Flooding

The variety of floods is as diverse as the nation’s natural resources, ranging from localized flash flood covering several city blocks to massive flooding encompassing up to a quarter of the area of the lower 48 states.  Causes and characteristics of flooding include:

Flash floods: Flash floods are typically caused by short, intense rainfall over areas as small as a city to larger than a state.  Flash flooding risks can occur well away from rivers and streams.  Water pools in low spots such as underpasses and basements because rain falls faster than the ground can absorb it.  Year in and year out, flash floods take more lives than any other type of flooding – most flood fatalities involve motorists.   The cumulative effect of widespread, prolonged flash flooding can lead to flooding of smaller streams and, if enough rain falls, major river systems.

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Riverine flooding: When runoff from widespread rain events flows into rivers and streams faster than the water moves downstream, excess water causes flooding along these waterways.  Once a river reaches flood stage, flood severity categories are used by the NWS to communicate the expected flood impact:

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• Minor Flooding: minimal or no property damage, but possibly some public threat or inconvenience.
  
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• Moderate Flooding: some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary.
  
  
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• Major Flooding: extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.
  
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Tropical cyclones: Hurricanes consistently capture the attention of the nation because of their destructive power.  While the effects of extreme winds and coastal storm surge flooding are widely recognized, the potential for severe flooding inland is not always appreciated.  Rainfall is typically heavier with slower moving storms that allow heavy rain to persist over a location.  In fact, weaker systems, while often not the focus of intense attention, can lead to some of the most serious flooding associated with tropical systems.  Inland freshwater floods accounted for more than half (59%) of U.S. tropical cyclone deaths over the past 30 years.

• Coastal flooding: Driving winds associated with hurricanes pushes water inland, causing storm surges that can exceed 20 feet, causing devastation in coastal areas.  Storm surges can be caused by any strong storm system that moves slowly.  Severe coastal erosion and flooding are wither-time threats along parts of the West Coast and in the northeastern U.S.

Snow melt floods: Snow melts slowly enough that, by itself, it seldom causes flooding in most parts of the country.  However, warm, moist conditions and heavy rain can combine with snow melt to cause dramatic winter and spring flooding.  In relatively flat areas in the Midwest, river beds drop very slowly along the length of the river.  As a result, the water in the river glides slowly downstream.  In such areas, accumulation of melt water from extensive snow-covered areas can cause significant flooding.  This situation is often compounded by the effects of ice jams.

Ice jam floods: When rivers clogged with ice rise rapidly due to rainfall and/or snowmelt, the ice breaks up into chunks, some larger than an automobile.  These chunks of ice move downstream and can jam at constrictions in rivers such as bends, bridge abutments or shallow areas.  The effect is much like a traffic jam that occurs if travel lanes are closed due to an accident.  The ice jam can act as a dam, causing water to back up behind it.  River levels behind the ice jam can rise rapidly.  On occasion, the ice jam can release quickly, sending huge chunks of ice downstream in the torrent, destroying everything in its wake.

•  Structural failure:  Flooding is sometimes also caused by dam breaks or levee failures, resulting in a sudden release of water.
  
  

•  Debris flow: Once the smoke clears from a wildfire, the danger is not over. Because of the lack of vegetation, moderate amounts of rain can cause flash floods and debris flows – or mudflows – on burned hill slopes. The powerful force of rushing water, soil, and rock, both within the burned area and downstream, can destroy culverts, bridges, roadways, and structures, and can result in injury or death.
  
  Slot canyons: Slot canyons are formed over the centuries by water gouging narrow slits into Southwestern sandstone.  Some are more than 100 feet deep and so narrow that one has to turn sideways to pass through them at their floor.  Their uniqueness and beauty, make them an increasingly popular destinations for outdoor lovers.  They can also be death traps, as water, sometimes falling miles away, can surge though them with ferocious power.