What is the definition of a once in a decade flood and a once in a century event?

1. Once in ten years refers to the average probability of the maximum flood flow of a river in a certain flow section is once every ten years, but it is not only a flood of this flow every ten years, for example, there was a flood in a certain place last year, and its flow and flood peak were set as once in ten years, and this year the river has a flood again, and its flow is basically equal to last year, which is also a once in ten years, so the next time the river has the same flow of flood, the time may be 20 years later or even longer. But it is also possible that a river has been flooding every year for several years in a row, and it is likely that it will be dry for a long time after that year.

2. The greater the flow of the flood, the smaller the frequency of its occurrence, which can be divided into a major flood once in a hundred years, a catastrophic flood once in a thousand years, or a rare flood once in 10,000 years. For example, the flood in the entire Yangtze River basin in '98 was once in a century, and the next time there was a flood as big as '98 might be in 100 years, but it was also possible that there would be a small flood once in five years and a flood once in 10 years or once in 100 years in the past few years, but there might not be a major flood once in a century for a long time in the future, and the last time the Yangtze River had a flood like '98 was in 1934. The design flood control standards for the Three Gorges Project are:

It can withstand a once-in-a-century flood like the 98th year, and can hold back the flood when the flood comes, without using the downstream flood distribution zone; The design flood capacity of the dam is to ensure that the land in Wuhan and the downstream plain will not be flooded with the cooperation of the Jingjiang River flood distribution area in the event of a major flood once in a thousand years: when a rare flood arrives once in 10,000 years, it will ensure that the dam will not burst, that is, the water conservancy profession often says that the mega water conservancy project is designed as: a thousand years of fortification, 10,000 years of school load.

For example, the Yellow River has not had a flood for more than 20 years, so the probability of a once-in-a-decade or once-in-a-century flood is also increasing.

Why are there so many once-in-a-century floods? What does a once-in-a-century event represent, listen to what the water conservancy people say.

Legal Analysis: When flooding large rivers, people often say that floods occur once in ten years, once in a hundred years, and once in a thousand years, which is a habitual name for the probability value of floods. When the flood flow from the upstream of a river or a certain river system reaches a flood flow with a probability of 10, people habitually call it a once-in-a-decade flood; When the flood flow with a probability of 1 is reached, it is called a once-in-a-century flood; And so on.

There are four levels of mega floods, which are as follows:

1. Floods with a return period of less than 5 years are small floods;

2. The return period of flood elements is 5 years and 20 years of floods, which are medium floods;

3. The return period of flood elements is 20 years and 50 years of floods, which are major floods;

4. A flood with a return period of more than 50 years is a catastrophic flood.

Legal basis: "Flood Control Law of the People's Republic of China" Article 18 In the prevention and control of river floods, both storage and discharge shall be carried out, giving full play to the flood carrying capacity of rivers and the functions of reservoirs, depressions and lakes in regulating and storing floodwaters, strengthening the protection of rivers, and taking measures such as regular dredging and dredging in accordance with local conditions to maintain the smooth flow of floodwater.

To prevent and control river floods, it is necessary to protect and expand the forest and grass vegetation in the river basin, conserve water sources, and strengthen the comprehensive management of water and soil conservation in the river basin.

The number of floods is divided according to the frequency of floods, for example, if the flood frequency is 1, it is a once-in-a-century flood.

For example, a once-in-a-century event is likely to recur once in a hundred years, with a cumulative frequency of 1%.

According to the law of nature, the occurrence of mega floods is small, and the number of general floods is high, which must be obtained based on long-term measured data and the use of mathematical statistical methods.

The hydrology industry generally uses it to measure floods of varying magnitude.

If the flood frequency is 1, it is a 100-year flood.

Flood frequency is the number of occurrences that the characteristic value equals or exceeds a certain number of occurrences over a multi-year period, and can also be converted into the probability of a flood occurring in each year, expressed as a percentage, and its reciprocal is the "return period".

According to the law of nature, the occurrence of mega floods is small, and the number of general floods is high, which must be obtained based on long-term measured data and the use of mathematical statistical methods.

The once-in-a-century flood is based on statistics alone. For geologists or hydrologists, it means a huge flood that occurs only once every 100 years along a part of the floodplain.

The return period of the literary phenomenon has the concept of statistical average, and it cannot be mechanically regarded as a must occur once in many years; For example, the "once in a hundred years" rainfall does not mean that the rainfall in a certain place is greater than or equal to this rainfall occurs exactly once in 100 years, in fact, such a value may appear many times in a hundred years, or it may not occur once. This is true only in a large number of processes or over a long period of time.

Supplementary Information:1. Floods with a return period of less than 5 years are small floods;

2. The return period of flood elements is 5 years and 20 years.

3. The return period of flood elements is 20 years and 50 years of floods, which are major floods;

4. A flood with a return period of more than 50 years is a catastrophic flood.

Legal Analysis: Four levels of catastrophic floods, as follows:

1. Floods with a return period of less than 5 years are small floods;

2. The return period of flood elements is 5 years and 20 years of floods, which are medium floods;

3. The return period of flood elements is 20 years and 50 years of floods, which are major floods;

4. A flood with a return period of more than 50 years is a catastrophic flood.

Legal basis: Constitution of the People's Republic of China Article 53 Citizens of the People's Republic of China must abide by the Constitution and laws, keep state secrets, protect public property, abide by labor discipline, abide by public order, and respect social morality.

A once-in-a-century flood means a flood that occurs once in a long time.

The so-called "once in a century" is a concept about frequency, which means that the frequency is low and rarely occurs. If the occurrence of an event cannot be predicted in advance, the probability of occurrence can only be estimated by mathematical statistics through a large number of measured data, which is called empirical frequency, and "frequency" in hydrological calculations, and "return period" is usually used instead of "frequency".

The return period of hydrological phenomena has the concept of statistical average, which cannot be mechanically regarded as a must occur once in many years; For example, the "once in a hundred years" rainfall does not mean that the rainfall in a certain place is greater than or equal to this rainfall occurs exactly once in a hundred years, in fact, such a value may appear many times in a hundred years, or it may not occur once, only in a large number of processes or for a long period of time is correct.

It's just that the flood is very big, and it happens once in a hundred years.

It's a form of expression, not necessarily exactly a hundred years.

It's a rare meaning, and I haven't seen it in recent years.

A larger flood is a flood that occurs once in 10-20 years.

The Emergency Plan classifies floods: general floods are floods that occur once in 5 10 years during the return period of peak flood flow or flood volume. The Great Flood is a flood that occurs once in 20 50 years. A catastrophic flood is a flood that occurs more than once in 50 years.

Flood is the flow of water bodies such as rivers, oceans, lakes and other water bodies that exceed a certain water level, threaten the safety of relevant areas, and even cause disasters, also known as large water. The word flood comes from the pre-Qin "Shangshu Yaodian" in China. Since then, there have been many recorded floods over the past 4,000 years, and the earliest record of a flood in Europe dates back to 1450 BC.

The Tigris Euphrates River in Western Asia and the Nile River in Africa date back to the 40th century BC.