100-year flood

Indication of the likelihood of a flooding thumb|Mississippi River at Kaskaskia, Illinois, during the Great Flood of 1993 A 100-year flood, also called a 1% flood,Holmes Robert R. Jr. Dinicola Karen USGS General Information Product 106: 100-Year Flood-It's All About Chance https://pubs.usgs.gov/gip/106/ August 5, 2026 U.S. Geological Survey 2010 or High Probability in the UK, is a flood event for a defined location at a level reached or exceeded once per hundred years, on average, but as there are many locations there are multiple independent 100-year floods within the same year. In the US, it is estimated on past records as having a 1 in 100 chance (1% probability) of being equaled or exceeded in any given year.Viessman Warren Introduction to Hydrology 1977 Harper & Row, Publishers, Inc. 0-7002-2497-1 160 registration https://archive.org/details/introductiontohy00vies_0/page/160 Porter Jeremy FEMA's flood maps often miss dangerous flash flood risks, leaving homeowners unprepared The Conversation July 16, 2025 https://theconversation.com/femas-flood-maps-often-miss-dangerous-flash-flood-risks-leaving-homeowners-unprepared-260990 July 20, 2025 The estimated boundaries of inundation in a 100-year or 1% flood are marked on flood maps.What are Flood Zones and Maps? https://www.floodsmart.gov/flood-zones-and-maps April 4, 2025 www.floodsmart.gov UK planning guidance defines Flood Zone 3a "High Probability" as Land having a 1% or greater annual probability of river flooding; or Land having a 0.5% or greater annual probability of sea.Flood risk and coastal change GOV.UK March 6, 2014 https://www.gov.uk/guidance/flood-risk-and-coastal-change#para77 July 26, 2025 Maps, elevations and flow rates For coastal flooding and lake flooding, a 100-year flood is generally expressed as a water level elevation or depth, and includes a combination of tide, storm surge, and waves.Coastal Flooding https://www.floodsmart.gov/floodsmart/pages/coastal_flooding/cf_overview.jsp FloodSmart National Flood Insurance Program March 7, 2016 NFIP coastal https://web.archive.org/web/20160308052549/https://www.floodsmart.gov/floodsmart/pages/coastal_flooding/cf_overview.jsp March 8, 2016 dead For river systems, a 100-year flood can be expressed as a flow rate, from which the flood elevation is derived. The resulting area of inundation is referred to as the 100-year floodplain. Estimates of the 100-year flood flow rate and other streamflow statistics for any stream in the United States are available.https://pubs.usgs.gov/publication/fs20083067 StreamStats: A water resources web application Ries Kernell G. III Guthrie John D. Rea Alan H. Steeves Peter A. Stewart David W. 2008 U.S. Geological Survey en July 12, 2015 A 100-year storm may or may not cause a 100-year flood, because of rainfall timing and location variations among different drainage basins, and independent causes of floods, such as snow melt and ice dams. In the UK, the Environment Agency publishes a comprehensive map of all areas at risk of a 100-year flood.http://maps.environment-agency.gov.uk/wiyby/wiybyController?x=460730.0&y=189442.0&topic=floodmap&ep=map&scale=9&location=OX10%200AJ&lang=_e&layerGroups=default&distance=&textonly=off Environment Agency Flood Map for Planning (Rivers and Sea) August 25, 2016 2016 https://web.archive.org/web/20160916225532/http://maps.environment-agency.gov.uk/wiyby/wiybyController?x=460730.0&y=189442.0&topic=floodmap&ep=map&scale=9&location=OX10%200AJ&lang=_e&layerGroups=default&distance=&textonly=off September 16, 2016 dead In the US, the Federal Emergency Management Agency publishes maps of the 100-year and 500-year floodplains. Maps of the riverine or coastal 100-year floodplain may figure importantly in building permits, environmental regulations, and flood insurance. These analyses generally represent 20th-century climate and may underestimate the effects of climate change. Risk thumb|Observed intervals between floods at Passau, 1501–2013 A common misunderstanding is that a 100-year flood happens once in a 100-year period. On average one happens per 100 years, five per 500 years, ten per thousand years. Any average hides variations. In any particular 100 years at one spot, there is a 37% chance that no 100-year flood happens, 37% chance that exactly one happens, and 26% chance that two or more happen. On the Danube River at Passau, Germany, the actual intervals between 100-year floods during 1501 to 2013 ranged from 37 to 192 years.Eychaner James H. 2015 Lessons from a 500-year Record of Flood Elevations https://asfpm-library.s3.us-west-2.amazonaws.com/ASFPM_Pubs/asfpmpubs-techrep7_2015.pdf November 20, 2021 Association of State Floodplain Managers. Technical Report 7 Madison, Wisconsin, United States A related misunderstanding is that floods bigger than 100-year floods are too rare to be of concern. The 1% chance per year accumulates to 10% chance per decade, 26% chance during a 30-year mortgage,February 4, 2021 100-Year Flood - It's All About Chance https://www.usgs.gov/media/images/100-year-flood-its-all-about-chance April 4, 2025 U.S. Geological Survey and 55% chance during an 80-year human lifetime. It is common to refer to 100-year floods as floods with 1% chance per year. It is equally true to refer to them as floods with 10% chance per decade. Over a large diverse area, such as a large country or the world, in an average year 1% of watersheds have 100-year floods or bigger, and 0.1% of watersheds have 1,000-year floods or bigger. There are more in wet years, fewer in dry years. Of 1.6 million kilometers of coastline in the world,Burke Lauretta 2001 Coastal Ecosystems http://pdf.wri.org/page_coastal.pdf April 5, 2025 World Resources Institute Kura Yumiko Kassem Ken Revenga Carmen Spalding Mark Mcallister Don in an average year 1,600 kilometers have 1,000-year floods or bigger, more in stormy years, fewer in calmer years. The US flood insurance program, starting in the 1960s, chose to foster rules in, and insure buildings in, 100-year floodplains, as "a fair balance between protecting the public and overly stringent regulation." After the North Sea flood of 1953, the United Kingdom mapped 1,000-year floods and the Netherlands raised its flood defenses to protect against up to 10,000 year floods.Caires Sofia 2011 Extreme Value Analysis: Still Water Level https://www.jodc.go.jp/info/ioc_doc/JCOMM_Tech/JCOMM-TR-058.pdf April 4, 2025 JCOMM Technical Report No. 58 In 2017 the Netherlands designed some areas against million-year floods.November 1, 2017 Fundamentals of Flood Protection, p.48 https://www.enwinfo.nl/publish/pages/183541/grondslagenen-lowresspread3-v_3.pdf April 4, 2025 Expertise Network Water Safety The American Society of Civil Engineers recommends designing some structures for up to 1,000-year floods,Coulbourne William L. August 1, 2024 Major Changes to ASCE 7-22 Flood Loads, p.53 https://www.structuremag.org/issue/august-2024/ April 4, 2025 National Council of Structural Engineers Associations Cox Daniel Mandrick Jessica while it recommends designing for up to 3,000-year winds.May 1, 2023 Supplement 2 for Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-22) https://ascelibrary.org/doi/epdf/10.1061/9780784415788.sup2 April 4, 2025 ascelibrary.org 10.1061/9780784415788.sup2 Per century, any one area has a 63% chance of a 100-year flood or worse, 10% chance of a 1,000-year flood, 1% chance of a 10,000-year flood, and 0.01% chance of a million-year flood. As David van Dantzig, working on the government response to the 1953 flood, said, "One will surely be willing to spend a multiple of the amount that would be lost by a flood if the flood can thereby be prevented."Danzig David van 1956 Economic Decision Problems for Flood Prevention https://ir.cwi.nl/pub/8233/8233D.pdf April 4, 2025 Econometrica, J of the Econometric Society Flood insurance Flood insurance In the United States, the 100-year flood provides the risk basis for flood insurance rates. 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 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. Coastal flood hazards have been mapped by a similar approach that includes the relevant physical processes. Most areas where serious floods can occur in the United States have been mapped consistently in this manner. On average nationwide, those 100-year flood estimates are sufficient for the purposes of the National Flood Insurance Program (NFIP) and offer reasonable estimates of future flood risk, if the future is like the past.24 Approximately 3% of the U.S. population lives in areas subject to the 1% annual chance coastal flood hazard.Crowell Mark others An estimate of the U.S. population living in 100-year coastal flood hazard areas Journal of Coastal Research 2010 26 2 201–211 10.2112/JCOASTRES-D-09-00076.1 9381124 http://www.floods.org/PDF/JCR_Est_US_Pop_100y_CFHA_2010.pdf March 6, 2016 Crowell October 17, 2016 https://web.archive.org/web/20161017232219/http://www.floods.org/PDF/JCR_Est_US_Pop_100y_CFHA_2010.pdf dead In theory, removing homes and businesses from areas that flood repeatedly can protect people and reduce insurance losses, but in practice it is difficult for people to retreat from established neighborhoods.Schwartz Jen Surrendering to rising seas Scientific American August 1, 2018 319 2 44–55 https://www.scientificamerican.com/article/surrendering-to-rising-seas/ October 2, 2018 10.1038/scientificamerican0818-44 30020899 240396828 subscription Probability The probability Pe that one or more floods occurring during any period will exceed a given flood threshold can be expressed, using the binomial distribution, as where T is the threshold mean recurrence interval (e.g. 100-yr, 50-yr, 25-yr, and so forth), greater than 1. If T is in years, then n is the number of years in the period, and Pe is the chance per year, Annual Exceedance Probability.2015 Key USACE Flood Risk Management Terms https://www.hec.usace.army.mil/publications/TrainingDocuments/TD-40.pdf April 5, 2025 US Army Corps of Engineers The formula can be understood as: Chance per year of a T-year flood is 1/T for example 1/100 = 0.01 Chance per year of no such flood is 1 − 1/T for example 1 − 0.01 = 0.99 Chance that n independent years have no such flood, by multiplying, is (1 − 1/T)n for example 0.99100 = 0.366 Chance of at least one flood in n years is 1 − (1 − 1/T)n for example 1 − 0.99100 = 0.634 = 63.4% The probability of exceedance Pe is also described as the natural, inherent, or hydrologic risk of failure.Mays, L.W. (2005). Water Resources Engineering, chapter 10, Probability, risk, and uncertainty analysis for hydrologic and hydraulic design. Hoboken: J. Wiley & SonsMaidment, D.R. ed.(1993). Handbook of Hydrology, chapter 18, Frequency analysis of extreme events. New York: McGraw-Hill However, the expected value of the number of 100-year floods occurring in any 100-year period is 1. Ten-year floods have a 10% chance of occurring in any given year (Pe =0.10); 500-year floods have a 0.2% chance of occurring in any given year (Pe =0.002); etc. The percent chance of a T-year flood occurring in a single year is 100/T, where T is bigger than 1. During this many years, chance of at least one storm of severity shown on left, or worseStorm with chance shown below, of being equaled or exceeded each year1 year10 years30 years50 years80 years100 years200 years1/100 1.0%9.6%26.0%39.5%55.2%63.4%86.6%1/500 0.2%2.0%5.8%9.5%14.8%18.1%33.0%1/1,0000.1%1.0%3.0%4.9%7.7%9.5%18.1%1/10,0000.0%0.10%0.30%0.50%0.80%1.00%1.98%1/1,000,0000.0001%0.001%0.003%0.005%0.008%0.010%0.020% The same formula above can give the chance of occurrence in less than a year. A 1-year flood is a 12-month flood, with a 1/12 chance each month, and the formula in months shows 65% chance each year. The field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering. This theory is most commonly applied to the maximum or minimum observed stream flows of a given river. In desert areas where there are only ephemeral washes, this method is applied to the maximum observed flow over a given period of time (24-hours, 6-hours, or 3-hours). The extreme value analysis only considers the most extreme event observed in a given year. So, between the large spring runoff and a heavy summer rain storm, whichever resulted in more runoff would be considered the extreme event, while the smaller event would be ignored in the analysis (even though both may have been capable of causing terrible flooding in their own right). Statistical assumptions There are a number of assumptions that are made to complete the analysis that determines the 100-year flood. First, the extreme events observed in each year must be independent from year to year. 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. The second assumption is that the observed extreme events must come from the same probability density function. 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. 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. This concept is referred to as stationarity.England John and seven others Guidelines for determining flood flow frequency—Bulletin 17C Guidelines for determining flood flow frequency — Bulletin 17C Techniques and Methods March 29, 2018 U.S. Geological Survey 10.3133/tm4B5 134656108 https://pubs.er.usgs.gov/publication/tm4B5 October 2, 2018 The first assumption is often but not always valid and should be tested on a case-by-case basis. The second assumption is often valid if the extreme events are observed under similar climate conditions. For example, if the extreme events on record all come from late summer thunderstorms (as is the case in the southwest U.S.), or from snow pack melting (as is the case in north-central U.S.), then this assumption should be valid. 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. The third assumption is only a problem when trying to forecast a low, but maximum flow event (for example, an event smaller than a 2-year flood). 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 (see next section). More broadly, substantial evidence of climate change strongly suggests that the probability distribution is also changingScience Magazine 319 5863 573–574 Stationarity is Dead Sciencemag.org February 1, 2008 10.1126/science.1151915 18239110 Milly P. C. D. Betancourt J. Falkenmark M. Hirsch R. M. Kundzewicz Z. W. Dennis P. Lettenmaier Lettenmaier D. P. Stouffer R. J. 206509974 and that managing flood risks in the future will become even more difficult.Intergovernmental Panel on Climate Change (2012). Managing the risks of extreme events and disasters to advance climate change adaptation, Summary for policymakers https://web.archive.org/web/20150719211442/http://ipcc-wg2.gov/SREX/images/uploads/SREX-SPMbrochure_FINAL.pdf July 19, 2015 Cambridge and New York: Cambridge University Press, 19 p. 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. 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. Direct statistical analysisBulletin 17C https://acwi.gov/hydrology/Frequency/b17c/ Advisory Committee on Water Information October 2, 2018 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.Water Data for the Nation https://waterdata.usgs.gov/ January 30, 2014 U.S. Geological Survey For comparison, the total area of the nation is about 3,800,000 sqmi km2, so there are perhaps 3,000 stream reaches that drain watersheds of 1,000 sqmi km2 and 300,000 reaches that drain 10 sqmi km2. In urban areas, 100-year flood estimates are needed for watersheds as small as 1 sqmi km2. For reaches without sufficient data for direct analysis, 100-year flood estimates are derived from indirect statistical analysis of flood records at other locations in a hydrologically similar region or from other 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.Obtaining Tide Gauge Data http://www.psmsl.org/data/obtaining/ Permanent Service for Mean Sea Level PSMSL March 7, 2016 PSMSL_data thumb|High-water scale 1501–2002 at 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.24 For the largest recorded flood at any specific location, or any potentially larger event, the recurrence interval always is poorly known.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.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.7,24 Observed intervals between floods Peak elevations of 14 floods as early as 1501 on the Danube River at Passau, Germany, reveal great variability in the actual intervals between floods.16–19 Flood events greater than the 50-year flood occurred at intervals of 4 to 192 years since 1501, and the 50-year flood of 2002 was followed only 11 years later by a 500-year flood. Only half of the intervals between 50- and 100-year floods were within 50 percent of the nominal average interval. Similarly, the intervals between 5-year floods during 1955 to 2007 ranged from 5 months to 16 years, and only half were within 2.5 to 7.5 years. See also Extreme value theory Extreme weather Flood forecasting Frequency of exceedance List of floods Lists of floods in the United States References 35em External links "What is a 100 year flood?". Boulder Area Sustainability Information Network (BASIN). URL accessed 2006-06-16. "Flood Extreme Analysis". GeoTide Extreme Analysis Software. URL accessed 2023-11-28. Category:Actuarial science Category:Extreme value data Category:Flood control Category:Hydrology Category:Flood insurance