Editing 100-year flood (section)

==Probability uncertainty==

When these assumptions are violated, there is an ''unknown'' amount of uncertainty introduced into the reported value of what the 100-year flood means in terms of rainfall intensity, or flood depth. When all of the inputs are known, the uncertainty can be measured in the form of a confidence interval. For example, one might say there is a 95% chance that the 100-year flood is greater than X, but less than Y.<ref name="Holmes"/>

Direct statistical analysis<ref name="B17C"/><ref name="ACWI">{{cite web |title=Bulletin 17C |url=https://acwi.gov/hydrology/Frequency/b17c/ |publisher=Advisory Committee on Water Information |access-date=2 October 2018}}</ref> to estimate the 100-year riverine flood is possible only at the relatively few locations where an annual series of maximum instantaneous flood discharges has been recorded. In the United States as of 2014, taxpayers have supported such records for at least 60 years at fewer than 2,600 locations, for at least 90 years at fewer than 500, and for at least 120 years at only 11.<ref name="NWIS">[http://waterdata.usgs.gov/nwis ''National Water Information System database''] U.S. Geological Survey. URL accessed 2014-01-30.</ref>  For comparison, the total area of the nation is about {{convert|3,800,000|sqmi|km2}}, so there are perhaps 3,000 stream reaches that drain watersheds of {{convert|1,000|sqmi|km2}} and 300,000 reaches that drain {{convert|10|sqmi|km2}}. In urban areas, 100-year flood estimates are needed for watersheds as small as {{convert|1|sqmi|km2}}. For reaches without sufficient data for direct analysis, 100-year flood estimates are derived from [[regression analysis|indirect statistical analysis]] of flood records at other locations in a hydrologically similar region or from other [[Hydrology#Modeling|hydrologic models]]. Similarly for coastal floods, [[tide gauge]] data exist for only about 1,450 sites worldwide, of which only about 950 added information to the global data center between January 2010 and March 2016.<ref>{{cite web|title=Obtaining Tide Gauge Data|url=http://www.psmsl.org/data/obtaining/|website=Permanent Service for Mean Sea Level|publisher=PSMSL|access-date=7 March 2016|ref=PSMSL_data}}</ref>

[[File:Passau flood scale Sept 2012.JPG|thumb|High-water scale 1501–2002 at [[Passau|Passau, Germany]], as of September 2012]]
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.

For an individual stream reach, the uncertainties in any analysis can be large, so 100-year flood estimates have large individual uncertainties for most stream reaches.<ref name="Eychaner"/>{{rp|24}}  For the largest recorded flood at any specific location, or any potentially larger event, the recurrence interval always is poorly known.<ref name="Eychaner"/>{{rp|20,24}}  Spatial variability adds more uncertainty, because a flood peak observed at different locations on the same stream during the same event commonly represents a different recurrence interval at each location.<ref name="Eychaner"/>{{rp|20}}  If an extreme storm drops enough rain on one branch of a river to cause a 100-year flood, but no rain falls over another branch, the flood wave downstream from their junction might have a recurrence interval of only 10 years. Conversely, a storm that produces a 25-year flood simultaneously in each branch might form a 100-year flood downstream. 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.<ref name="Eychaner"/>{{rp|7,24}}