreme floods occurred in Yangtze River valley of China in 1998. On the basis of oceanography and meteorology the reasons for abnormal climate are analyzed in this paper. Flood regime in Yangtze River valley, along with disaster situation in the middle and lower areas is presented. In addition, this paper deals with the reasons for a lasting high stage in Yangtze trunk river, effects of deterioration of ecological resources caused by man in this area, as well as the role of Three Gorges Project in flood control. Finally, the measures of comprehensive control are suggested.

 

Keywords: devastative flooding, Yangtze River, reasons for the floods

 

INTRODUCTION

From ancient times, the riversides are the main places where the mankind lives and multiplies. Effects of rivers on human activities are extremely profound and lasting. On the one hand, people maintain their life and production depending on rivers and lakes, on the other hand, people suffer from flood disasters. Flood is a natural phenomenon, it is difficult to completely control floods. A design standard for flood control engineering can easily be exceeded by the next flood. The utmost that one can do is to take measures to minimize the disaster losses due to floods. Even if we have flood control experience for many years, highly advanced engineering technique, advanced communication facilities and monitoring means today, floods continue to wantonly inundate and cause enormous losses in many places of the world. Therefore, flooding disaster is still one of the major natural calamities perplexing human life.

In the past four decades, the comprehensive improvements in flood control for seven major rivers in China have successively been carried out, including construction of 240,000 km long levees, 25,000 sluices, over 80,000 reservoirs, setting up 86 detention basins, and dredging 100,000 km long channels [3]. Through these engineering and non-engineering measures mentioned above, channel capacity for flood control in China was significantly enhanced, so that middle and small scales floods can be effectively controlled, and hazards due to devastating floods decreased.

The Yangtze River is the longest one in China as shown in Figure 1. It occupies the third place in the world. It is over 6300 km long, has a 1.8 million kmcatchment area and over 9.710mmean annual runoff. In Yangtze River valley, the climate is temperate and humid, precipitation is plentiful, terrain exhibits high in the west and low in the east. From the river source to the estuary, the height difference is about 5400 m . In the middle and lower areas of Yangtze River, industry and agriculture are highly developed, there are a number of key cities such as Wuhan, Nanjing and Shanghai, and also some key railway trunks such as Beijing to Guangzhou, Beijing to Shanghai and Beijing to Hong Kong. The areas are mainly protected by levees of 30,000 km length , located along 3600 km of the Yangtze river. The most dangerous reach for flood control in Yangtze River is Jingjiang Levee, which is 180 km long, meanly 12 m high and up to 16 m high in some places, protecting the vast an abundant Jianghan Plain. Once the levee breaks, the lives and properties of local residents will suffer enormous losses and threaten the security of Wuhan City.

 

Fig.1 Sketch of Yangtze River Valley

 

As known to all, the third whole valley-wide extraordinary flood of Yangtze River of this center occurred in 1998 (the first two is 1931 and 1954, respectively), millions of people were affected. These three historical floods had the following characterizes (1) The floods covered the whole valley. In addition the flood exceeded in the upper, in the middle reaches, f.i., water levels of river systems for Poyang Lake the highest records. Floods of river systems for Dongting Lake occurred for several times. High floods also occurred in Yangtze tributaries such as Hanjiang and Qingjiang Rivers. (2) Interaction of trunk and branch floods, and superposition of flood peaks. Under the condition of the earlier flood occurrence in the two lakes with lasting high water level, repeated flood occurrences in the upper reach increasingly interact with ones in the middle and lower reaches. (3) High stage. The stages in the whole middle and lower reaches exceeded the warning lines. Water levels of reaches between Shashi and Luoshan, Wuxue and Jiujiang, as well as in Dongting and Poyang Lakes exceeded the historically highest ones. Especially the water level in Shashi reach was up to 45.22 m in far excess of flood diversion. (4) Successive flood peak occurrences and a long high stage duration. Successive 8 flood peaks occurred in the upper reaches. Especially from early- August to mid-August, the successive 3 flood peaks occurred in a ten-days time. The water level in the middle reach had exceeded the warning stage two months, and exceeded the historically highest one for over a month. Because floods in 1998 were of a mighty order, covered a vast area and had a long duration, the disaster was rather serious. However, the losses due to the floods were much less than that in 1931 and 1954(see Table 1).

 

 

Table1 Comparison of Disaster Situations for Three Extraordinary This Century

 

Notes:(1) Beijing to Wuhan held up a long-term, Tianjin to Nanjing held up for 54 days; (2) Beijing to Guangzhou held up for 100 days; (3) closure in a couple of days.

 

INFLUENCES OF EL NINO AND LANNINA

Interaction of atmosphere with ocean would directly result in change in the global climate. If the ocean surface temperature rise due to the interaction in equatorial areas is higher 0.5 than the mean annual value for a six-month, we can refer it to as El Nino phenomena or events; On the contrary, if the ocean surface temperature is lower 0.5than the mean annual value for a six-month, then we can refer it to as Lannina phenomena.

The greatest temperature rise event of a vast surface seawater in the middle and east tropic Pacific areas took place in 1997 since a century record history, that is El Nino. Oceanographers and meteorologists in the world are very much shocked at the El Nino that is rapid and intense. Because specific heat of water is as about four times as that of air, and water density as over 800 times as air one, heat released by slight change in seawater temperature (for example, 0.5) can make atmospheric circulation intense change so that result in abnormal climate. That event caused in some places in the world serious drought, and the other ones serious flood and waterlogging.

The end of 1997, the El Nino temperature rise phenomena tended to steady state. Oceanographers and meteorologists guess the event ended in May or June of 1998, so that the global climate will return to normal. However, it was unforeseen that the occurrence of Lannina made oceanographers and meteorologists more astonishal than the warm water event of El Nino in 1997. Though 1997's El Nino event created the highest historic value, including a vast seawater surface temperature in excess of 5 than the normal value and even part of areas in excess of about 7, such a high temperature attained through a eight-month from April of 1997, far inferior to Lannina's swift and violent.

As a result of the tropic ocean cooling, atmospheric heat obtained from ocean decreases as compared with normal situation. Under the condition, the behaviour of atmospheric circulation will deviate from normal state and result in abnormal climate. However, the abnormal climate due to Lanina event is not quite opposite to El Nino one, which has its intrinsic law. As far East-Asia, intensity and latitude of subtropic high inclined to strong and high during El Nino, respectively, whereas during Lanina to weak and low, respectively. At that time effects of El Nino have not yet completely vanished due to time delay, in the meantime effects of Lannina have already started, which make China's climate extremely complicated.

In 1998 such a long-term heavy precipitation occurs in Yangtze River valley, the "culprit" was subtropic high. The late-July in a normal year, the middle and lower areas of Yangtze River should be under subtropic high, in a hot season drought, and in the weather of air temperature high and less rain, rainfall belt should move to north. However, under the combination of El Nino and Lanina events, subtropic high move to south-east, so that make the middle and lower areas being the leading edge of subtropic high, in which warm and wet currents meet with northern cold air, thus resulting necessarily in rain.

 

REASONS FOR A LASTING HIGH STAGE IN YANGTZE TRUNK RIVER

As a matter of fact, flood disaster in 1998 preliminarily gave an inkling in early 1997's winter. The winter flood in December of 1997, and later spring flood in March of 1998(a month ahead of time) made water level of Yangtze River widespreadly higher.

First of all, a longer lasting rainstorm between mid-June and late-June, the areas in Dongting and Poyang Lakes situated in the middle and lower of Yangtze River exhibit rainstorms in succession, in addition, increasingly received water from tributary rivers, so that an 800 km long reach exceeded warning stage. At this moment, the first flood peak of Yangtze River had not occurred yet. In addition, over a month later, the eastern in Sichuan Province and areas of Three Gorges situated in the upper of Yangtze River successively exhibit rainstorms. In the meantime, several hard rain in the whole drainage of Yangtze River occurred. In this way, a vast amount of floods torrentially surged into Yangtze River.

Secondly, occurred oncoming flood peaks with tremendous force and a short interval between the peaks. Eight flood peaks successively took place in the upper reach of Yangtze River (see Table 2), only between mid-August and late-August, three flood peaks occurred in a ten-day. After the first two flood peaks, the middle and lower reaches beyond Yichang in Hubei Province exceeded warning stage, the middle reach in excess of warning stage lasted two months. Under the condition of oncoming floods successively in the upper and discharge slowly in the lower reach, water levels in the middle rise steeply.

Thirdly, the poor soil and water conservation in the upper reach resulted in the flood disaster in the middle and lower areas. Because the vegetation in Minjiang and Jialingjiang catchments of the upper Yangtze River have been seriously destroyed, along with the weaken ability for forests to receive rain, a vast runoff directly converged into rivers. Soil and water loss resulted in sedimentation in channels and lakes, which made flood control in the middle and lower reaches of Yangtze River more serious.

 

first peak	Second Peak	third peak	fourth peak	fifth peak	sixth peak	seventh peak	eighth peak
July 2	July 18	July 24	Aug. 7	Aug.12	Aug.16	Aug. 25	Sept. 2

 

Table 2 Date of Flood Peak at Yichang Station in 1998 [4]

 

Fourthly, conveying and storage areas of floods are largely decreasing. Area of eight lakes subjected to flood storage in Yangtze River valley decreased by 33% compared to the early 1950's, which reached over 5500 km. As far as Dongting Lake is concerned [2], between 1951 and 1957 mean annual 45% of sediment discharge in the middle areas of Yangtze River was diverged into the lake, which had made lake bed level aggradation rapidly. The fertile flood plain of aggradation was promptly embanked for polder. By rough estimate, area of Dongting Lake was about 7000 kmone hundred years ago, on the basis of measured data in 1936, at that time the area was only left over 4700 km, and up to 1947 only 3100 km. In a short eleven-year, the area of lake reduced by 1600 km, at that period of time, however, the cultivated area of lake plain increased by about 167 thousand ha. Thus it can be seen that natural sediment aggradation and man-made embanked polder make natural volumes of flood storage of Dongting Lake sharply decreased, so that greatly reduced effectiveness of diversion and storage of Yangtze floods than before. Dykes of some tributaries of Yangtze River were heightened and Reinforced, as well as capacity for flood storage sharply reduced, which made water impounded in the trunk dykes, in addition, tidal lockup in the lower, thus making water level of trunk river high.

Fifthly, flood recession developed slowly in the middle and lower of Yangtze River. The slow flood recession was directly related to low head between middle and lower reaches of Yangtze River. Yichang in Hubei Province is the demarcation between upper and lower reaches of Yangtze River, and 1800 km from the estuary, however, the fall is only over 50 metres. In addition, spring of lunar calendar make water of lower Yangtze River locked up. In this case, the channel of storage flood in the lower and middle Yangtze River receded slowly.

 

ECOLOGICAL RESOURCES SUFFERING DESTRUCTION

After Yangtze River entering main flood period, rain in the upper, middle and lower occurred simultaneously, and floods in the whole Yangtze River took place simultaneously, which made flood amount sharply increase in excess of the amount in earlier years, which was the principal reason for the flood disaster. However, in contrast to the past hydrological records we can find that flood discharge in 1998 was quite large, but not the maximum. In Yichang, for example, the maximum discharge of the first three flood peaks passing through in 1998 was 56400. However, according to the hydrological records the maximum peak discharge was in the order of 60000 in the last 23 years. That was not the maximum discharge of floods, but created repeatedly new stage records, which was directly related to man-made destruction of ecological environment in Yangtze River valley for a long time past, characterized by three aspects: First, destruction of vegetative cover and wetland in Yangtze valley; Second, non-rational use of channels and lakes; Third, land surface hardening making runoff coefficient increase.

 

REFER TO VEGETATION AS GREEN RESERVOIR

It is investigated that a water storage capacity of 1000 ha forests is analogous to water storage of a million mreservoir. Yangtze River valley was always the area where forest cover rate was larger in the past. However, of late several decades, forests suffered a vast cutting, especially at drainage of Jinshajiang, Minjiang Rivers. Though at present the areas of artificial forest, middle and young growths, to some extent, increase, the mature and natural forests of ecological effects on conserving water, soil conservation, and creature variety protection etc. have still suffered from cutting, and residual natural woods are also under degeneration.

Effects of vegetation destroyed directly bring about two harms: first, it reduces capacity for conserving water so that precipitation immediately is turned into surface runoff, which increase pressure of flood into conveying channels; second, it results in serious soil and water losses. The statistical data show that the area of soil and water losses in Yangtze River valley increased by 56.6% over the past 30 years, a vast amount of sediment is discharged into the river, which degragated channel bed level and aggravated siltation of channels and lakes. In this case, a higher water level will easily attain in spite of a not too big discharge, which endanger security of the people' lives and properties along riversides.

Decrease in amount and area of wetlands were also the main aspects of ecological resources subjected to destruction in Yangtze River valley. Wetland is an important part in storage and regulation of floods, regulating climate, soil and water conservation, purifying water quality and creature variety protection. It's a pity that with economic development and sharp increase of population the wetland resources in Yangtze valley increasingly reduce, which largely lower capacity for storage and conservation of water and flood regulation.

 

THE NON-RATIONAL USE OF CHANNELS AND LAKES

It is in the middle and lower areas where freshwater lakes concentratedly distribute the water. Numerous lakes became the natural regulating regions of Yangtze water. However, blind polders shrink lakes and aggravate ecology. In 1950's the number of lakes in Hanjiang River areas were 1056, up to in the early 1990's they reduced to 182. According to statistics, in recent several decades the area of lake loss in lower and middle Yangtze River was about twelve million ha, the loss rate being 34%, so that water storage is less and less and capacity for flood regulation is lower and lower. In addition, in the areas of middle and lower Yangtze River, phenomena of artificial occupying channel for reclamation, building housing on channels, industrial and mineral enterprises arbitrarily discharging tailings, slags etc. into rivers are very serious. All of these seriously affected the capacity for flood retention.

 

URBANIZING DEVELOPMENT

A considerable change in the state of urban surface occurred, a great part of areas from originally pervious surface have been turned into an impervious one so that due to the increase of runoff coefficient and decrease area of water storage, increase of runoff occurred.

1998's flood disaster told us the lesson that protecting ecological environment to realize sustainable development is an important aspect which may not be ignored at social development. In the present time of developing economics ecological effects must be paid germane attention, if not, man will meet with the merciless retaliation of nature!

 

THE ROLE OF THREE GORGES PROJECT IN FLOOD CONTROL

Flood and waterlogging disaster in lower and middle Yangtze River, to a certain extent, are attributed to high peaks and discharges of floods in the upper reach, exceeding the flood conveying capacity of the river. Safety discharge in the middle and lower reaches is 60,000 to 70,000 , and 80,000, respectively. However, as far as the floods in 1931 and 1954are measured, flood discharge from stream up at Yichang plus the one from Dongting Lake exceeded a million. Flood volume, in 1954, for example, was 102 thousand billion . 70 thousand billion must be retained by reservoirs or diverted to secure dikes. At present, regions of flood diversion and plain lakes only accommodate 50 thousand billionof flood volume. Therefore, the other 20 thousand billionvolume need to be retained by reservoirs[1]. It should be noted that, there is a six-million ha cultivated land in flood diversion areas, some of the areas are populated densely, so that flood diversion will result in heavy losses. For Three Gorges Project under construction, the total reservoir capacity is over 40 thousand billion, the flood control capacity is over 20 thousand billion. In view of the favourable geographical position of Three Gorges reservoir, it can control a catchment area of 100and all five major tributaries. For this reason, it will play an important role in flood control for the middle and lower reaches.

 

TO STRENGTHEN COMPREHENSIVE IMPROVEMENT

The floods in Yangtze River in 1998 have taught a bitter lesson. Overall, the ability to fight against flood and waterlogging in China has not attained a high level. A lot of problems exist in hydraulic engineering and comprehensive control methods. They mainly concern: (1) The low standard, ageing, poor maintenance of flood control works at a number of dangerous sections of the river; (2) With increase of population the phenomena of competing with interests concerning flood control and land reclamation appear to become increasingly serious. An amount of flood plains of lakes and channels have been embanked for polder lowering the capacity for both storage and regulation of flood; (3) The phenomena of reclamation of wasteland and deforestation is evident and makes soil and water losses increasing and aggradation of lakes and rivers serious.

To control flood disasters in Yangtze River needs to be tackled in a comprehensive way including engineering and structural measures, and social science. It can mainly consists of heightening and reinforcing levee systems, rational planning of diversions and retarding regions of flood, building reservoirs in trunk and tributary streams, soil and water conservation, training and dredging of channel, as well as flood control according to law etc. In addition, the advanced science and technology should be adopted in both construction of flood control engineering, and aspects of emergency measures to strengthen existing dikes Relations between economic development and environment protection should be scientifically coordinated.

 

REFERENCES

[1]    Yan, Kai: Three Gorges Projecta comprehensive use engineering of rational planning water resources, Information of Science and Technology of Hohai University, No.1, 1990(in Chinese)

[2]    Shi, Jiayang: A comprehensive use of water resources, China Waterpower Press, 1996(in Chinese)

[3]    Yao, Leren: Flood control engineering, China Waterpower Press, 1997(in Chinese)

[4]    Bo, Er: Floodssketch of Chinese soldiers and civilians fighting floods in 1998, Chinese Industrial and Commercial Union Press, 1998(in Chinese)