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Home < FRM Welcome < FRM Table Of Contents. < FRM Flooding Introduction

Causes of Flooding

The most common cause of flooding is when the volume of water exceeds the capacity of the river or stream channel. Rivers are natural drainage channels for surface waters. Surface waters comprise two components: runoff and base flow. Runoff is that part of precipitation that flows toward the rivers or streams on the ground surface or within the soil (subsurface runoff or interflow). Base flow is the part of stream flow that enters the stream channel from groundwater.

Stream flow is affected by a number of factors (The Corps' Hydrologic Engineering Center (HEC) offers Hydrologic Engineering for Planning a hydrology course for non-hydrologists for those interested in more details than are provided here). The most important of these for the purposes of this manual are the amount and type of precipitation, the nature and condition of the drainage basin and climate. During a rainstorm, the amount, intensity and duration of the rain as well as the area of the storm and its path, all determine the surface water runoff that reaches a stream.

The amount, intensity and duration of rain affect the ability of the land to absorb the precipitation, which further affects the rate of runoff. The area and path of the storm in relation to the size of the watershed determine the area contributing runoff. The runoff rate and the area affected together determine the volume of water that will pass a given point downstream. The volume of water moving through the channel and the channel's dimensions and conditions determine the nature and extent of the flood.

The shape, size, soil type and topography of the drainage basin are other factors that can affect the quantity of water reaching the stream and the timing with which it arrives. Although some of these factors are constant, some (like the absorptive or shedding properties of the soil) vary with vegetation cover, season and previous rainfall.

Climate can also influence the relationship between precipitation and runoff. Frost makes most soil impenetrable if the soil contains moisture. Parched soil can also influence runoff rates. A large part of the year's precipitation may be stored in the form of snow in the Northern U.S. during winter. Heavy ice formation on rivers can also influence flooding.

Floods may result from one or more of the following causes:

  • Rainfall
  • Snowmelt runoff
  • Urban stormwater runoff
  • Coastal storms, tsunamis, cyclones, hurricanes
  • Ice jams and other obstructions
  • Dam failure or the failure of some other hydraulic structure
  • Catastrophic outbursts

Rainfall Flooding

Picture of flash flood warning sign.

Picture of flooded houses.

As noted above, rainfall is the most common cause of flooding in the U.S. The volume of water in the stream or river's channel simply exceeds its capacity to convey the water. As a result water begins to spill out of the channel onto the adjoining lands of the natural floodplain, which may have been significantly altered by human activity.

Floods can rise slowly or quickly. In many areas they may develop over a period of days. Flash floods can be extremely dangerous. Unanticipated, they usually happen on small watersheds as a result of a torrential downpour, often caused by heavy thunderstorm activity. In a flash flood, stream flow peaks within hours of the rainfall. Estimating damages due to rainfall floods is now a straightforward process.

Snowmelt Flooding

During winter in some parts of the U.S., most of the precipitation may be stored as snow or ice on the ground. As temperatures rise huge quantities of water are released. These floods are most common in spring but can occur as a result of sudden winter thaws. Heavy runoff can result from the rapid melting of the snow under the combined effect of sunlight, winds and warmer temperatures. If the ground is frozen, the water produced by the melting snow is unable to penetrate and runs off into streams and lakes. Flooding becomes even more severe if the snowmelt runoff is compounded by runoff from concurrent heavy rainfall. The later the spring thaw, the greater the risk of this compound flood problem. Snowmelt explains the prevalence of heavy spring runoff and flooding in some parts of the country.

Picture of stream bed and snow covered ground.

Urban Drainage (Stormwater Runoff) Flooding

Picture of car driving through flooded street. Urbanization drastically alters the drainage characteristics of the land. The slanted roofs, downspouts, storm gutters and stormwater conveyance systems increase the volume and rate of surface runoff. The urban runoff from intense rainfall can exceed the carrying capacity of the sewer system, creating a backup in the system. This backup often causes flooding of basements and low lying roads. Urban stormwater runoff can also cause local rivers to flood as well as the urban area itself. Although the impact on a major river may be minimal, the carrying capacity of small streams can be quickly exceeded, causing localized flooding and erosion problems.

Coastal Storm Flooding

High winds and wave action have created flood conditions on the seashores as well as on the shores of the Great Lakes and other large water bodies throughout the U.S. A related cause of flooding includes the interaction between high estuarine flows and tides. Storm surge or seiches occurring simultaneously with high waves can cause shoreline flooding. Every body of water has a set of natural periods of oscillation at which it is easy to set up motions called seiches. Surges are caused by sudden changes in atmospheric pressure and by the wind stress accompanying moving storm systems.

Picture of moored boat in a hurricane.

Storm systems occur frequently and some have the potential to cause abnormal water levels at coastlines. Determining water elevations during storms is a complex problem. It involves interactions between wind and water and differences in atmospheric pressure. Erosion damage can be a significant category of losses in these kinds of floods. For examples of erosion damage on the Great Lakes see Great Lakes Issues You are leaving a Federal Government web site. Click this icon for more information.. This makes estimating damages for such events complex and difficult.

Lake flooding can be complicated by the fact that it is often a weir flow that can last for extended periods of time in areas afflicted by high lake levels.

Tsunami Flooding

Tsunami is a Japanese term for "harbor wave." A tsunami, also known as a tidal wave, is the most spectacular coastal flooding event. A tsunami actually has nothing to do with the tides. An undersea movement such as an earthquake or a landslide causes a disturbance that gives a vertical motion to the water column resulting in a tsunami.

An earthquake of 7.0 on the Richter scale can generate a series of waves. In the Pacific Basin these waves have been known to travel at almost 570 mph over long distances with little loss of energy. Crests can be several hundred miles apart. As the wave approaches the coast it grows as it slows down. The mass of water that hits the shore can have both tremendous velocity as well as force behind it.

Estimating damages from these kinds of floods is very difficult because tsunamis are unique with respect to location, amplitude of waves and time between troughs. Because the source of the wave is always unknown, modeling these events remains a crude approximation. For an overview of recent tsunami events see Recent Tsunami Events You are leaving a Federal Government web site. Click this icon for more information.. The December 2004 tsunami in the Indian Ocean is well documented. See NOAA and the Indian Ocean Tsunami for a starting point. Informative publications can be found at After the Tsunami: Human Rights of Vulnerable Populations (PDF Size:39.8Kb) You are leaving a Federal Government web site. Click this icon for more information. and Hope for Renewal: Photographs from Indonesia After the Tsunami You are leaving a Federal Government web site. Click this icon for more information.. Several informative animations are also available on-line including: Savage Earth Animation You are leaving a Federal Government web site. Click this icon for more information., National Geographic You are leaving a Federal Government web site. Click this icon for more information. and NOAA.

Hurricane Flooding

The following materials were taken from the FEMA Hurricanes site.

A hurricane is a tropical storm with winds that have reached a constant speed of 74 mph or more. Hurricane winds blow in a large spiral around a relative calm center known as the "eye." The "eye" is generally 20 to 30 miles wide, and the storm may extend outward 400 miles. As a hurricane nears land, it can bring torrential rains, high winds and storm surges. A single hurricane can last for more than two weeks over open waters and can run a path across the entire length of the eastern seaboard. August and September are peak months during the hurricane season that lasts from June 1 through November 30. Hurricanes are called "typhoons" in the western Pacific Ocean, while similar storms in the Indian Ocean are called "cyclones."

Moving ashore, they sweep the ocean inward while spawning tornadoes and producing torrential rains and floods. Even more dangerous than the high winds of a hurricane is the storm surge, a dome of ocean water that can be 20 feet at its peak and 50 to 100 miles wide. The surge can devastate coastal communities as it sweeps ashore. Nine out of 10 hurricane fatalities are attributable to the storm surge.

Heavy rains and ocean waters brought ashore by strong winds can cause flooding. The runoff systems in many cities are unable to handle such an increase in water because of the gentle topography in many of the coastal areas where hurricanes occur. Hurricanes are capable of producing copious amounts of flash-flooding rainfall. During landfall, a hurricane rainfall of 10 to 15 inches or more is common. If the storm is large and moving slowly, less than 10 mph, the rainfall amounts from a well-organized storm may be even greater. To get a generic estimate of the rainfall amount (in inches) that can be expected, divide the storm's forward motion by 100, i.e., Forward Speed/100 = estimated inches of rain. Tropical Storm Claudette (1979) brought 45 inches of rain to an area near Alvin, Texas, contributing to more than $600 million in damage.

Picture of buildings being flooded by stormwater.

Estimating damages for hurricane floods is more difficult than for fluvial floods. Estimating wave damages, for example is one problem, separating out wind damage from water damage is another challenge. Nonetheless, hurricane flood damages are estimated routinely. To see some of the latest advances in this area see the Corps' Storm Damage Reduction Model.

2005 Hurricanes

Storm damage and flooding from Hurricanes Katrina and Rita during the 2005 hurricane season have produced extensive information documented in data, text and photographs. Some useful files and links include:

Hurricane Katrina: Analysis of the Impact on the Insurance Industry (PDF Size:537Kb)

Hurricane Katrina-Harvard Medical School You are leaving a Federal Government web site. Click this icon for more information.

Katrina Index: Tracking Variables of Post-Katrina Reconstruction (PDF Size:537Kb) You are leaving a Federal Government web site. Click this icon for more information.

NOVA: Storm That Drowned a City You are leaving a Federal Government web site. Click this icon for more information.

PBS: The Storm You are leaving a Federal Government web site. Click this icon for more information.

Be sure to check the FEMA and Corps' links to these storm events as they are completed.












Ice Jam Flooding

Ice jams are a major concern in some cold region parts of the country. Jams form during both the freeze-up and breakup periods of ice formation. They result from the accumulation of ice fragments that build up in a logjam fashion to restrict the flow of water. The jams act as a temporary obstruction to stream flow. The mechanics of ice jam flooding can be quite complex, for more information see the Ice Jam and Ice Flooding Clearinghouse. A brief overview is provided below.

Picture of ice floes left behind by floodwaters.

Ice floes left behind by floodwaters

Ice jam


Picture of ice jam.

During freeze-up ice jams usually form where floating ice slush or blocks, formed by frazil ice, encounter a stable ice cover. The beginning of the ice jam is the toe and the upstream end is the head. The stable ice is usually frozen to the banks or is restricted from moving by the channel configuration. Generally, incoming ice fragments either submerge and deposit under the stable ice cover, pile up behind it, or both. Bridge piers, islands, bends, shallows, slope reductions and constrictions can increase the likelihood of a jam forming. Ice jams in the spring result from accumulated ice from the breakup of the upstream ice cover.

Ice jams cause flooding for two reasons. First, ice jams can be very thick, many feet thick in some cases. Second, the underside of the ice cover is usually very rough. In an open stream the streambed is the only source of friction retarding the flow of water. The rougher the streambed, the greater the depth required to pass a given stream discharge. With an ice jam in place frictional resistance is greatly increased and the flow depth has to be much greater than for open water. Add the depth of water needed to float the ice jam to the depth required to maintain the discharge and extremely high water levels can occur, even at relatively small discharges.

When an ice jam suddenly is released it produces a surge of flow that can move at very rapid speeds. This surge can carry and deposit chunks of ice as large as automobiles, presenting a significant increase in damage potential for these kinds of floods. Estimating damages for ice jam floods is made difficult by the fact that it is very difficult to estimate the frequency of occurrence of an ice jam and the significance of damages caused by floating ice floes.

Dam Failure Flooding

Picture of dam.

Flooding can result from the failure of dams or other hydraulic structures. These failures can result in a wall of water being released in a surge down the river channel. The suddenness and magnitude of such an event can have disastrous results.

Catastrophic Outburst Flooding

Outburst floods are more common in western Canada and other parts of the world than they are in the U.S. An outburst flood occurs when lakes dammed by glaciers or moraines suddenly drain and tons of water, mud and debris are released. The resulting floodwaters can pick up large quantities of sediments and transform into destructive debris flows. The random and often unpredictable nature of these kind of events make the estimation of damages resulting from them as difficult as estimating damages from dam failures.

Glossary of Terms

A Glossary of Lake and Water Words You are leaving a Federal Government web site. Click this icon for more information. is available from the North American Lake Management Society. Also see the Glossary from the National Flood Insurance Program.

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Last Updated:

9/13/2012 9:32:58 AM

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