The National Weather Service has declared March 16 through March 22 as Flood Safety Awareness Week. The National Weather Service in Jackson will feature a different educational topic each day during the awareness week.
Today's Topic: Types of floods and flood hazards
It floods somewhere in the United States or its territories nearly every day of the year. Flooding causes more damage in the United States than any other weather related event, with an average of eight billion dollars a year and an average of 89 fatalities per year in the past thirty years. Flooding can occur anywhere at anytime of the year, sometimes very quickly. Being prepared in advance and knowing a few flood safety tips will help you and your family survive a flood if it happens in your area.
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 or intense rainfall over a short period of time. Other factors that can cause or worsen flooding are melting snow in the winter and early spring such as what happens along the upper reaches of the Mississippi River, 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, or a 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 creek 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 (RFC), and 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 evacuation 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 evacuation 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 does not mean they do not flood!
Contact your Jackson NWS office for more information on flood categories.
Coastal flooding is another type of flood. When it comes to tropical cyclones, 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 1970s, 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. As it slowly moved inland, it caused significant flooding across portions of the Jackson Mississippi metro area as well as east Mississippi. The remnants of Tropical Storm Lee proceeded to move slowly to the northeast and caused heavy rainfall and historic flooding in Washington D.C., 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 rain associated with decaying tropical cyclones, and in extreme cases, river floods can last a week or more. Just because you do not live near the coast does not mean you will not be impacted by tropical cyclone flooding. In the last 100 years, Mississippi, Northeast Louisiana, and Southeast Arkansas had not experienced major flooding from tropical storm systems. That is until 2008 when Hurricane Gustav dumped 10 to 20 inches of rainfall in a short time on Southeast Arkansas, Northeast Louisiana and west Mississippi. Much damage occurred, but thankfully there were no fatalities. Storms like Gustav could cause devastating results across Mississippi. Most recently, Hurricane Isaac in 2012 produced significant flash flood damage across south portions of Mississippi as it moved inland.
Flooding due to snowmelt most often occurs in the spring when warming temperatures quickly melt the snow. The water runs off the still partially frozen or already saturated ground into nearby streams and rivers, causing them to rapidly rise and sometimes overflow their banks. Larger floods on the Mississippi River are usually the product of heavy rainfall and large volumes of snow melt.
Dam Break and Levee Failure...
A break or failure can occur with little to no warning. Most often they are caused by water overtopping the structure, excessive seepage through the surrounding ground, or a structural failure. The ARKLAMISS region has had its share of Dam Breaks over the years. During the 1927 Mississippi River Flood, a levee breach above Greenville Mississippi flooded much of the Lower Yazoo Delta Region. During Mississippi high water, many agencies monitor the levees along the Mississippi River and in the Yazoo River region.
Knowing the different types of floods, how to prepare for them, and knowing the actions to take during, and afterward can save you time, money, and even your life. Prepare now and be a force of nature!
Important NWS flood websites:
National Weather Service Partners:
- FEMA: http://www.fema.gov/
- Safety Information: http://www.ready.gov/
- Safety Kit Assembly: http://www.ready.gov/build-a-kit/
- Property Protection and Flood Insurance: http://www.floodsmart.gov/
- FEMA Flood Map Center: http://www.msc.fema.gov/