# 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.

### 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.