1998, another basin-wide flood occurred in the Yangtze River since 1954.

Since mid-June, the flow of the Yangtze River has increased rapidly due to the continuous heavy rain and heavy rain in Dongting Lake and Poyang Lake. Affected by the upstream inflow and flood season, the tidal level along the Yangtze River in Jiangsu Province has exceeded the warning level since June 25th. The climax of Nanjing Station is located on July 6th, reaching 9.90 meters. The area along the Yangtze River in southern Jiangsu entered Mei from June 24th to July 6th. Because of the high tide level along the river, the drainage of inland river is blocked, which has formed a severe situation of flood disaster. The highest water level at Dongshan Station of Qinhuai River 10.28m is the third in history. The highest water level at Xiaoqiao Station of Chuhe River reached 1 1.29 m, which exceeded the warning water level 1.79 m. ..

From late July to early September, influenced by seven consecutive flood peaks in the upper reaches of the Yangtze River and the confluence of tributaries in the middle reaches, the maximum flow of Datong Station reached 82,300 m3/s on August 2, second only to the flood peak flow 1954, ranking second in history. On July 29th, the highest tidal level at Nanjing Station was 10. 14m, ranking second in history, and it lasted for 17 days above/kloc-0.0m. The high tide level at Zhenjiang Station on August 24th was 8.37m, only lower than 1954/kl.

1998 compared with the historical flood, the main difference is that the flood peak water level in the middle and lower reaches of the Yangtze River and the main control stations of Dongting Lake and Poyang Lake during the flood of 1998 is obviously higher than the long-term high water level; Reduce flood diversion and breach.

Reasons for flood investigation:

Rising water level:

1998, another basin-wide flood occurred in the Yangtze River since 1954. Although the flood has passed, all kinds of arguments are still pouring in, and what's more, it is attributed to "man-made disasters." From a scientific point of view, although the flood is huge in scale, involving a wide range, lasting for a long time and suffering from serious disasters, the losses caused are much smaller than those caused by 193 1 and 1954.

The analysis shows that the Yangtze River 1998 flood is mainly caused by abnormal climate, excessive rainstorm, decreased storage capacity of rivers and lakes, reduced peak clipping and elevated water level. The comparison between the maximum flood peak discharge and the maximum flood volume in the 60 days below Jingjiang River shows that 1998 flood is generally less than 1954, and the three floods in the whole basin in this century (193 1, 1954, 1998

The flood level in the middle and lower reaches of the Yangtze River 1998 greatly exceeded the measured water level in 1954. The main reasons for the high flood level are as follows:

At 1 and 1998, the flood regulation capacity is greatly reduced compared with 1954. 1954, the total amount of flood diversion breaches in the middle and lower reaches of the Yangtze River reached102.3 billion cubic meters, while 1998 was only 100 billion cubic meters.

2. The water storage capacity of the lake has decreased. Since the founding of New China, the area of the same rivers and lakes in the middle and lower reaches of the Yangtze River has decreased by about 65.438+0.8 million square kilometers, and Dongting Lake and Poyang Lake have decreased by more than 65.438+0.8 billion cubic meters due to siltation and reclamation.

Deforestation:

The flood in the Yangtze River is the direct result of soil erosion caused by deforestation and land reclamation in the Yangtze River basin and excessive occupation of rivers in the middle and lower reaches. There are 400 million people living on both sides of the Yangtze River. In the mid-1950s, the forest coverage rate in the upper reaches of the Yangtze River was 22%. Due to continuous land reclamation, factory construction and urbanization, 80% of the forests on both sides of the Yangtze River have been cut down. Of the 193 counties in Sichuan Province, only 12 counties have forest coverage rate over 30%, and some counties have forest coverage rate less than 3%. Because of this, 20% of the 6.5438+0.8 million square kilometers of land in the Yangtze River Basin is subject to soil erosion, and 2.4 billion tons of topsoil is lost every year. Every year, more than 500 million tons of soil and sand are carried from the upstream and flow into the East China Sea along the Yangtze River.

Due to the accumulation of sediment year after year, the riverbed of the Yangtze River has been above the ground many years ago, becoming another "suspended river" after the Yellow River. The "clear water" of the Yangtze River is long gone, and its "muddy yellow" degree can be "comparable" to that of the Yellow River. On the other hand, lakes with flood storage function in the middle and lower reaches of the Yangtze River are rapidly shrinking. The water area of Dongting Lake decreased from 4,350 square kilometers in 1949 to 2 145 square kilometers, and Poyang Lake decreased by15 in 40 years, and hundreds of small and medium-sized lakes disappeared from the map forever. These are the reasons for the flood of the Yangtze River.

Meteorological anomaly:

1998 climate anomaly in China. During the main flood season, the rainfall in the Yangtze River basin is frequent, intense, wide in coverage and long in duration. The rainy season in Songhua River basin is ahead of schedule, and the rainfall is obviously high. The main factors of climate anomalies are:

1, El Nino event (that is, the water temperature near the equatorial eastern Pacific Ocean rises abnormally). 1May, 1997, the strongest El Ni? o event occurred since this century, reaching its peak at the end of that year and basically ending in1June, 1998. The analysis of statistical data shows that in the second year of each El Ni? o event, there are two rain belts in summer in China, one in the Yangtze River and its south, and the other in the north. This unusually strong El Nino event is one of the main reasons for the rainy summer in China Yangtze River Basin from 65438 to 0998.

There is more snow on the plateau. According to the analysis of climate law, when it is snowy in Europe, Asia and Qinghai-Tibet Plateau in winter and spring, the East Asian monsoon is generally delayed, the summer monsoon is weak, the main rain belt is located in the south, and the Yangtze River basin is rainy. 1997 The unusually heavy snow on the Qinghai-Tibet Plateau in winter is an important factor affecting the heavy rainfall in the Yangtze River and Jiangnan areas in summer 1998.

3. The western Pacific subtropical high (hereinafter referred to as subtropical high) is abnormal. Subtropical high is an important factor affecting the location and intensity of rain belt in China. 1From June to August, 1998, the subtropical high was extremely strong, and the ridge line continued to be south-west and showed a stable northeast-southwest trend. This phenomenon is rare in recent 40 years. In the middle and late June, the subtropical high is still normal, and the rainfall belt is mainly located in the middle and lower reaches of the Yangtze River. From the end of June to the beginning of July, the subtropical high briefly moved northward; From mid-July, the subtropical high suddenly withdrew to the south, and its position was unusually south-southwest and remained stable for more than a month, making the middle and upper reaches of the Yangtze River always at the intersection of southwest airflow and cold air, with frequent rainstorm weather, resulting in many flood peaks in the middle and upper reaches of the Yangtze River and rising water levels in rivers and lakes in the middle and lower reaches.

4. Asian mid-latitude circulation is abnormal, and blocking high pressure is frequent. From June to August, 1998 repeatedly appeared blocking high pressure in the Urals Mountain, the west side of Lake Baikal and the Sea of Okhotsk in Asia, especially in the Sea of Okhotsk. The meridional circulation in the westerly belt of Asia was absolutely dominant, which made the Siberian cold air move southward frequently and affected China, which was the cold air condition of continuous rain in the Yangtze River basin.

Extended data:

Several major floods in history are as follows:

During the period of 1870 (the ninth year of Tongzhi in Qing Dynasty), the flood in the Yangtze River basin was mainly caused by the water coming from the main stream of the upper reaches. The highest flood level occurred in Chongqing-Yichang section of the main stream of the upper reaches of the Yangtze River once in a hundred years, and it still maintains the record of the highest value in history. The peak flow of Yichang Station reaches 1000500 cubic meters per second, and the maximum flood volume in 30 days is165 billion cubic meters, which is the largest flood since153 (the 23rd year of Song Shaoxing).

In the same year, Dongting Lake and Hanjiang River in the middle reaches of the Yangtze River also suffered from catastrophic floods, with a large number of breaches between Yichang and Hankou, and dikes generally burst. Although there is no breach in Jingjiang levee, there are many execution openings on the north bank of Jingjiang below Jianli. Jianghan Plain and Dongting Lake area break in Wang Yang, and Huangjiabu and Pangjiawan in Songzi County on the south bank, forming the passage of Songzi River into Dongting Lake today. In the case of stagnant lake depression, the measured peak water level at Hankou Station is 27.36 meters, and the peak flow rate is 66,000 cubic meters per second.

193 1 year, the climate is abnormal, and long-term rainfall has caused floods all over the country. Among them, Hunan, Hubei, Jiangxi, Zhejiang, Anhui, Jiangsu, Shandong, Henan and other provinces in the middle and lower reaches of the Yangtze River and Huaihe River Basin suffered the most, which was the most extensive and serious year in the 20th century. In that year, the flood season in the Yangtze River basin was advanced, and the floods in Xiangjiang River and Ganjiang River in the middle reaches of the two lakes were the largest in April, while the floods in Yongjiang River in the upper reaches were larger. The peak flow of Cuntan Station in the main stream is 63,600 m3/s, Yichang Station is 64,600 m3/s, the highest water level of Shashi Station is 43.85 m, and the maximum flow of Zhicheng Station is close to 70,000 m3/s..

In mid-July, when the water level of Hankou Station reached 26.93 meters, the dike of Danshuichi burst and the urban area of Hankou was flooded. After the upstream flood came, the peak water level of Hankou Station was 28.28 meters, and the peak flow rate was 59,900 cubic meters per second. In the case that there were multiple outlets along the river and the lake. The maximum 60-day flood above Hankou is 330.2 billion cubic meters, slightly less than 1954. If there is no river and lake breach for flood storage, the maximum discharge of Hankou Station will reach 1 13000 cubic meters, which greatly exceeds the flood discharge capacity of the river.

During the period of 1954, the Yangtze River was flooded, and the floods in the middle and lower reaches of the Yangtze River met the Sichuan water. That year, the rainy season in the middle and lower reaches of the Yangtze River came ahead of schedule, and the flood occurred earlier than usual. In April, the Dongting Lake and Poyang Lake water systems entered the flood season, and the high water level in the middle and lower reaches of the Yangtze River lasted for a long time. The water level in Hankou-Nanjing section has exceeded the alert since June 25th, from 100 to 135 days. The flood level in the middle and lower reaches of the Yangtze River exceeded the historical maximum at that time.

In that year, the maximum peak flow of Yichang Station in the upper reaches of the Yangtze River was 66,800 cubic meters per second. After Jingjiang adopted flood diversion measures, the highest water level in Shashi still reached 44.67 meters. The highest water level of Hankou Station in the middle reaches is 29.73 meters, which exceeds the highest water level of 193 1 year, and the corresponding flow is 76 100 cubic meters per second; The highest water level at the downstream Datong station is 16.64m, the corresponding flow is 92,600m3/s ... the maximum 30-day flood volume is 1.954, and the flood diversion and water leakage amount reaches 1.023 million cubic meters.

1998 is another big flood in the whole basin after 1954. The water level in the 359-kilometer reach from Shashi to Luoshan and Wuxue to Jiujiang in the middle and lower reaches of the Yangtze River exceeded the highest water level in history. After floods occurred in five rivers in Poyang Lake system and four rivers in Dongting Lake system, floods occurred in the middle and upper reaches of the Yangtze River, and eight flood peaks appeared in the upper reaches of the Yangtze River. According to the preliminary analysis, in July and August of 1998, the inflow of the upper reaches of the Yangtze River was slightly larger than 1954, and the inflow of the middle and lower reaches was slightly smaller than 1954.

The maximum 30-day floods of Yichang, Hankou and Datong stations are137.9 billion cubic meters,188.5 billion cubic meters and 2 193 billion cubic meters respectively, while the maximum 30-day floods of the above three stations are138.6 billion cubic meters and 1954 respectively. Overall, the flood of 1998 is less than that of 1954.

Baidu encyclopedia-1998 catastrophic flood