The USGS is developing methods to improve data collection during floods to gain new insight into the rise and fall of flood waters. In the past, the only data left behind after a flood was how high the water got, or the peak of the flood.
What are the two types of floods?
There are two basic types 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 water height (stage) of a stream or normally-dry channel. Flash floods are more common in areas with a dry climate and rocky terrain because lack of soil or vegetation allows torrential rains to flow overland rather than infiltrate into the ground.
River flooding is generally more common for larger rivers in areas with a wetter climate, when excessive runoff from longer-lasting rainstorms and sometimes from melting snow causes a slower water-level rise over a larger area. Floods also can be caused by ice jams on a river or high tides, but most floods can be linked to a storm of some kind.
Learn more:
Related Content
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 significant 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. Learn more: The 100-Year Flood The 100-Year Flood--It's All About Chance
How are floods predicted?
Flood predictions require several types of data: The amount of rainfall occurring on a real-time basis. The rate of change in river stage on a real-time 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...
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 rainstorm 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 off into streams, but if the ground is dry, it will...
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 with the same 1-percent chance that a 100-year...
What is a 1,000-year flood?
The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. These statistical values are based on observed data.
Where can I find flood maps?
FEMA is the official public source for flood maps for insurance purposes: FEMA’s Flood Map Service Center FEMA’s National Flood Hazard Layer NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data: National Weather Service - Advanced Hydrologic Prediction Service River Forecasts Long-Range River Flood Risk Coastal Inundation Dashboard : Real...
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...
Understanding Floods | Long-term Streamflow Data Collection
The USGS is developing methods to improve data collection during floods to gain new insight into the rise and fall of flood waters. In the past, the only data left behind after a flood was how high the water got, or the peak of the flood.
USGS hydrologic technician Amy Simonson measuring floodwaters at East Meadow Brook at Freeport NY
Ice Jams
Ice Jams in Redwater River near Vida, MT.
Ice Jams in Redwater River near Vida, MT.
Ice Jams
Ice Jams in Powder River at Arvada, WY, downstream of the bridge.
Ice Jams in Powder River at Arvada, WY, downstream of the bridge.
A Flooded Field in East Boulder, Colo.
Numerous rivers flooded during a significant September 2013 rain event along Colorado's Front Range, damaging or destroying several USGS streamgages. In response, USGS field crews measured flood flows, made gage repairs, and assessed sites to replace those gages destroyed.
Numerous rivers flooded during a significant September 2013 rain event along Colorado's Front Range, damaging or destroying several USGS streamgages. In response, USGS field crews measured flood flows, made gage repairs, and assessed sites to replace those gages destroyed.
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.
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.
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.
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.
Flooded Street Sign in Moorhead MN
Traffic sign surrounded by floodwater from Red River of the North at Moorhead, MN. Photo taken at 2nd Ave and 3rd St, Moorhead, MN.
Traffic sign surrounded by floodwater from Red River of the North at Moorhead, MN. Photo taken at 2nd Ave and 3rd St, Moorhead, MN.
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.
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.
What is a "500 year flood", and how do scientists determine if a flood is considered a "500 year flood"?
linkListen to hear the answer.
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.
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.
Flooded Neighborhood
Flooded neighborhood near Wilson Ave. SW and Hamilton Street SW
Flooded neighborhood near Wilson Ave. SW and Hamilton Street SW
Are there different types of flooding?
Listen to hear the answer.
We had a "100-year flood" two years in a row. How can that be?
Listen to hear the answer.
A flooded road in Highland County, Virginia.
Streamwater flowing over a rural road in Highland County, Virginia, June, 2006.
Streamwater flowing over a rural road in Highland County, Virginia, June, 2006.
Flash Floods: USGS Against the Water
A glimpse at the often dangerous task USGS employees make every year to obtain stream flow measurements from flood events.
A glimpse at the often dangerous task USGS employees make every year to obtain stream flow measurements from flood events.
Related Content
- FAQ
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 significant 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. Learn more: The 100-Year Flood The 100-Year Flood--It's All About Chance
How are floods predicted?
Flood predictions require several types of data: The amount of rainfall occurring on a real-time basis. The rate of change in river stage on a real-time 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...
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 rainstorm 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 off into streams, but if the ground is dry, it will...
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 with the same 1-percent chance that a 100-year...
What is a 1,000-year flood?
The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. These statistical values are based on observed data.
Where can I find flood maps?
FEMA is the official public source for flood maps for insurance purposes: FEMA’s Flood Map Service Center FEMA’s National Flood Hazard Layer NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data: National Weather Service - Advanced Hydrologic Prediction Service River Forecasts Long-Range River Flood Risk Coastal Inundation Dashboard : Real...
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...
- Multimedia
Understanding Floods | Long-term Streamflow Data Collection
The USGS is developing methods to improve data collection during floods to gain new insight into the rise and fall of flood waters. In the past, the only data left behind after a flood was how high the water got, or the peak of the flood.
The USGS is developing methods to improve data collection during floods to gain new insight into the rise and fall of flood waters. In the past, the only data left behind after a flood was how high the water got, or the peak of the flood.
USGS hydrologic technician Amy Simonson measuring floodwaters at East Meadow Brook at Freeport NY
Ice JamsIce Jams in Redwater River near Vida, MT.
Ice Jams in Redwater River near Vida, MT.
Ice JamsIce Jams in Powder River at Arvada, WY, downstream of the bridge.
Ice Jams in Powder River at Arvada, WY, downstream of the bridge.
A Flooded Field in East Boulder, Colo.Numerous rivers flooded during a significant September 2013 rain event along Colorado's Front Range, damaging or destroying several USGS streamgages. In response, USGS field crews measured flood flows, made gage repairs, and assessed sites to replace those gages destroyed.
Numerous rivers flooded during a significant September 2013 rain event along Colorado's Front Range, damaging or destroying several USGS streamgages. In response, USGS field crews measured flood flows, made gage repairs, and assessed sites to replace those gages destroyed.
The Anatomy of Floods: The Causes and Development of 2011's Epic Flood EventsFlooding 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.
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.
2011: The Year of the FloodDevastating 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.
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.
Flooded Street Sign in Moorhead MNTraffic sign surrounded by floodwater from Red River of the North at Moorhead, MN. Photo taken at 2nd Ave and 3rd St, Moorhead, MN.
Traffic sign surrounded by floodwater from Red River of the North at Moorhead, MN. Photo taken at 2nd Ave and 3rd St, Moorhead, MN.
Streamgages: The Silent SuperheroWhether 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.
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.
What is a "500 year flood", and how do scientists determine if a flood is considered a "500 year flood"?What is a "500 year flood", and how do scientists determine if a flood is considered a "500 year flood"?
linkListen to hear the answer.
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.
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.
Flooded NeighborhoodFlooded neighborhood near Wilson Ave. SW and Hamilton Street SW
Flooded neighborhood near Wilson Ave. SW and Hamilton Street SW
Are there different types of flooding?Listen to hear the answer.
We had a "100-year flood" two years in a row. How can that be?Listen to hear the answer.
A flooded road in Highland County, Virginia.Streamwater flowing over a rural road in Highland County, Virginia, June, 2006.
Streamwater flowing over a rural road in Highland County, Virginia, June, 2006.
Flash Floods: USGS Against the WaterA glimpse at the often dangerous task USGS employees make every year to obtain stream flow measurements from flood events.
A glimpse at the often dangerous task USGS employees make every year to obtain stream flow measurements from flood events.
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