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A one-hundred-year flood is a flood event that has a 1% probability of occurring in any given year. The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%.〔Holmes, R.R., Jr., and Dinicola, K. (2010) ''100-Year flood–it's all about chance '' (U.S. Geological Survey General Information Product 106 )〕 The 100-year flood is generally expressed as a flowrate. Based on the expected 100-year flood flow rate in a given creek, river or surface water system, the flood water level can be mapped as an area of inundation. The resulting floodplain map is referred to as the 100-year floodplain, which may figure very importantly in building permits, environmental regulations, and flood insurance. Estimates of the 100-year flood flowrate and other streamflow statistics for any stream in the United States are available.〔Ries, K.G., and others (2008) ''StreamStats: A water resources web application '' (U.S. Geological Survey, Fact Sheet 2008-3067 ) (Application home page ) URL accessed 2015-07-12.〕 ==Probability== A common misunderstanding exists that a 100-year flood is likely to occur only once in a 100-year period. In fact, there is approximately a 63.4% chance of one or more 100-year floods occurring in any 100-year period. On the Danube River at Passau, Germany, the actual intervals between 100-year floods during 1501 to 2013 ranged from 37 to 192 years.〔Eychaner, J.H. (2015) ''Lessons from a 500-year record of flood elevations '' (Association of State Floodplain Managers, Technical Report 7 ) URL accessed 2015-06-27.〕 The probability Pe that one or more floods occurring during any period will exceed a given flood threshold can be expressed, using the binomial distribution, as where T is the threshold return period (e.g. 100-yr, 50-yr, 25-yr, and so forth), and n is the number of years in the period. The exceedance probability Pe is also described as the natural, inherent, or hydrologic risk of failure.〔Mays, L.W (2005) ''Water Resources Engineering, chapter 10, Probability, risk, and uncertainty analysis for hydrologic and hydraulic design'' Hoboken: J. Wiley & Sons 〕〔Maidment, D.R. ed.(1993) ''Handbook of Hydrology, chapter 18, Frequency analysis of extreme events'' New York: McGraw-Hill 〕 However, the expected value of the number of 100-year floods occurring in any 100-year period is 1. Ten-year floods have a 10% chance of occurring in any given year (Pe =0.10); 500-year have a 0.2% chance of occurring in any given year (Pe =0.002); etc. The percent chance of an X-year flood occurring in a single year can be calculated by dividing 100 by X. A similar analysis is commonly applied to rainfall data. The recurrence interval of a storm is rarely identical to that of an associated flood, because of rainfall timing and location variations among different drainage basins. The field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering. This theory is most commonly applied to the maximum or minimum observed stream flows of a given river. In desert areas where there are only ephemeral washes, this method is applied to the maximum observed rainfall over a given period of time (24-hours, 6-hours, or 3-hours). The extreme value analysis only considers the most extreme event observed in a given year. So, between the large spring runoff and a heavy summer rain storm, whichever resulted in more runoff would be considered the extreme event, while the smaller event would be ignored in the analysis (even though both may have been capable of causing terrible flooding in their own right). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「100-year flood」の詳細全文を読む スポンサード リンク
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