"A heavy rain that happens once in a hundred years" does not mean that it will happen once in a hundred years

 Guan Yang/3M Chief Traffic Safety Education and Policy Liaison Officer

2019-08-15 17:38 Source: The Paper

The recent typhoons and heavy rains have caused waterlogging in many places. There are also many complaints about the municipal drainage capacity. Some people think that if the torrential rain that occurs once in a hundred years cannot be withstood, then forget it, why can't it withstand the torrential rain once in five years?

There is actually a misunderstanding of technical terminology here. Not only the public, but also many professionals and government departments may not be clear. Many technical terms are introduced technical concepts. If the translation is not careful, it will cause misunderstandings. The "storm once in a hundred years" is a typical example.

"Once in a hundred years" is an imported term in hydrometeorology, such as 100-year flood, which is usually directly translated as "flood in a hundred years." Many people think that if you encounter it once in a hundred years, you will never encounter it again. So strange, why do you often encounter heavy rains that are "once in a hundred years"?

On the U.S. Geological Survey website, there is an official explanation after the term "100-year flood" has been misread by the public: hydrologists don’t like to hear the term "100-year flood" because, scientifically speaking, it is Misunderstanding of terminology. The preferred expression of hydrologists is that this kind of flood has a repetition interval of 100 years. In other words, the probability of occurrence is 1% every year, rather than once every 100 years.

On this website, some members of the public asked why two 500-year floods occurred in 15 years?

Bob Holmes, the National Flood Coordinator of the U.S. Geological Survey, discussed this issue in a podcast (from the U.S. Geological Survey's core broadcast podcast and video series).

He said: This question points out the importance of correct terminology. The term "flood once in a hundred years" is used to simplify the definition of the scale of the flood. From a statistical point of view, the probability of a flood occurring in any given year is 1%. Similarly, the term "100-year storm" is used to define a rainstorm event with a statistically 1% probability of occurrence. In other words, in 1 million years, these events are expected to occur 10,000 times. However, if there were 10 inches of rain in a day last year, it does not mean that there will be no more 10 inches of rain on a certain day this year.

In this regard, the official term is called Annual Exceedance Probability (AEP), which is the percentage of the probability of an event exceeding the carrying capacity or limit.

For example, "a flood caused by a storm of 0.2 AEP" means that the probability of occurrence in any given year is 20%. Corresponding to floods with repeated intervals of 5 years, China is accustomed to call it the "five-year encounter".

Of course, for comparison of flood intensity, the terminology of repetition interval is easier to understand than the terminology of AEP. However, if a certain place is designed with a drainage facility based on the amount of rain that occurs once in five years, that is, there is a 20% probability of heavy rain every year, the probability of being flooded is actually not low.

In the 1960s, the US government decided to make the 1% probability of exceeding the limit per year (AEP) the basis of the national flood insurance plan, because 1% of AEP floods were considered a fair balance between protecting the public and strict supervision. The 1% AEP flood is commonly referred to as the "one-in-a-hundred-year flood".

It should also be noted that a torrential rain that occurs once in a hundred years does not necessarily cause a flood that occurs once in a hundred years. This year’s probability of exceeding the limit is only statistical data. The accuracy and authenticity of the data have a lot to do with the uniformity of rainfall in the entire basin, the density of collection, and the quality of historical data. It also has a lot to do with the soil and water content of the basin, and the previous rainfall. The amount is related. These statistics do not represent the present and the future, but a tool of scientific management.

According to the assessment on the U.S. Geological Survey's website, for a 30-year mortgage house, if the house is located in a 1% AEP floodplain, there is a 26% chance that it will be flooded at least once in the 30 years. The value of 26% is based on the calculation result of probability theory. The calculation basis is that the probability of flooding every year in 30 years is 1%. (Data comes from the official website of the US Geological Survey)

To sum up, "heavy rain that occurs once in a hundred years" is a simplified expression of the magnitude of a heavy rain. It does not mean that it is encountered only once in a hundred years, but that the chance of this magnitude of rain occurring in a year is 1%.

If, when studying a city's flood control project, feel that a hundred years of torrential rains have occurred in the previous few years, and no longer invest human and financial resources to resist floods, it is obviously misguided. Preventing flood disasters is a kind of safety work and a modern management technique, seeking a more scientific balance between investment and safety. From this perspective, urban flood control has a long way to go.