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Flooding Awareness Week – Day 3: Flood Hazards

| by Henry Luker

By National Weather Service Des Moines

The National Weather Service (NWS) has declared March 16 through March 22 as Flood Safety Awareness Week. The NWS in Des Moines will feature information about a different flood topic each day during the awareness week.

Today’s Topic: Flood Hazards

A flood is defined as any high flow…overflow or inundation of water that causes or threatens damage. Flooding can occur with prolonged rainfall over several days…intense rainfall over a short period of time or when water from an existing source moves too quickly (i.e. snowmelt, dam break, etc.).

Brief descriptions of the various types of flooding you may experience are found below. Note that while not all of the below types of flooding may be found in Iowa they are still included here for your reference. More information about these flood hazards can be found on the NWS Flood Safety Web site at http://www.floodsafety.noaa.gov/.

  • Flash Flooding
    • Flash flooding is 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 (i.e. intense rainfall, dam failure, or ice jam).
  • River Flooding
    • River flooding occurs when rivers rise and overflow their banks inundating areas that are normally dry.
  • Tropical Systems and Coastal Flooding
    • At any time of year a storm from over the ocean can bring heavy precipitation to the U.S. coasts. Whether such a storm is tropical or not, prolonged periods of heavy precipitation can cause flooding in coastal areas as well as further inland as the storm moves on shore.
  • Burn Scars/Debris Flows
    • Wildfires burn away the vegetation of an area leaving behind bare ground that tends to repel water. When rain falls it runs off a burn scar towards a low lying area, sometimes carrying branches, soil, and other debris along with it. Without vegetation to hold the soil in place flooding can produce mud and debris flows.
  • Snowmelt
    • 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.
  • Ice and Debris Jams
    • A backup of water into surrounding areas can occur when a river or stream is blocked by a build-up of ice or other debris.
  • Dry Wash
    • When heavy rain falls over dry land the water rushes towards low-lying areas, which may include dried up canyon or stream beds. This can quickly turn a dry channel into a raging river.
  • 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.

Understanding the different flood hazards and knowing the actions to take before, during and afterwards can help you protect your life, the lives of your loved ones and your property. Prepare now by visiting http://www.floodsafety.noaa.gov/.

Join us tomorrow for information on flood-related services provided by the NWS.