20 10 causes of floods in Jiangxi, Fujian and Hunan

Hunan is located in the south of the Yangtze River and north of Nanling, between 24 degrees 29 minutes and 30 degrees 28 minutes north latitude and between 108 degrees 27 minutes and 1 14 degrees 15 minutes east longitude. The southeast is close to the coast, with high temperature and abundant rainfall all year round. The average annual rainfall is about1100 ~1800 mm, with the most rainfall in April-July, accounting for more than half of the annual rainfall. North Dongting Lake accepts Hunan and foreign investment. There are four water sources, namely, Li Shen, Songzi, Lotus Pond, Taiping and Tiaoxian (Tiaoxian was blocked at 1958, so it is hereinafter referred to as the four sources). There are more than 800 tributaries in Hunan, Guizhou, Shenyang and Lishui, with a total length of more than 43,000 kilometers. More than 95% of the towns in the province are built along the river, and one third of the cultivated land is distributed between Dongting Lake and these tributaries.

Due to the restriction and influence of climate, geography, water system and other factors, Hunan has always been a region with frequent natural disasters, especially floods. This paper makes a brief introduction and analysis of the floods in Hunan in recent 50 years for reference only.

I. Overview of floods in history

According to historical records, from 155 BC (the second year of Emperor Han Jingdi) to 20 14 1949 AD, there were 468 floods, with an average of 1 times every five years, among which there were 43 floods in the whole province, accounting for1times. In the past 300 years, there have been 245 floods, with an average of 15 months and 1 time. The interval is 4 years 1 time, 2 years1time, 3 years1time, and the rest are continuous. In 38 years of the Republic of China alone, there were 32 floods, with an average of 1.2 times and 1 times, of which 1, 1.935, 1.948, 1.949 were the most serious. For example, 1935, 38 counties and cities in the province were affected, 1659 was cut off, and the affected area was 389 15000 mu, and another 283 1660800 mu was seriously damaged, 4101. Livestock and property losses cannot be counted.

Another example is the floods in 1948 and 1949 for two consecutive years, and the losses are even more shocking. 1948 from may to June, there was a general rainstorm in the whole province, and the four waters of Hunan, Guizhou, Yuan and Li surged at the same time. Zishui rose by 5.03 meters overnight, Yiyang county was flooded by 2 meters, and most of the dikes on the lakeside burst, Yuanjiang River. The whole city of Xiangyin County was flooded, with a water depth of more than ten feet. Jiang Shen drowned more than 200 people, Xiangyin drowned 1000 people. Lishui River Basin is a Zeguo, three hundred miles below the Yangtze River in Tianjin, and Nanxian County is a Wang Yang. Binhu County 1 1 has 306 broken alkanes, with an affected area of more than 6.06 million mu, 2.67 million tons of rice lost, 0.6 million head of cattle/kloc-0, more than 20,000 houses washed away and more than 8,300 people drowned. Xiangtan. Liuyang. Flash floods occurred in 37 counties including Pingjiang, Huxi and Qiyang. The province was affected by more than 8 million mu, with more than 400,000 victims and property losses totaling 207 trillion yuan (referring to old coins). 1949, the disaster in the whole province is more serious, Changsha and Hengyang. Shaoyang and counties along the lake were seriously affected. From April to May, the total rainfall in Changsha was 964.8 mm, the water level in Changsha reached 22.5 m on May 26th, and the general water depth in Changsha was 1 m.. Ships along the north gate are unimpeded, and there are more than 60,000 people affected by the disaster in the city. More than 50 kinds of alkanes collapsed in 62 dikes along the Yangtze River in Changsha County, causing the biggest loss in Tianhe. The Dong 'an Street in Hengyang City is flooded and the boats are unimpeded. 0/7 people in 40 townships in Hengyang County were affected, and almost all the fields and houses along Xiangjiang River, Laishui River and Zhengzhou River were destroyed. A total of 8,737 houses were damaged in various counties and cities, and 984,600 mu of farmland was affected. 20,026 people could not escape from drowning. The continuous rainstorm in Shaoyang from June to July 135.7mm, and all the towns along the river were severely affected. The second floor of Shaoyang County was flooded, and buildings 1 156 1 were washed away in the county, which affected 469,500 mu, drowned 5 136 people and raised 65 19 cattle. Binhu county 1 1 suffered heavy losses. Huarong County alone burst its banks 16, flooding more than 200,000 mu of farmland, and Hanshou County's affected area accounts for 70% of the county's cultivated land. In addition, Leiyang and Changning in southern Hunan, Chaling in eastern Hunan, Chen Ling and Gancheng in western Hunan, Wugang and Hongjiang in southwestern Hunan and Xiangtan in central Hunan. Forty-six counties such as Xiangxiang were also seriously affected. More than 57,877 people were killed, 4,465,438 levees were broken, and 20 bridges 1535 million mu were washed away, resulting in countless property losses.

Cause of flood

The flood disaster in Dongting Lake area is the most serious in Hunan. In recent decades, the flood disaster in hilly areas is a natural disaster that cannot be ignored. Frequent floods are caused by degassing, geography, rivers and lakes and other factors. Sedimentation in Dongting Lake, destruction of ecological environment, disrepair of water conservancy projects and acceleration of human social activities are also important reasons.

Due to siltation and blind reclamation, the flood storage function of Dongting Lake has declined.

First, the floods in the People's Lake with four ports and four waters carry a lot of Dongting Lake sediment every year. The sediment volume in the first four estuaries of Caiwan in Jingjiang is 2 1, 21.2000 tons/year. Although the sediment in the last four estuaries of Caiwan in Jingjiang has decreased, the sediment in Renmin Lake is still 1654.38+0086 million tons per year. Together with the sediment of Sishui People's Lake, the average sediment of Dongting Lake is one year. In recent years, there are 5 billion cubic meters of sediment deposition. According to the scope and depth of sediment deposition, it is calculated that the total amount of sediment deposition is about 6 million cubic meters, the average deposition height of lake bed is1.4m, the average deposition height for many years is 0.035m, and the maximum deposition height is 6.12m, including the decade from 1976 to 1985. As a result, the lake surface becomes narrower, the lake bed becomes shallower and the flood storage function decreases.

Second, the area of natural lakes has been artificially reduced through blind reclamation and dike repair. Before liberation, feudal leaders and landlords rushed to reclaim land and build dams, which were all over the place. By 1949, there were 993 dikes in the lake area, with an area of 5,935,300 mu of cultivated land. Only the beaches of the Yangtze River and Hanjiang River have cultivated 65,439 mu of cultivated land. For a period of time after liberation, the county governments in the lake area reclaimed the lake and built dikes on the edge of Gaozhou in Dongting Lake, which had little effect on the smooth flow of flood diversion. But in the later period, there were no restrictions on dikes in lakeside counties. In particular, on the grounds of snail control and flood control, they blindly and inappropriately expanded the scope of the lake, which accelerated the reduction of the natural area of Dongting Lake, such as Yuanjiang 198. Xiangyin 1978 is around the lake, covering an area of 25 1 km2. Up to now, there are 3470 levees in the lake area, with a total area of 10042.5 square kilometers, accounting for 66.07% of the land area of the lake area.

Due to a large number of silting and land reclamation, the water surface and capacity of Dongting Lake decreased rapidly in a short time. According to records, from 1524 to 1860, the water surface of Dongting Lake was 6,270 square kilometers, and by 1949, it was reduced to 4,350 square kilometers, with a flood storage capacity of 79.3 billion cubic meters. In 1990, the water level dropped by 30.6%, from 1949 to 1949. The storage capacity is reduced to 65.438+0.94 billion cubic meters, the water surface is reduced by 38.065.438+0%, the storage capacity is reduced by 34%, and the corresponding water level is raised by 654.38+0.5 ~ 2.5 meters, which is one of the main reasons for frequent floods in the lake area.

2. The lake mouth is silted up, reeds are overgrown, and the flood discharge function is reduced. Dongting Lake is flooded, with an average annual water volume of 31900 million cubic meters. However, when the total flood storage capacity of Dongting Lake is19.4 billion cubic meters and the storage capacity is14.73 billion cubic meters, a large number of floods will inevitably be discharged. As the only Tongjiang exit in Dongting Lake-Fanhukou, Chengling, the water flow is blocked by silt and nearby reeds. After Jingjiang River cut off the bay, the sediment in Sikou Renhu Lake decreased. The sediment in these sparsely populated lakes did not stay in the Xiajing River, where the sediment carrying capacity was enhanced, but rushed to the river below Chenglingfan, and then all deposited on the riverbed. According to statistics, from 1972 to 1987, the accumulated sediment is10 million tons, the river section is reduced by 4,000-6,000 square meters, the flood discharge is reduced by 4,000-6,000 cubic meters, and the alum water level in Chengling is raised by 0.8- 1.0 meters. 1983 from June 20th to July 20th * * 28 days, it can be seen that the number of people returning from Dongting Lake is much larger than that from Dongting Lake. In these 28 days, the total amount of human lake water was 67.7 billion cubic meters, while in the same period, Fanhukou of Chenglingji discharged 52.97 billion cubic meters, and the amount of lake water discharged was 65.438+0.473 billion cubic meters less than that of human lake water.

Dongting Lake is not only blocked by sediment at the mouth of the lake and the river below Chenglingji, but also has siltation in the inner lake, reeds and stagnant water, which also seriously affects the flood discharge function. Due to sediment deposition, Dongting Lake has produced a large area of beach land, which is increasing at the rate of 60,000 mu per year. A large area of reeds grows on the beach, which blocks water and accelerates sediment deposition. There is a vicious circle that the faster the beach develops, the more reeds there are, the more water is blocked and the more sediment is deposited. During the period of 1965 ~ 198 1 year, the reed area of Dongting Lake has expanded by about 5000 mu. So far, the reed area of Dongting Lake has grown to about one million mu, accounting for a quarter of the whole lake area.

It can be seen that the decline of flood discharge function and the shrinkage of the total storage capacity of Dongting Lake are the fundamental reasons for the frequent flood disasters and the deterioration of high flood level in Dongting Lake year by year.

3. The ecological balance has been destroyed and soil erosion is serious.

First of all, the vegetation was severely damaged. Over the years, due to various factors and man-made deforestation, vegetation has been seriously damaged, especially in the middle and upper reaches of the Yangtze River, resulting in a lot of soil erosion. During the rainstorm, the sediment rushed into the rivers and lakes with the rolling flood without hindrance, and finally deposited at the bottom of the rivers and lakes, resulting in excessive siltation in the river bed, blocked water flow and rising water level, causing disasters. In addition, the destruction of vegetation and ecological imbalance have brought about climate anomalies in some or most areas, such as long sunny days without rain, long heavy rains or short-term heavy rains, which is another reason for the frequent occurrence of climate anomalies and floods in Hunan in recent decades.

Secondly, people built dikes along the river banks to cut and straighten the bay, which accelerated the frequency of floods in the downstream areas. Before embankment construction and bend straightening, the flood slowly flows downstream in the wide and winding riverbed, making the riverbed narrow and straight. In case of rainstorm, the flood has to surge downstream in the narrow riverbed designated and designed by people, thus causing flood disasters in the downstream and low-lying areas. In recent decades, the floods in the lake area are closely related to the construction of dikes in the middle and upper reaches of the Yangtze River, Si Shui and Dongting Lake, and the cutting of the bay for straightening. The disaster brought by the formation of Jingjiang levee to Dongting Lake area is the best example. Before Jingjiang levee was formed, Dongting Lake was flooded once in 90 years, once in 20 years after Jingjiang levee was formed, and once in 9-4 years after Songzi burst. This shows that the flood disaster in the lake area is closely related to Jingjiang River, as well as to the straightening of the upper reaches of the Yangtze River and the four tributaries and the construction of dikes. On the other hand, it also reflects that while people are transforming nature, they are also destroying the balance between nature and ecology, bringing unexpected and greater disasters.

Third, the growth of population, the expansion of human living space and the acceleration of human social and economic activities are also one of the reasons for accelerating the destruction of living environment and frequent floods.

4. The influence of geographical environment on floods in Hunan. Hunan is surrounded by mountains in the east, west and south, and gradually inclines to the middle and northeast. DJ has a large area, active and complex landforms, steep slopes and deep valleys, strong surface cutting ability and weak anti-corrosion ability of some soils. After rainfall, the surface runoff is easy to collect quickly, the flood velocity is high, the rising trend is fierce, the erosion and soil erosion are serious, and floods are often caused. In addition, the old farmland in Dongting Lake area has a low terrain, and the production ground elevation is generally 5-8 meters lower than the flood level of the outer river, and it exceeds 10 meter in some places. In the second half and the first half of the production site, the production and life are completely protected by dikes. Therefore, it is inevitable that Dongting Lake will be flooded before its roots are completely carved out. This is because the geographical environment determines that Dongting Lake has more floods and a greater threat than other areas, and this threat is aggravated with the change of water regime and the rise of water level, and worsens with the deterioration of flood storage conditions in rivers and lakes.

5. Climatic conditions affect floods in Hunan. From April to August, the climate of our province is often controlled by the convergence of subtropical high, westerly circulation, southeast monsoon and southwest monsoon. The subtropical climate jumps north and moves south, the westerly circulation sinks south and retreats north, and the southeast monsoon and southwest monsoon meet. Under this effect, the annual average rainfall exceeds 1.462 mm, which is one of the rainy areas in China. The distribution of rainfall in four seasons is extremely uneven, with great interannual variation. The rainfall from April to September accounts for about 69% of the annual rainfall, and the rainfall from April to June is the most concentrated, accounting for about 50% of the annual rainfall, which often causes floods in hilly areas many times a year and floods in Dongting Lake area for a long time. July and August are vulnerable to typhoons, during which high-intensity and concentrated rainfall often occurs. The annual average maximum seven-day rainstorm 140 ~ 300mm is also the main reason for flash floods and steep rise of rivers.

6. Water conservancy projects are in disrepair. From the early days of liberation to the 1970s, although some water conservancy projects were built, which alleviated the flood disaster, since the 1980s, the water conservancy projects have been in disrepair for a long time, and even the original ponds, reservoirs and lake dikes in hilly areas lack maintenance and reinforcement, especially Dongting Lake, where the old fields are low and the dike quality is poor, which has lost and reduced the ability of flood storage and flood control, resulting in great disasters in wet years and small disasters in dry years.