100y

100y is a shorthand notation used in statistics, hydrology, and engineering to denote a time horizon or event with a nominal return period of one hundred years. The letter y stands for year, and the term is commonly encountered in discussions of extreme events such as floods or high winds, or in structural design criteria.

In practice, "100y" typically implies an event with about a 1% chance of being exceeded in any given year, assuming a stationary statistical process. A related concept is the 100-year flood, which is the magnitude of flood expected to be equaled or exceeded on average once per century. In design contexts, systems or structures may be specified to withstand the 100-year event to provide a target level of reliability.

Limitations and interpretation: a 100y designation is not a precise forecast or guaranteed schedule. The probability of experiencing such an event in a longer period increases with time, and the real-world applicability of a 100-year return period can be affected by nonstationarity, climate change, and event clustering. Misinterpretations are common, especially among the public, who may assume an event will occur exactly every 100 years rather than having a 1% annual probability.

See also: return period, flood frequency analysis, risk assessment.