|Flood Safety Awareness Week
|Today's Topic: Types of Floods
A flood is defined as any high flow, overflow or inundation by water which causes or threatens damage. This usually occurs with prolonged rainfall over several days, intense rainfall over a short period of time or when an ice or debris jam causes a river or stream to overflow and flood the surrounding area. Other factors that can cause or worsen flooding are melting snow in the winter and early spring, thunderstorms in the spring and summer and tropical cyclones in the summer and fall.
A flash flood is defined as 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. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters. Commonly it occurs within six hours of a heavy rain event. However flash floods can also occur within hours or even minutes if a dam or levee fails, following a sudden release of water held by an ice or debris jam or rapid ponding of water caused by torrential rainfall. Flash floods can even occur in areas away from the causative event. For example, an otherwise dry arroyo or canyon can fill quickly from an upstream rain event. Flash floods can catch people off guard and unprepared. You may only have a few minutes warning that these deadly, sudden floods are coming. If you live in areas prone to flash floods plan now to protect your family and property.
With river flooding, the NWS uses different categories to convey the expected flood severity. These categories are: minor flooding, moderate flooding and major flooding. Each category has a definition based on property damage and public threat and are closely coordinated by the local NWS office, the servicing River Forecast Center, or RFC, and other emergency and public officials.
Here are the official definitions of the NWS flood categories:
Minor flooding usually has minimal or no property damage, but there could be some level of public threat or inconvenience. Many times you will hear this referred to as nuisance flooding.
Moderate flooding generally has some inundation of structures and roads near streams and rivers. Some evacuations of people and transfer of property to higher elevations can become necessary with moderate flooding.
Major flooding occurs with extensive inundation of structures and roads. Significant evacuations of people and transfer of property to higher elevations may become necessary.
An additional category of flooding is record flooding. This is flooding which equals or exceeds the highest stage or discharge observed at a given site during the period of record. The highest stage on record is not necessarily above the other three flood categories, especially if the period of record is short.
One thing to note: flood categories do not exist for all forecast points or flood locations. That doesn’t mean they don't flood! Contact your local NWS office more information on flood categories.
Coastal flooding is another type of flood. When it comes to tropical cyclones, the generic term for a hurricane, typhoon or tropical storm, wind speeds do not tell the whole story. Along the coast, storm surge is often the greatest threat to life and property from a tropical cyclone. In the past, large death tolls have resulted from the rise of the ocean associated with many of the major hurricanes that have made landfall. Hurricane Katrina in 2005 is a prime example of the damage and devastation that can be caused by storm surge. At least 1500 people lost their lives during Hurricane Katrina and many of those deaths occurred directly, or indirectly, as a result of storm surge. By knowing your vulnerability and what actions you should take, you can reduce the effects of a coastal flooding disaster.
Since the 1970’s, inland flooding has been responsible for more than half of the deaths associated with tropical cyclones in the United States. Typically greater rainfall amounts and flooding are associated with tropical cyclones that have a slow forward speed or stall over an area. These areas can be right along the coast but they may be even hundreds of miles away from where the storm made landfall. Tropical storm lee is a recent example of inland flooding several hundreds of miles away from where the eye of the storm ultimately landed. In 2011, Tropical Storm Lee made landfall along the Louisiana coast. But the remnants from the storm caused heavy rainfall and historic flooding in Washington DC, Pennsylvania, and as far away as New York. Ten fatalities, all in Pennsylvania, were confirmed as flood related during the flooding associated with Tropical Storm Lee.
Rainfall from tropical cyclone remnants can cause flooding if conditions are right. Flash flooding can occur in creeks, streams and urban areas within a few minutes or hours of excessive rainfall. Streets can become swift moving rivers and underpasses can become death traps. River flooding can occur from heavy rains associated with decaying tropical cyclones and in extreme cases, river floods can last a week or more. Just because you don’t live near the coast doesn’t mean you won’t be impacted by tropical cyclone flooding.
Knowing the different types of floods, how to prepare for them and knowing the actions to take during and afterwards can save you time, money and even your life. Prepare now and be a force of nature!
Join us tomorrow for information on Turn Around Don’t Drown.