FAQ's about Floods

As with many natural phenomena, tsunamis can range in size from micro-tsunamis detectable only by sensitive instruments on the ocean floor to mega-tsunamis that can affect the coastlines of entire oceans, as with the Indian Ocean tsunami of 2004. If you hear a tsunami warning or if you feel strong shaking at the coast or very unusual wave activity (e.g., the sea withdrawing far from shore), it is important to move to high ground and stay away from the coast until wave activity has subsided (usually several hours to days). For more general information on tsunamis and what to do during a tsunami warning, please visit sites sponsored by FEMA, the National Weather Service, NOAA, and the USGS.

The term "100-year flood," is used to describe the recurrence interval of floods. As the table below shows, 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 event will occur.

But, just because a 100-year flood happened last year doesn't mean that it won't happen this year, too. In other words, future rainfall and floods don't depend on the rainfall and floods that happened in the past. The past records are mainly used to show what kind of river flows can be expected. So, when you hear about a 100-year flood, at least you have a general idea that it does mean a BIG flood, and if you hear of a 200-year flood you know that it means one even BIGGER! As an example, in July of 1994, some places in south Georgia received more than 20 inches of rainfall in a few days -- the floods they produced were tremendous... way over the 100-year flood. At Senoia, Ga., the maximum amount of water flowing by the Line Creek gage was 2.4 times greater than the 100-year flood level.

Go to the Natural Hazards Gateway, which includes:

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Several types of data can be collected to assist hydrologists predict when and where floods might occur. The first is monitoring the amount of rainfall occurring on a realtime basis. Second, monitoring the rate of change in river stage on a realtime basis can help indicate the severity and immediacy of the threat. Third, knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, is valuable for determining possible severity of the flooding. And fourth, 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, can help to predict how extensive and damaging a flood might become.

The National Weather Service collects and interprets rainfall data throughout the United States and issues flood watches and warnings as appropriate. The National Weather Service uses statistical models and flood histories to try to predict the results of expected storms. The USGS maintains a network of streamflow-gaging stations throughout the country for which the discharge and stage are monitored. Flood estimation maps are generally produced by estimating a flood with a certain recurrence interval or probability and simulating the inundation levels based on flood plain and channel characteristics. More information on floods is available from the USGS Hydrologic Information Center at http://www.nws.noaa.gov/oh/hic and from the USGS national home page at http://water.usgs.gov. For more information on real-time flood monitoring, please see USGS Fact Sheet FS-209-95, which is available on-line at http://water.usgs.gov/public/wid/FS_209-95/mason-weiger.html.

Although Mount Rainier (Washington) has not produced a significant eruption in the past 500 years, it is potentially the most dangerous volcano in the Cascade Range because of its great height, frequent earthquakes, active hydrothermal system, and extensive glacier mantle. Mount Rainier has 26 glaciers containing more than five times as much snow and ice as all the other Cascade volcanoes combined. If only a small part of this ice were melted by volcanic activity, it would yield enough water to trigger enormous lahars. Mount Rainier's potential for generating destructive mudflows is enhanced by its great height above surrounding valleys. -- From: Scott, et.al., 1990, Sedimentology, Behavior, and Hazards of Debris Flows at Mount Rainier, Washington: USGS Open-File Report 90-385, and Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

How high and how fast a river will rise during a storm depends on many things. Most important, of course, is how much rain is falling. But also we have to look at other things, such as the stage of the river when the storm begins, at what the soil is like in the drainage basin where it is raining (is the soil already saturated with water from a previous storm?), and at how hard and in what parts of the basin the rain is falling. The USGS has studied these things at many places across the country for many years, and thus is often able to make predictions about if and where a flood will occur and how bad that flood will be.

With the advent of modern computer and satellite technology, the USGS can monitor the stage of many streams almost instantly. Since some streams, especially those in the normally arid Western U.S., can rise dramatically in a matter of minutes during a major storm, it is important to be able to remotely monitor how fast water is rising "in real time" in order to warn people that might be affected by a dangerous flood. Recreational users of streams, such as kayakers, also use "real-time" stream-stage data to tell them if certain streams are at the right height for kayaking. The USGS can now gather data on stream stage and even produce graphs showing stage as the rain is falling. In fact, some of these real-time data and graphics are being made available for you to use via the World Wide Web. You can access current stream conditions for your state right now.

The current El Niño will probably surpass the greatest El Niño of century, that of 1982-83. During the past 40 years, nine El Niños have affected the western coasts of North and South America. Most of them raised water temperatures along 5000 miles of coast. The weaker events raised sea temperatures only a few degrees Fahrenheit and caused mild changes in weather. But the strong ones, like the El Niño of 1982-83, left a climatic imprint that was global in extent.

El Niño recurs irregularly, from two years to a decade, and no two events are exactly alike. Before the 1982-83 El Niño event, scientists did not collect detailed information on El Niños, so information is scanty for making high-quality predictions about the effects of the current El Niño of 1997-98.

The impacts of El Niños can be devastating, as illustrated by some of the effects of the unusually strong El Niño of 1982-83:

- Drought (sometimes with associated wildfires) in many nations (particularly in the western Pacific Rim, southern and northern Africa, southern Asia, southern Europe, and parts of South and Central America);- Severe cyclones that damaged island communities in the Pacific;- Flooding over wide areas of South America, western Europe, and the Gulf Coastal states; - Severe storms in the western and northeastern United States.

A: For flood insurance maps, contact the Federal Emergency Management Agency (FEMA), Flood Map Division.

Telephone: 1-800-358-9616
FAX: 1-800-333-1363
URL: http://store.msc.fema.gov

For prints of historical flood-prone area maps on microfilm, contact:

U.S. Geological Survey
Earth Science Information Center
507 National Center
Reston, VA 20192

Telephone: 1-888-ASK-USGS (1-888-275-8747) or 703-648-5953
FAX: 703-648-5548
TDD: 703-648-4119
E-mail: ask@usgs.gov

Please see Can It Happen Here?

The U.S. Geological Survey (USGS) stream-gaging program provides streamflow data for a variety of purposes that range from current needs, such as flood forecasting, to future or long-term needs, such as detection of changes in streamflow due to human activities or global warming. The development of data on the flow of the Nation's rivers mirrors the development of the country. From the establishment of the first stream-gaging station operated by the USGS in 1889, this program has grown to include 7,292 stations in operation as of 1994. Data from the active stations, as well as from discontinued stations, are stored in a computer data base that currently holds mean daily-discharge data for about 18,500 locations and more than 400,000 station-years of record. The stream-discharge data base is an ever-growing resource for water-resources planning and design, hydrologic research, and operation of water-resources projects.

The USGS stream-gaging program provides hydrologic information needed to help define, use, and manage the Nation's water resources. The program provides a continuous, well-documented, well-archived, unbiased, and broad-based source of reliable and consistent water data. Because of the nationally consistent, prescribed standards by which the data are collected and processed, the data from individual stations are commonly used for purposes beyond the original purpose for an individual station. Those possible uses include the following:

• Enhancing the public safety by providing data for forecasting and managing floods
• Characterizing current water-quality conditions
•  Determining input rates of various pollutants into lakes, reservoirs, or estuaries
• Computing the loads of sediment and chemical constituents
• Understanding the biological effects of contamination
• Delineating and managing flood plains
• Setting permit requirements for discharge of treated wastewater
• Designing highway bridges and culverts
• Setting minimum flow requirements for meeting aquatic life goals
• Monitoring compliance with minimum flow requirements
• Developing or operating recreation facilities
• Scheduling power production
• Designing, operating, and maintaining navigation facilities
• Allocating water for municipal, industrial, and irrigation uses
• Administering compacts or resolving conflicts on interstate rivers
• Defining and apportioning the water resources at our international borders
• Evaluating surface- and ground-water interaction
• Undertaking scientific studies of long-term changes in the hydrologic cycle

Data for one or more of these purposes are needed at some point in time on virtually every stream in the country, and a data-collection system must be in place to provide the required information. The general objective of the stream-gaging program is to provide information on or to develop estimates of flow characteristics at any point on any stream. Streamflow data are needed for immediate decision making and future planning and project design. Data, such as that needed to issue and update flood forecasts, are referred to as "data for current needs." Other data, such as that needed for the design of a future, but currently unplanned, bridge or reservoir or development of basinwide pollution control plans, are referred to as "data for future or long-term needs." Some data, of course, fit into both classifications; for example, a station that supplies data for flood forecasting and also provides data to define long-term trends. Reference: Wahl, K.L., Thomas, W.O., Jr., and Hirsch, R.M., 1995The stream-gaging program of the U.S. Geological Survey: U.S. Geological Survey Circular 1123, 22 p.

For more information on the National Streamflow Information Network (NSIP) go to: http://water.usgs.gov/nsip/

During the past 10,000 years, about 60 giant debris flows from Mount Rainier have filled river valleys to a depth of hundreds of feet near the volcano, and have buried the land surface under many feet of mud and rock sixty miles downstream. Seven debris flows large enough to reach Puget Sound have occurred in the past 6,000 years. -- From: Walder and Driedger, 1995, Living With a Volcano in Your Backyard - Volcanic Hazards at Mount Rainier: USGS Open-File Report 95-421.

The Pacific Tsunami Warning Center is responsible for tsunami monitoring in the Pacific Basin. Their website is at http://www.prh.noaa.gov/ptwc/. Tragically, no such system existed for the Bay of Bengal where the devastating earthquake and tsunami occurred in December 2004.

1. 1797: A magnitude 8.4 earthquake near the central part of the western Sumatra generated a tsunami that flooded Padang. More than 300 fatalities.

2. 1833: A magnitude 8.7 earthquake near the south coast of the western Sumatra triggered a huge tsunami that flooded the southern part of western Sumatra. Numerous victims.

3. 1843: A tsunami that came from the southeast and flooded the coast of the Nias Island. Many fatalities.

4. 1861: A magnitude 8.5 earthquake affected all the western coast of Sumatra. Several thousand fatalities.

5. 1881: A magnitude 7.9 earthquake in the Andaman Island region generated a 1 m high tsunami on India's eastern coast. (http://cires.colorado.edu/~bilham/Oldham1881account.htm

6. 1883: Krakatau explosion. 36,000 fatalities, primarily on the islands of Java and Sumatra.

7. 1941: A magnitude ~7.7 Adaman Islands earthquake. Anecdotal accounts exist of a tsunami, however, no official records exist.

References:
Tsunami Laboratory, Institute of Computational Mathematics and Mathematical Geophysics
(http://tsun.sscc.ru/tsulab/20041226tsun.htm)
National Geophysical Data Center (http://www.ngdc.noaa.gov/seg/hazard/hazards.shtml)
K. Sieh (http://www.gps.caltech.edu/~sieh/publications.htm)
R. Bilham (http://cires.colorado.edu/~bilham/IndonesiAndaman2004.htm)
Oritz and Billham, 2003 JGR, VOL. 108, NO. B4, pp 2215

WaterQualityWatch is a USGS web site that provides access to real time water-quality monitor data collected in surface waters throughout the United States as part of the USGS mission to describe water resources.

Measurements include streamflow (through WaterWatch) water temperature, specific conductance, pH, dissolved oxygen, and turbidity. These measurements are available at more than 1,300 sites in streams with watersheds as small as a few square miles to more than 1,000,000 square miles in the Mississippi River as it enters the Gulf of Mexico.

Continuous real-time water-quality data are used for decisions regarding drinking water, water treatment, regulatory programs, recreation, and public safety. Additionally, links to other USGS technical resources and how these measurements are used as surrogates to obtain real-time computations or estimates of other water quality constituents are provided.

Most currently available flood maps are used to assist planners in identifying and preparing for flooding scenarios. These maps portray statistics based on long-term historical records to estimate and forecast an approaching weather system. For more information on the techniques of flood statistics reports see: http://pubs.usgs.gov/twri/.

The U.S. Geological Survey (USGS) and the National Weather Service (NWS) have developed a way to bring flood forecasting and flood mapping together, using available technologies, to produce maps which can be served on the Internet in time to allow communities to prepare for potential flooding. More information on this project can be obtained by downloading the USGS fact sheet at: http://pubs.water.usgs.gov/fs2004-3060/; or checking out the informational page at: http://wa.water.usgs.gov/projects/pugethazards/urbanhaz/MappingNWS.htm. For a demonstration of the system go to: http://wa.water.usgs.gov/cgi/flood_snoqualmie.cgi.

WaterWatch, the USGS information on current water resources conditions, is found at: http://water.usgs.gov/waterwatch/.

FEMA Flood Maps are available free at: http://msc.fema.gov. These maps are often used in regard to property damage and for local planners defining escape routes.

There are two basic kinds of floods, flash floods and the more widespread river flooding. 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 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, but over a larger area. Floods also can be caused by ice jams on a river, or high tides. However, most floods can be linked to a storm of some kind.

The USGS provides access to water-resources data collected at approximately 1.5 million sites in all 50 States, the District of Columbia, and Puerto Rico. Online access to this data is organized around the categories listed to the left.

The USGS investigates the occurrence, quantity, quality, distribution, and movement of surface and underground waters and disseminates the data to the public, State and local governments, public and private utilities, and other Federal agencies involved with managing our water resources.

This information is available at: http://waterdata.usgs.gov/nwis

Go to the USGS National Water Information System Web site (NWISWeb) for a tutorial on how to use NWISWeb.