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 in 100 chance (1% probability) of being equaled or exceeded 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 in 100 chance (1% probability) of being equaled or exceeded 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.

Passau

Passau, GermanyBatavisSchalding-Heining
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.

Extreme value theory

rare eventextreme eventsextreme value analysis
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.

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.

National Flood Insurance Program

Biggert–Waters Flood Insurance Reform Act of 2012NFIPCommunity Rating System
Complete information on the National Flood Insurance Program (NFIP) is available here. A regulatory flood or base flood is routinely established for river reaches through a science-based rule-making process targeted to a 100-year flood at the historical average recurrence interval.
In this sense, a base flood is synonymous with a 100-year flood and a floodplain is synonymous with a special flood hazard area.

Frequency of exceedance

probability of exceedanceexceedance frequencyexceeded
The probability of exceedance P e is also described as the natural, inherent, or hydrologic risk of failure.

Wind wave

waveswaveocean wave
For coastal or lake flooding, the 100-year flood is generally expressed as a flood elevation or depth, and may include wave effects. Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves.

Water

H 2 OHOliquid water
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 AgencyUK Environment Agency Environment Agency’s
In the UK The Environment Agency publishes a comprehensive map of all areas at risk of a 1 in 100 year flood.

Tide

tidalhigh tidelow 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.

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 RiverRiver DanubeDanubian
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 modelBinomial probability
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 flooding

coastal floodCoastal 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

assumptionsStatistical assumptionsdistributional assumption
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 significantsignificantsignificance level
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

correlationcorrelatedcorrelations
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 valueaveragepopulation mean
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.