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= Zhujiang River Flood =

The Zhujiang River Basin is one of China's largest river basins, which located in South China within total area approximately 453,700 square kilometers (excluding the Leizhou Peninsula region) in Guangdong province, Guangxi province, Guizhou province, Yunnan province, Hunan province, Jiangxi province and Fujian province and partly Northeastern Vietnam.

Also, the Zhujiang River basin is consisting by three main tributaries: the Xi River, Bei River, and Dong River, and other a lot of small rivers with the Zhujiang River Delta. Furthermore, the Xi River is the largest tributary within total length of 2,214 kilometers, and the average slope of the river is 0.58%. The Bei River with a total length of 468 kilometers has the average slope of the river is 0.26%. The Dong water system has the main stream 520 kilometers long with the average slope of the river is 0.388%. Total annual water volume of the whole basin is 345.8 billion cubic meters, which is the second only to the Yangtze River in China.

The floods in the Zhujiang River are mainly caused by heavy rains. Since the area of the river basin is wide and the intensity of heavy rain is high, the floods in the alpine hilly areas of the upper and middle reaches are fast, and there are no lakes in the middle reaches. Therefore, in the case of continuous heavy rain, the floods are often formed with high peaks and large quantities in long duration, endangering the middle and lower reaches in the cities and towns with low land and large population, and vast farmland along the river, which restricts economic development and affects social stability.

Flood Characteristics
The Zhujiang River belongs to the rain-fall river and the flood comes from heavy rain. The causes of heavy rain are:

Atmospeheric Circulation
Since the heavy rain is affected by the subtropical monsoon, and the cold and warm groups are in harmony. Also, it is affected by the topography, resulted in frontal type of heavy rain, which usually occurs in April to July, named the first flood season.

The typhoon
The heavy rain is also affected by the typhoon, which usually occurs in August to September, called the later flood season. Rainfall in flood season accounts for about 80% of the whole year. Its characteristics are: a large amount of rainfall, a large intensity and a long duration. The regular pattern rule: Bei River first, then Dong River and Xi River. The rainstorm center: Bei River is in the area from Yingde to Qingyuan, Dong River is in the area of Jiulian Mountain, Xunwu and Shangping, while Xi River is in the Miao Mountain in the upper reaches of the Gui River, the Xingren in Laowang Mountain and Duan in Daming Mountain. The torrential rainstorms at the edge of the basin includes the city of Haifeng to Huidong in the east of Guangdong, Enping to Yangjiang in the west of Guangdong, and Qinzhou to Dongxing in southern Guangxi.

The maximum average annual rainfall of 24h in the basin is 100-200mm, the coefficient of variation is 0.35~0.65, and the maximum annual rainfall of 24h is 848mm (Jinjiang Encheng Station). The annual variation of runoff in the basin corresponds to the rainfall, that the water volume in the flood season from April to September accounts for 70% to 80% of the total annual water. Since the large amount of rainfall and high intensity during the flood season, many tributaries are fan-shaped, and floods are easy to collect into the main stream at the same time. There are many hills in the upper and middle reaches, and the flood convergence speed is relatively fast. There is no lake storage in the middle reaches, which it is easy to form floods with high peaks and large quantities. The biggest flood peaks in Bei River and Dong River often appear from May to June, and a flood lasts about 7 to 15 days. The largest flood peak in the Xi River often appears from June to August, while the largest floods mostly occur from June to July, lasting for 30 to 45 days. The Xi flood is the main source of floods in the Zhujiang River Delta. Sometimes the floods of Xi River and Bei River cause serious disasters in the Pearl River Delta.

Every year the typhoon season is from May to October. The typhoon landed in Shenzhen, Guangdong, to the Zhujiang River Estuary in Taishan City. The typhoon with a wind level of 8 or above in the center has an average of 2 to 3 times per year, and a strong typhoon of 10 to 11 levels every 2 to 3 years. Under the invasion of the typhoon, sometimes resulted in the storm surges.

Typical Flood
In July 1949: The typical floods encountered in the Xi River Flood and the Bei River. The flood was a basin flood of the Xi River caused by heavy rain formed by the front shear weather system. The flood mainly comes from the Lancang River and the Liujiang River had the largest proportion. The flood peak flow of the Wuzhou station is 48,900 cubic meters per second, which was equivalent to a 50-year flood. It is a single-peak type with a flow rate of over 30,000 cubic meters per second. After 18 days, the 30-day flood was 88.4 billion cubic meters, which was the first in the series of the largest 30-day flood.

In June 1959: Dong River basin flood caused by heavy rain formed by frontal shear weather system. The flood mainly comes from Boluo. The Boluo station has a measured peak flow of 12,800 cubic meters per second and a reduction value of 14,100 cubic meters per second, which is equivalent to a flood in 100 years.

Flood Disaster
The floods in the Zhujiang River Basin are frequent, especially in the middle reaches, lower reaches and deltas. Historical literature had described a lot of floods. According to statistics, the floods occurred 125 times in the Ming Dynasty in the Xi River Basin, 181 times in the Qing Dynasty, and 17 times in the Republic of China. There were 24 floods occurered in the Bei River Basin before 1949 and 14 major floods in the Dong River Basin from 1864 to 1985. In the lower reaches of the Zhujiang River and in the delta, more than 32,200 hectares of floods were affected or affected by arable land. There were 26 times in the 18th century and 36 times in the 19th century. From 1915 to 1949, there were 22 times of arable land flooding area exceeding 66,700 hectares. There have been 12 heavy floods since the 1949. After the 1990s, the "94.6", "96.7" and "98.6" floods occurred, which the damages were very serious.

Precaution
There are 926,600 feet of the cultivated land threatened by floods in the Zhujiang River Basin, affecting 20 million population. The Zhujiang River floods are mainly concentrated in the confluence zones of the Zhujiang River Delta, the Lancang River, and the Liutun River. These areas are densely populated and economically developed. Although most of these areas have the embankment protection, the flood control standards are not strictly. Except for the Bei embankment in Guangzhou, which has been defended against floods for 20 years, most of them are only defense for 10 years of flooding. According to the principle of “combination of embankment, venting and storage”, in the 1980s, three flood control projects combined with dikes were planned:

Preserving the natural vegetation upstream of the river, regulating the deforestation and planting more trees to cure the floods, which will grasp the soil to prevent the sediment from being washed downstream and avoid excessive sediments downstream of the river.