100-year flood
[4] Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves.
[5] Maps of the riverine or coastal 100-year floodplain may figure importantly in building permits, environmental regulations, and flood insurance.
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.
The field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering.
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 third assumption is that the probability distribution relates to the largest storm (rainfall or river flow rate measurement) that occurs in any one year.
If, however, there are some extreme events taken from thunder storms, others from snow pack melting, and others from hurricanes, then this assumption is most likely not valid.
Since this is not typically a goal in extreme analysis, or in civil engineering design, then the situation rarely presents itself.
The final assumption about stationarity is difficult to test from data for a single site because of the large uncertainties in even the longest flood records[6] (see next section).
[11] The simplest implication of this is that most of the historical data represent 20th-century climate and might not be valid for extreme event analysis in the 21st century.
[14] Much longer records of flood elevations exist at a few locations around the world, such as the Danube River at Passau, Germany, but they must be evaluated carefully for accuracy and completeness before any statistical interpretation.
[6]: 24 For the largest recorded flood at any specific location, or any potentially larger event, the recurrence interval always is poorly known.
During a time of flooding, news accounts necessarily simplify the story by reporting the greatest damage and largest recurrence interval estimated at any location.
The public can easily and incorrectly conclude that the recurrence interval applies to all stream reaches in the flood area.
In addition to historical flood data, the process accounts for previously established regulatory values, the effects of flood-control reservoirs, and changes in land use in the watershed.
[6]: 24 Approximately 3% of the U.S. population lives in areas subject to the 1% annual chance coastal flood hazard.
