100-year flood

100 year flood100-year500-year floodhundred-year flood100 year1 percent chance50-year flood500 year floodhundred year flood1 in 500 year event
A one-hundred-year flood is a flood event that has a 1% probability of occurring in any given year.wikipedia
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Flood

floodingfloodsflood control
A one-hundred-year flood is a flood event that has a 1% probability of occurring in any given year.
Coincident events may cause extensive flooding to be more frequent than anticipated from simplistic statistical prediction models considering only precipitation runoff flowing within unobstructed drainage channels.

Extreme value theory

Extreme Value Analysisextreme eventsextreme value distribution
The field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering.
For example, EVA might be used in the field of hydrology to estimate the probability of an unusually large flooding event, such as the 100-year flood.

Passau

BatavisInnstadtPassau, Germany
On the Danube River at Passau, Germany, the actual intervals between 100-year floods during 1501 to 2013 ranged from 37 to 192 years.
On 2 June 2013, the old town suffered from severe flooding as a result of several days of rain and its location at the confluence of three rivers Peak elevations of floods as early as 1501 are displayed on a wall at the Old City Hall.

Return period

recurrence interval1-in-100 year event1 in 100 year
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 recurrence interval of a storm is rarely identical to that of an associated riverine flood, because of rainfall timing and location variations among different drainage basins.
100-year flood

List of floods

19752013 flood2015 China floods
List of floods
In Alaska, United States, from May to September 1992 it was unusually wet, causing the 100 year flood. Snow melt only made the floods worse.

Frequency of exceedance

probability of exceedanceexceededprobability of exceeding
The probability of exceedance P e is also described as the natural, inherent, or hydrologic risk of failure.
100-year flood

Water

H 2 Oliquid wateraqueous
Based on the expected 100-year flood flow rate, the flood water level can be mapped as an area of inundation.

Floodplain

flood plainfloodplainsflood plains
The resulting floodplain map is referred to as the 100-year floodplain.

Environment Agency

the Environment Agency Environment Agency’sAir Quality Modelling and Assessment Unit (AQMAU)
In the UK The Environment Agency publishes a comprehensive map of all areas at risk of a 1 in 100 year flood.

Tide

tidallow tidehigh tide
Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves.

Storm surge

storm tidetidal surgestorm surges
Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves.

Wind wave

waveswavewave dominated
Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves.

Flood insurance

floodflood risk
Maps of the riverine or coastal 100-year floodplain may figure importantly in building permits, environmental regulations, and flood insurance.

Danube

Danube RiverDanubianRiver Danube
On the Danube River at Passau, Germany, the actual intervals between 100-year floods during 1501 to 2013 ranged from 37 to 192 years.

Binomial distribution

binomialbinomial probability distributionbinomial random variable
The probability P e that one or more floods occurring during any period will exceed a given flood threshold can be expressed, using the binomial distribution, as

Expected value

expectationexpectedmean
However, the expected value of the number of 100-year floods occurring in any 100-year period is 1.

Coastal flood

coastal floodingCoastal Flood Warningcyclone-generated wave washover
A similar analysis is commonly applied to coastal flooding or rainfall data.

Drainage basin

watershedbasincatchment area
The recurrence interval of a storm is rarely identical to that of an associated riverine flood, because of rainfall timing and location variations among different drainage basins.

Statistical assumption

assumptionsmodel assumptionsstatistical assumptions
There are a number of assumptions that are made to complete the analysis that determines the 100-year flood.

Independence (probability theory)

independentstatistically independentindependence
First, the extreme events observed in each year must be independent from year to year.

Statistical significance

statistically significantsignificantsignificantly
In other words, the maximum river flow rate from 1984 cannot be found to be significantly correlated with the observed flow rate in 1985, which cannot be correlated with 1986, and so forth.

Correlation and dependence

correlationcorrelatedcorrelate
In other words, the maximum river flow rate from 1984 cannot be found to be significantly correlated with the observed flow rate in 1985, which cannot be correlated with 1986, and so forth.

Probability distribution function

probability functionprobability functions
The second assumption is that the observed extreme events must come from the same probability distribution function.

Mean

mean valuepopulation meanaverage
The fourth assumption is that the probability distribution function is stationary, meaning that the mean (average), standard deviation and maximum and minimum values are not increasing or decreasing over time.

Standard deviation

standard deviationssample standard deviationsigma
The fourth assumption is that the probability distribution function is stationary, meaning that the mean (average), standard deviation and maximum and minimum values are not increasing or decreasing over time.