Flood Types
Alluvial Fan
Flooding occurring on the surface of an alluvial fan or similar landform which originates at the apex and is characterized by high-velocity flows; active processes of erosion, sediment transport, and deposition; and unpredictable flowpaths. Alluvial fans, which occur mainly in dry mountainous regions, are deposits of rock and soil that have eroded from mountainsides and accumulated on valley floors in a fan-shaped pattern.
Coastal
Flooding of coastal areas due to the vertical rise above normal water level caused by strong, persistent onshore wind, high astronomical tide, and/or low atmospheric pressure, resulting in damage, erosion, flooding, fatalities, or injuries. Coastal areas are defined as those portions of coastal land zones (coastal county/parish) adjacent to the waters and bays of the oceans. This includes flooding of inland lakeshore areas.
Flash
A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam-related), on a widespread or localized basis. Flash floods are also characterized by a rapid rise in water, high velocities, and large amounts of debris.
Ponding
Flooding that develops more gradually, usually from prolonged and persistent moderate to heavy rainfall. This results in a gradual ponding or buildup of water in low-lying, flood prone areas, as well as small creeks and streams. The flooding normally occurs more than six hours after the rainfall begins, and may cover a large area. However, even though this type of flooding develops more slowly than flash flooding, it can still be a threat to life and property.
Riverine
Flooding or inundation of a normally dry area caused by the rise of a river to an elevation such that the river overflows its natural banks causing or threatening damage. Also described as overbank flooding when river exceeds the bankfull capacity of the channel and spills over to the floodplain where property and structures can be damaged.
Urban
Urban flooding is the inundation of land or property in a built environment, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems, such as storm sewers. Flash flooding or snowmelt can trigger urban flooding as well, but urban flooding is a condition characterized by repetitive and systemic impacts on a community. Urban flooding can happen regardless of community location within formally designated floodplains or near any body of water.
Keywords
dam failure
Flooding due to the failure of the dam structure, which can be attributed to the construction of an inadequate dam or to a flood that exceeds the design protection level or capacity. Dam failure can often be traced either to a poor decision made during design and construction or to inadequate maintenance or operational mismanagement. Failure may also result from natural hazards, such as earthquakes, or from flow volumes that exceed capacity. Damage from dam failure is especially severe because of the high velocity of floodwater. Breaching often occurs within hours after the first visible signs of dam failure, leaving little or no time for evacuation.
debris flow
A combination of either water, soil, rock and other material that forms on the sides of hill slopes and moves rapidly downhill. Debris flows are fast moving and highly destructive due to the amount of material being carried with the flow. Large boulders, trees and massive amounts of sediment can be carried in a debris flow. In most cases, lahars or mudflows from volcanic activity are considered a debris flow.
erosion
The process by which soil and rock are removed from the Earth's surface by processes such as wind or water flow, and then transported and deposited in other locations. River and stream erosion causes channel migration and downward, deepening of the channel. Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role.
groundwater (high water tables)
Groundwater is water that is found underground in cracks and spaces in soil, sand and rocks. The area where water fills these spaces is called the saturated zone. The top of this zone is called the water table and it may be only a foot below the ground’s surface or it may be hundreds of feet down.
heavy rain
Unusually large amount of rain resulting in urban and/or small stream flooding, which does not cause a flash flood.
high tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. The total water level from a storm is caused by all contributing factors, including surge, tide, waves, and rivers. This rise in total water level can cause extreme flooding in coastal areas, particularly when storm surge coincides with high tide. This occurrence makes the storm surge even more devastating.
hurricane (typhoon)
A tropical cyclone in which the maximum 1-minute sustained surface wind is 64 knots (74 mph) or greater. In the Atlantic Ocean or the North Pacific Ocean east of the International Date Line this event would be labeled a Hurricane, and in the North Pacific Ocean west of the International Dateline this event would be classified as a Typhoon. The hurricane/typhoon will usually include many individual hazards, such as storm tide, freshwater flooding, tornadoes, rip currents, etc.
ice jam
Flooding caused by ice jams is similar to flash flooding – the formation of a jam results in a rapid rise of water both at the point of the jam and upstream. Failure of the jam results in sudden flooding downstream. The formation of ice jams depends on both the weather and the physical conditions in the river channel. Flooding due to ice jams (or other ice conditions) can occur at different times and in different ways. The classic ice jam occurs at spring breakup due to a combination of ice conditions.
levee failure
Flooding due to the failure of a levee structure, which can be attributed to the construction of an inadequate levee or to a flood that exceeds the design protection level or capacity. Many private or locally built levees may provide only limited flood protection or are sometimes poorly designed and maintained. Many were built with no design standards.
nor 'easter
A macro-scale storm along the upper East Coast of the United States and Atlantic Canada; it gets its name from the direction the wind is coming. A nor’easter (also northeaster) is a low pressure area that often passes just off the New England and southeast Canada Atlantic coastline. Winds in the left-forward quadrant rotate onto land from the northeast. Nor’easters can cause severe coastal flooding, coastal erosion, hurricane force winds or blizzard conditions; usually accompanied with very heavy rain or snow, depending on when the storm occurs.
seiche
A standing-wave oscillation in any enclosed lake which continues after a forcing mechanism has ceased and results in shoreline flooding and/or damage. In the Great Lakes and large inland lakes, large pressure differences, high winds, or fast-moving squall lines may act as the forcing mechanism. In addition, earthquakes or debris flows can initiate a seiche. When the forcing mechanism ends, the water sloshes back and forth from one end of the lake to the other, causing water level fluctuations of up to several feet before damping out.
snow melt
Water from melting snow that flows over the surface of the ground into streams and rivers.
storm
A storm is any disturbed state of the earth’s atmosphere affecting the surface, and strongly implying severe weather. It may be marked by significant disruptions to normal conditions such as strong wind, hail, thunder and/or lightning (a thunderstorm), heavy precipitation (snowstorm, rainstorm), heavy freezing rain (ice storm), strong winds, etc.
storm surge
For coastal and lakeshore areas, the vertical rise above normal water level associated with a storm of tropical origin (e.g., hurricane, typhoon, or tropical storm) caused by any combination of strong, persistent onshore wind, high astronomical tide and low atmospheric pressure, resulting in damage, erosion, flooding, fatalities, or injuries.
stormwater
Water that originates from rainfall or snowmelt that enters the stormwater system. Stormwater that does not soak into the ground becomes surface runoff, which either flows directly into surface waterways or is channeled into storm sewers, which eventually discharge to surface waters. Stormwater is of concern as it relates to the volume and timing of runoff water (flooding) and the other related to potential contaminants that the water is carrying, i.e. water pollution.
tropical storm
A tropical cyclone in which the 1-minute sustained surface wind ranges from 34 to 63 knots (39 to 73 mph) inclusive. The tropical storm will usually include many individual hazards, such as storm tide, freshwater flooding, tornadoes, debris flows, rip currents, etc.
tsunami
A series of very long waves generated by any rapid, large-scale disturbance of the sea (e.g., an underwater earthquake, landslide, or volcanic eruption) resulting in a fatality, injury or damage. When the wave reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.
