Dams and Landslides

Flood types on the Yangtze River

Chen Guojie

September 12, 2002

The big dam will be ‘totally useless’ in the face of the most common type of Yangtze flood, a senior Chinese Academy of Sciences researcher writes.

Professor Chen Guojie is a senior researcher at the Institute of Mountain Hazards and Environment, the Chinese Academy of Sciences and Ministry of Water Resources, Chengdu, Sichuan province. This article, which appears here in English for the first time, has been translated by Three Gorges Probe from the Chinese-language book, Flood disasters in the Yangtze basin and solutions from a scientific and technological perspective, edited by Xu Houze and Zhao Qiguo, and published by Beijing’s Sciences Press in 1999.

Chengdu, Sichuan: A mention of floods on the Yangtze River normally brings to mind the image of floodwater sweeping through the middle and lower reaches of the river, especially in the most dangerous Jingjiang section, Dongting Lake and Honghu Lake areas, which are affected most severely and most frequently by devastating floods. However, to gain a better understanding of the floods in the middle and lower reaches, and to deal with them more effectively, we must take a look at the floods that occur in the upper reaches of the river.

1. Characteristics of floods in the upper reaches

As in the middle and lower reaches of the Yangtze, floods are also regular visitors to the upper reaches, arriving almost every year. Based on statistics kept from 1952 to 1989 (excluding 1969, 1970 and 1971, three years [during the Cultural Revolution] for which no data are available) floods were recorded in every one of the other 35 years. Floods claimed at least 200 lives in every flood season in those years, and in 1961, 1981 and 1989, killed more than 1,000 people. The floods affected more than 80 counties in the upper reaches in most of those years, damaging large areas of farmland. These records indicate that the middle and lower reaches are not the only part of the river to experience flood disasters.

At the same time, the upper reaches are frequently hit by drought. In most counties in this part of the Yangtze, the twin disasters of flood and drought occur almost every year, in an alternating pattern of flood and drought.

Moreover, the floods and drought are becoming more severe, occurring more frequently and causing greater losses. For example, the area of farmland affected by drought in the upper reaches has risen steadily from 330,000 hectares in the 1950s, 870,000 hectares in the 1960s, 1.2 million hectares in the 1970s, and 1.6 million hectares in the 1980s. Meanwhile, the economic losses caused by floods increased from US$300 million in 1981 to US$1.4 billion in 1998.

The floods in the upper reaches are characterized by a sudden rise and fall, and a shorter period of inundation compared with the floods that occur in the middle and lower Yangtze. With a large water-flow volume and high peak discharge (or high water level) plus the sudden waxing and waning, floods in the upper reaches often trigger geological disasters such as landslides, riverbank collapses, mud-rock flows and so forth. A joint survey by Chongqing municipality and Sichuan province identified about 100,000 locations considered at risk of landslides, riverbank collapses and mud-rock flows in the Three Gorges area, the upstream regions along the Yangtze and mountainous areas around the Sichuan basin, threatening more than 300 townships in 120 cites and counties. It appears that there is a close correlation between floods and such geological disasters. The 1981 flood in the upper reaches, for example, was largely responsible for 68,000 landslides and mud-rock flows that occurred in 82 counties in the July-September period that year. The severe flood of 1982 triggered most of the 64,000 landslides that summer, incidents that claimed 100 lives and affected one million people.

2. The relationship between floods in the upper reaches and downstream

It is true that [as Li Bai wrote in his poem about the Yellow River] “the water of the Yangtze comes from the heavens” and not just from the upper reaches. Whether the floods in the upper reaches result in floods downstream is a complicated issue and depends on a variety of factors. Based on historical data, at least five types of floods can be identified that show the connection – or lack thereof – between events in the upper reaches and in the middle and lower reaches:

1) Rare floods in the upper reaches that cause floods downstream: In 1870, an extremely big flood in the upper reaches led to an exceptionally big flood in the middle and lower reaches. Heavy rainstorms in the Jialing River valley and the Three Gorges area combined to contribute to the biggest flood ever recorded in the Yangtze basin. The peak discharge in the upper reaches was 100,000 cubic metres at Chongqing and 110,000 cubic metres at Shashi, with devastating consequences in the middle and lower reaches. However, a flood such as the 1870 disaster occurs only once every 1,000 years, and it is actually extremely rare for floods in the upper reaches of the Yangtze River to be directly responsible for severe floods in the middle and lower reaches.

2) Floods caused by storms throughout the river basin: A second type of flood occurred in 1954, the biggest Yangtze flood of the past century. In that instance, half the floodwater originated in the upper reaches and half in the middle and lower reaches. A total water-flow volume of 803 billion cubic metres was recorded during the May-August period, with 337 billion cubic metres of that originating in the Three Gorges area and upstream, about 42 per cent of the total. The middle Yangtze had been hit by an unbroken spell of wet weather, and the situation there was exacerbated by floodwater from the upper reaches. About 30,000 people were killed and 19 million affected by the flood. Three million hectares of farmland were seriously damaged, leading to economic losses of US$1.2 billion. By contrast, the flood in the upper reaches that year was not particularly serious and caused only slight damage.

3) Floods more or less confined to the upper reaches: Floods can be severe in the upper reaches but have no significant impact on downstream areas, which experience much smaller floods or even none at all. The flood of 1981 was a typical case. Major storms in the heart of the Sichuan basin affected the Min, Tuo, Fu, Qu and Jialing river valleys in particular. The floodwater level peaked at 193 metres at Chongqing, and 20 million of people in 139 counties were seriously affected in the former Sichuan province. [Chongqing was carved out of Sichuan province in 1997, and designated a municipality under the central government.] The 1981 flood killed 1,358 people, left one million homeless, and inundated one million hectares of farmland. However, this devastating flood had little impact on areas downstream. The peak discharge at Chongqing of 85,700 cubic metres per second was already reduced to 72,800 cubic metres per second by the time it got to Yichang. With a much lower water level at Shashi (44.5 metres) and Wuhan (25.2 metres) compared with the 1954 flood, the middle and lower reaches basically were not affected by the 1981 disaster. This shows that in the absence of heavy rainstorms in the middle and lower Yangtze and its tributaries, even major floods in the upper reaches are unlikely to cause serious floods downstream.

4) Floods originating in the middle and lower reaches: With no floodwater arriving from the upper reaches, big storms and incessant, heavy rain in the middle and lower reaches can be entirely responsible for flood disasters in this area. Indeed, this type of flood is the most frequent in the Yangtze River valley. We know that one occurred in 1935 and, more recently, in 1980, 1983,1988, 1991, 1994, 1995, 1996 and 1997. The 1996 flood was the result of four big storms that hit Hunan province in July and August, and caused higher floodwater than the 1954 peak level. At the Qilishan hydraulic monitoring station, the water level reached 35.3 metres on July 22, 1996, three-quarters of a metre higher than the 1954 peak. The water level at 11 other monitoring stations in the Dongting Lake area exceeded the recorded historical highs for each place. Obviously, this kind of flood must not be underestimated. And, more significantly, the Three Gorges dam currently under construction will be totally useless against this type of Yangtze flood disaster.

5) Big floods of long duration affecting the whole river basin: The major flood of 1998 lasted two months, with the first peak discharge passing Yichang [near the Three Gorges dam] on July 2 and the eighth peak crest passing the city on Aug. 31. Floodwater from the upper reaches exacerbated the flooding in the middle and lower reaches. The successive peak crests were so strong that they swept down from Sichuan and Chongqing in the upper reaches to Hubei, Hunan, Jiangxi, Anhui and even Jiangsu along the middle and lower Yangtze, putting 1,000 kilometres of dykes in danger. The 1998 flood differed from the 1954 flood in two significant ways: First, while the consequences of the 1954 flood were felt most acutely on the Jianghan plain in Hubei province, the 1998 flood affected not only the middle and lower reaches but also the upper reaches of the Yangtze. (In Sichuan province alone, damage was inflicted on 150 counties and 1.4 million hectares of farmland, resulting in losses of US$1.2 billion.) Second, the 1998 flood had a higher flood level but smaller flow volume than in 1954. At the Shashi monitoring station, for example, the peak discharge in 1998 was 60,000 cubic metres per second compared with but 71,200 cubic metres per second in 1954. Meanwhile, the highest flood level recorded in 1998 was 45.2 metres, 0.55m higher than in 1954.

3. Conclusions

By examining past floods, particularly the 1998 flood, several conclusions can be drawn.

1) Even if heavy rainstorms in the Jinsha, Dadu, Min, Tuo, Fu, Qu and Jialing river valleys upstream of Chongqing produce a large water-flow volume, the middle and lower reaches will not be significantly affected if no storms occur simultaneously in the area below Chongqing, especially the Three Gorges section.

2) If heavy rain in the Three Gorges area swells the Yangtze, but no large amount of floodwater arrives from the area upstream of Chongqing to push the Three Gorges floodwater downstream, the middle and lower reaches will hardly be affected by the flooding in the upper reaches. For example, the devastating flood of 1982 caused enormous losses in the Three Gorges area: In Wanxian prefecture, 217 hydropower stations were destroyed and in Yunyang county, more than 50,000 houses collapsed. However, there was no flooding in the downstream area that year.

3) Floods originating in the upper reaches usually do not cause serious floods in the middle and lower reaches unless they happen to combine with floods occurring simultaneously in the downstream areas. The 1981 flood was one such case. From the above analysis, it is clear that the Three Gorges dam will not completely resolve the flood problem in the middle and lower reaches of the Yangtze. Thus, I argue that building a single giant dam in the Three Gorges area is not the ultimate solution to flood management in the Yangtze basin. In the long term, we should seek a new development path for the Yangtze valley that glues economic prosperity and environmental conservation together. In my opinion, we have to focus on the following three aspects:

First, perspective is everything, and we need to deal with the Yangtze flood problem from a completely different perspective. It is imperative to recognize that the Yangtze basin is a complex, complete ecosystem, so the flood problems in the upper, the middle and the lower reaches should not be dealt with in isolation. In addition, the whole flood-control context needs to be broadened to include physical, economic and social factors. A serious challenge facing us is how to deal with the relationships between the upper, middle and lower reaches, between the provinces and municipalities along the river, and between various regions and different industrial sectors and administrative departments.

Second, flood management cannot be separated from the sedimentation issue. Successfully managing sedimentation will greatly reduce the risks brought about by floods in the river basin. In the upper reaches, the first priority must be soil and water conservation through afforestation in the headwaters zone, to prevent soil erosion along the banks of the river. In the middle and lower reaches, the focus needs to be on making more room for floodwater to pass, by returning some farmland to the floodplain, controlling population growth and farmland expansion in some areas, and strengthening the dykes along the river, and so forth.

Finally, and more importantly, perhaps, we should seek a new development path for the entire river valley based on the philosophy and principles of sustainable development. I would argue for a shift away from the development model that has concentrated on traditional agricultural, toward a new strategy that focuses on developing sustainable tourism, organic foods, herbal medicine, and low-impact animal husbandry and hydropower. The key to successfully dealing with the Yangtze flood problem lies in following a sustainable development path that meets socio-economic development goals while also protecting the environment.

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