Why do the values for the 100-year flood seem to change with every flood?
The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a "flood quantile". For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The estimates of the flood quantiles are calculated using actual data collected at a site.
For a particular river, the USGS collects data over time, determines the largest flood in each year, and then calculate statistical data for that river. The more years of data available, the more accurate the estimates for the various flood quantiles.
As more years of data become available, the estimates become more refined, which can result in the quantiles changing.
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What is a 1,000-year flood?
The term “1000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1000 chance of occurring in any given year. In terms of probability, the 1000-year flood has a 0.1% chance of happening in any given year.
These statistical values are based on observed data.
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Does an increase in the 100-year flood estimate originate from climate or land-use change?
Climate variability (dry cycles to wet cycles) and land-use change play a large role, but there is a large amount of uncertainty around the flood quantile estimates (the value of discharge corresponding to the 100-year flood), particularly if there isn’t a long record of observed data at a stream location.
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How can a 1,000-year rainfall not result in a 1,000-year flood?
It comes down to a number of factors, including the pattern of movement of the rain storm in each particular watershed, the conditions of the soil and plant matter in the watershed, and the timing of the rainstorm in one watershed versus other watersheds.
For example, if the ground is already saturated before a
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We had a "100-year flood" two years in a row. How can that be?
The term "100-year flood" is used to describe the recurrence interval of floods. The 100-year recurrence interval means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100.
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Where can I find flood maps?
FEMA’s Flood Map Service Center is the official public source for flood hazard information for insurance purposes.
NOAA (National Oceanic and Atmospheric Administration) has flood forecast maps:
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How are floods predicted?
Flood predictions require several types of data:
- The amount of rainfall occurring on a realtime basis.
- The rate of change in river stage on a realtime basis, which can help indicate the severity and immediacy of the threat.
- Knowledge about the type of storm producing the moisture, such as
What are the two types of floods?
There are two basic kinds of floods, flash floods and the more widespread river floods. Flash floods generally cause greater loss of life and river floods generally cause greater loss of property.
A flash flood occurs when runoff from excessive rainfall causes a rapid rise in the stage (water height) of a stream or
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Guidelines for determining flood flow frequency—Bulletin 17C
Accurate estimates of flood frequency and magnitude are a key component of any effective nationwide flood risk management and flood damage abatement program. In addition to accuracy, methods for estimating flood risk must be uniformly and consistently applied because management of the Nation’s water and related land resources is a collaborative...
England, John F.; Cohn, Timothy A.; Faber, Beth A.; Stedinger, Jery R.; Thomas, Wilbert O.; Veilleux, Andrea G.; Kiang, Julie E.; Mason, Robert R.
Popular myths about flooding in Western Washington
Floods are the most destructive natural hazard in the Nation, causing more deaths and financial loss in the 20th century than any other natural disaster. The most significant 20 riverine floods of the 20th century for which data are available have killed more than 1,843 people and caused more than $50 billion (uninflated) in damages (Perry, 2000...
Jones, Joseph L.
100-Year flood–it's all about chance
In the 1960's, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of...
Holmes, Robert R.; Dinicola, Karen
The world's largest floods, past and present: Their causes and magnitudes
Floods are among the most powerful forces on earth. Human societies worldwide have lived and died with floods from the very beginning, spawning a prominent role for floods within legends, religions, and history. Inspired by such accounts, geologists, hydrologists, and historians have studied the role of floods on humanity and its supporting...
O'Connor, Jim E.; Costa, John E.
Large Floods in the United States: where they happen and why
O'Connor, Jim E.; Costa, John E.
The "100-Year Flood"
Dinicola, Karen Floods and flood plains
Moreland, Joe A.
USGS Science – Leading the Way for Preparedness
Learn About USGS Hazards Science and More About National Preparedness Month: The very nature of natural hazards means that they have the potential to impact a majority of Americans every year. USGS science provides part of the foundation for emergency preparedness whenever and wherever disaster strikes.
Study Links Major Floods in North America and Europe to Multi-Decade Ocean Patterns
The number of major floods in natural rivers across Europe and North America has not increased overall during the past 80 years, a recent study has concluded. Instead researchers found that the occurrence of major flooding in North America and Europe often varies with North Atlantic Ocean temperature patterns.
Stormy weather: How the USGS goes to work monitoring its effects
Atmospheric rivers are a global weather phenomenon that can bring large amounts of rain or snow to the U.S. West Coast each year. These rivers of wet air form over the Pacific Ocean near Hawaiʻi and pick up large amounts of moisture from the tropics and on their way to the West Coast. This moisture is carried in narrow bands across the Pacific Ocean to California, Oregon, Washington and Nevada.
Fragmented Patterns Seen in the Recent History of U.S. Floods
Some regional trends; no widespread national pattern
A tow truck driver wades through waist-deep water to assist motorists as Stewart Creek overtops Morehead Street. (From FS 036-98; Photograph from The Charlotte Observer/Kent D. Johnson)
Devastation from the 1913 flood is shown primarily through photographs taken during the March 1913 flood. In the aftermath of the 1913 flood, State and Federal funds were allocated for the installation of a streamgage network to monitor the water level and flow of Ohio's rivers and streams. The modern streamgaging network and uses of streamflow data are described.
Flooding costs the United States more than $7 billion per year and claims more than 90 lives annually. During the Spring and Summer of 2011, the central U.S. experienced epic flooding, while Hurricane Irene followed by Tropical Storm Lee caused severe flooding in the east and northeastern U.S, setting numerous flood records at USGS streamgages. Dr. Robert Holmes discusses cause and effect of flooding, including a look at aspects of the 2011 epic flooding, and how USGS science assists in the overall flood mitigation efforts of the United States.
Peak flood height of 1996 flood, as observed in the Tualatin River, OR.
Devastating floods across much of the U.S. were severe and unrelenting during the spring and summer of 2011. When floods happen, USGS crews are among the first-responders. Often working in dangerous conditions, USGS scientists measure streamflow and river levels, repair and install streamgages, measure water quality and changes in sediment flow, and assess river changes. From flood forecasts to difficult decisions about flood management, science is critical for flood preparations and response.
USGS hydrographer Jerrod Wheeler (in cablecar) measures flood flows right before the gagehouse washes away.
06225500 Wind River near Crowheart, WY: Jul 01 2011; 13,900 ft3/s
Releases of 80,000 cubic feet per second from Oahe Dam near Pierre, SD, on May 27, 2011, during 2011 Missouri River flood.
Releases of 80,000 cubic feet per second from Oahe Dam near Pierre, SD, on May 27, 2011, during 2011 Missouri River flood.
Interview with USGS crews regarding the 2009 flooding events in Fargo, ND.
