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
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.
|
|
Urban Drainage (Stormwater Runoff) Flooding
|
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.
|
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
 .
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
.
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)
and
Hope for Renewal: Photographs from Indonesia After the Tsunami
.
Several informative animations are also available on-line including:
Savage Earth Animation
,
National Geographic
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.
|
|
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.
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.
|
Ice floes left behind by floodwaters
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
|
|
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
is available from the North American Lake Management Society. Also see the
Glossary
from the National Flood Insurance Program.
