Missouri River Flood of 2011.
Photo Courtesy of the Iowa State Patrol Air Wing
Flash Flooding across HW149 near Ottumwa in 2010.
Photo Courtesy of Josh Stevens, Wapello County EMA

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Floods & Flash Floods: Introduction

Introduction
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Staying Safe

 


Everyone has experienced a flood in one way or another. Whether it is water ponding on a sidewalk or a river surging through an entire city, a flood is simply defined as water existing in locations that are normally dry. The unforgiving nature of water makes floods the second deadliest weather phenomenon in the United States.

The National Weather Service in Des Moines alerts the public to two types of flooding: river floods and flash floods.


River Floods

River Flood As the name implies, this type of flooding is associated with the rivers, lakes, and streams that flow across the state. While fluctuations in the height of these waterways throughout the year is quite normal, there comes a point at which rising water begins to adversely impact the surrounding towns and roadways. At this point, it is said that the river has reached flood stage. Over 100 river and stream gauges exist across central Iowa, and a flood stage has been determined for most of these sites based on analyzing the impacts of prior floods. River floods can be caused by any number of reasons, including:

  • Heavy Rain: Once the ground has become saturated, any excess rainfall will run off straight into the river system. A single downpour from a thunderstorm can cause flooding along smaller streams, but it usually takes a series of heavy rain events to cause flooding along major rivers. Floods caused by heavy rain are the most common type of river flood in Iowa.
  • Spring Snow Melt: If the winter snow pack melts too fast or the ground cannot absorb the melted snow, this excess water runs off into the river systems.

River floods can vary widely in duration and intensity. Some may only briefly touch flood stage and last a few days while others can reach record heights and last for months on end, such as the Mississippi River Flood of 1993 and the Missouri River Flood of 2011.

Terms to Know:

  • Crest: The maximum height a river rises to before it begins falling. There may be multiple crests during a prolonged flood event.
  • The Different Flood Stages
  • Action Stage Usually a few feet below flood stage and serves as a heads up that a river is rising close to flood stage.
    Minor Flood Stage
    (or Flood Stage)
    Minimal or no property damage occurs, but possibly some public impacts.
    Moderate Flood Stage Some inundation of structures and roads near the river or stream. Some evacuations of people and/or transfer of property to higher elevations.
    Major Flood Stage Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.

Flash Floods

A flash flood is defined as a flood that develops in under six hours, though they can form in a matter of minutes. They tend to occur in low-lying areas with poor drainage, with urban areas particularly at risk. Some causes of flash flooding include:

  • Heavy Rain: Precipitation falling too fast for the soil to absorb is the most common cause of a flash flood. The resultant runoff flows downhill and collects in low-lying regions; thus, a flash flood could occur miles from where the precipitation actually falls.
  • Ice Jams: As river ice breaks up in the spring, slabs of ice can pile up against bridges and other structures and form a dam, causing a rapid rise in water levels behind the build up. When these build ups break, water levels can also rapidly rise downstream.
  • Dam or Levee Break: A failure of a man-made dam or levee can flood the surrounding area in a matter of minutes.

Flash Flood

Flood waters associated with flash floods tend to be fast moving, further increasing the danger for anyone attempting to navigate through them. Many people who die in flash floods are swept away as they attempt to walk or drive through the flood waters.


Additional Links and Info:

Official NWS Flood Safety Page
American Red Cross Flood Safety Checklist (pdf)
FEMA Flood Page
National Flood Insurance Program
Centers for Disease Control and Prevention - Floods

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