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 duration, intensity and areal extent, which can be valuable for determining possible severity of the flooding. 
  • Knowledge about the characteristics of a river's drainage basin, such as soil-moisture conditions, ground temperature, snowpack, topography, vegetation cover, and impermeable land area, which can help to predict how extensive and damaging a flood might become.

The National Weather Service (an agency within NOAA) collects and interprets rainfall data throughout the United States and issues flood watches and warnings as appropriate. They use statistical models that incorporate USGS streamflow data to try to predict the results of expected storms.

The USGS maintains a network of streamflow-gaging stations throughout the country.

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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.

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...

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.

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 rainstorm, much of the rain will run...

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. Statistically, each year begins...

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: National Flood Forecasts Interactive Flood information Map Maps of flood events from the USGS: WaterWatch: Map of floods and high-water conditions Using...

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 normally dry channel. Flash...
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Date published: September 18, 2018

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.

Date published: February 22, 2017

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.

Attribution: Natural Hazards, Pacific
Date published: August 22, 2016

Fighting the Floods

The USGS response to the Louisiana floods is part of the larger USGS flood science mission...

Date published: January 14, 2011

ARkStorm: California’s other "Big One"

For emergency planning purposes, scientists unveiled a hypothetical California scenario that describes a storm that could produce up to 10 feet of rain, cause extensive flooding (in many cases overwhelming the state’s flood-protection system) and result in more than $300 billion in damage.

Date published: June 20, 2008

Two 500-Year Floods Within 15 Years: What are the Odds?

The term "500-year flood" has been used to describe the recent flooding in the Midwest.

Filter Total Items: 12
Image shows a flooded street with green median
August 13, 2016

Flooded Street in Denham Springs

Backwater flooding across Florida Blvd near the Amite River Bridge in Denham Springs, LA.

Picture of tow truck driver assisting car stuck in flood waters.
August 1, 2016

Flooded Morehead Street, Charlotte North Carolina

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)

video thumbnail: The Anatomy of Floods: The Causes and Development of 2011's Epic Flood Events
July 31, 2012

The Anatomy of Floods: The Causes and Development of 2011's Epic Flood Events

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

...
video thumbnail: 2011: The Year of the Flood
August 8, 2011

2011: The Year of the Flood

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.

...
October 14, 2009

Streamgages: The Silent Superhero

Whether you drink water from your tap, use electricity or canoe down your local river, chances are you benefit from USGS streamgage information. So what is a streamgage and what does it do for you? This CoreCast episode gives you the inside scoop on your silent superhero.

Transcript and captions available soon.

USGS
June 18, 2008

Two 500-Year Floods Within 15 Years?

We talk to Bob Holmes about some of the recent flooding events occuring in the Midwest, how does a 500-year flood occur twice in 15 years, and what do the recent events have in store for folks downriver.

USGS
June 4, 2008

How are floods predicted?

Listen to hear the answer.

USGS
February 5, 2008

We had a "100-year flood" two years in a row. How can that be?

Listen to hear the answer.