Significant Floods in the United States During the 20th Century - USGS Measures a Century of Floods

The USGS currently (2000) maintains more than 7,000 stream-gaging stations throughout the United States, Puerto Rico, and the Virgin Islands that monitor streamflow and provide data to various Federal, State, and local cooperating agencies as well as the general public. Some of these stream-gaging stations have been in operation since before 1900, providing more than a century of water informaton for the Nation. In addition to providing critical information on flood heights and discharges, these stations provide data used in the effective management of water-supply and water-quality needs, protection of aquatic habitat, recreation, and water-resources research.

The basic building block for a stream-flow data network is the stage-discharge relation that is developed at each gaging-station location. Measurements of the flow (discharge) are related graphically to the respective water depths (stage), which then enables discharge to be determined from stage data.

Discharge measurements can either be direct, using a current meter, or indirect, using mathematical flow equations. Both methods require that an elevation of the floodwater surface be determined by a water-depth gage or by a detailed survey of high-water marks. If time allows and conditions are safe, a direct measurement by USGS hydrographers is preferred. However, during major floods, direct measurements are often impossible or extremely dangerous, and indirect methods must be used.

During the 20th century, floods were the number-one natural disaster in the United States in terms of number of lives lost and property damage. They can occur at any time of the year, in any part of the country, and at any time of the day or night. Most lives are lost when people are swept away by flood currents, whereas most property damage results from inundation by sediment-laden water. Flood currents also possess tremendous destructive power, as lateral forces can demolish buildings and erosion can undermine bridge foundations and footings leading to the collapse of structures. The accompanying map and table locate and describe 32 of the most significant floods of the 20th century.

Some regional floods occur seasonally when winter or spring rains coupled with melting snow fill river basins with too much water too quickly. The ground may be frozen, reducing infiltration into the soil and thereby increasing runoff. Such was the case for the New England flood of March 1936 in which more than 150 lives were lost and property damage totaled $300 million.

Extended wet periods during any part of the year can create saturated soil conditions, after which any additional rain runs off into streams and rivers, until river capacities are exceeded. Regional floods are many times associated with slow-moving, low-pressure or frontal storm systems including decaying hurricanes or tropical storms. Persistent wet meteorological patterns are usually responsible for very large regional floods such as the Mississippi River Basin flood of 1993 wherein damages were $20 billion.

Flash floods can occur within several seconds to several hours, with little warning. Flash floods can be deadly because they produce rapid rises in water levels and have devastating flow velocities.

Ice-jam floods occur on rivers that are totally or partially frozen. A rise in stream stage will break up a totally frozen river and create ice flows that can pile up on channel obstructions such as shallow riffles, log jams, or bridge piers. The jammed ice creates a dam across the channel over which the water and ice mixture continues to flow, allowing for more jamming to occur. Backwater upstream from the ice dam can rise rapidly and overflow the channel banks. Flooding moves downstream when the ice dam fails, and the water stored behind the dam is released. At this time the flood takes on the characteristics of a flash flood, with the added danger of ice flows that, when driven by the energy of the floodwave, can inflict serious damage on structures. An added danger of being caught in an ice-jam flood is hypothermia, which can quickly kill. Ice jams on the Yukon River in Alaska contributed to severe flooding during the spring breakup of 1992.

Storm-surge flooding is water that is pushed up onto otherwise dry land by onshore winds. Friction between the water and the moving air creates drag that, depending upon the distance of water (fetch) and the velocity of the wind, can pile water up to depths greater than 20 feet. Intense, low-pressure systems and hurricanes can create storm-surge flooding. The storm surge is unquestionably the most dangerous part of a hurricane as pounding waves create very hazardous flood currents.

Nine out of 10 hurricane fatalities are caused by the storm surge. Worst-case scenarios occur when the storm surge occurs concurrently with high tide. Stream flooding is much worse inland during the storm surge because of backwater effects. In September 1900, the hurricane and storm surge at Galveston, Texas, killed more than 6,000 people, making it the worst natural disaster in the Nation's history.

Dams and levees are built for flood protection. They usually are engineered to withstand a flood with a computed risk of occurrence. For example, a dam or levee may be designed to contain a flood at a location on a stream that has a certain probability of occurring in any 1 year. If a larger flood occurs, then that structure will be overtopped. If during the overtopping the dam or levee fails or is washed out, the water behind it is released to become a flash flood. Failed dams or levees can create floods that are catastrophic to life and property because of the tremendous energy of the released water. Warnings of the Teton Dam failure in Idaho in June 1976 reduced the loss of life to 11 people.

Debris or landslide floods are created by the accumulation of debris, mud, rocks, and (or) logs in a channel, which form a temporary dam. Flooding occurs upstream as water becomes stored behind the temporary dam and then becomes a flash flood as the dam is breached and rapidly washes away. Landslides can create large waves on lakes or embayments and can be deadly. Mudflow floods can occur when volcanic activity rapidly melts mountain snow and glaciers, and the water mixed with mud and debris moves rapidly downslope. These mudflow events are also called lahars and, after the eruption of Mt. St. Helens in 1980, caused significant damage downstream along the Toutle and Cowlitz Rivers in southwest Washington.

Flood Insurance-http: //www.fema.gov/library/frnfip.htm
Dam Safety-http://www.usbr.gov/laws/damguide.html
Hurricane Safety-http://www.nws.noaa.gov/om/nh-hurr.htm
Flash-Flood Safety-http://www.nws.noaa.gov/om/nh-flsfd.htm
Regional Flood Safety-http://www.nws.noaa.gov/om/nh-flood.htm
River Forecast Centers-http://info.abrfc.noaa.gov/rfc_wfo.html
U.S. Army Corps of Engineers regional sites and districts-
    http://www.usace.army.mil/inet/organization/actlist2.htm
USGS Streamflow Data
    Historical-http://waterdata.usgs.gov/nwis-w/US/
    Peak flow-http://h2o.usgs.gov/public/realtime.htm
USGS Homepage-http://www.usgs.gov/

• Most flood-related deaths are due to flash floods.
• Fifty percent of all flash-flood fatalities are vehicle related.
• Ninety percent of those who die in hurricanes drown.
• Most homeowners insurance policies do not cover floodwater damage.
• Individuals and business owners can protect themselves from property losses by purchasing flood insurance through FEMA's National Flood Insurance Program.