Floods: The Worst Geological Hazard
Flood Styles: Flash Floods
Flash floods are typically caused by:
- Intense rainfall
- Short period of time
- Topography, soil conditions, and ground cover
There are numerous ways that flash floods can cause damage:
- Roll boulders
- Tear out trees
- Destroy buildings and bridges
- Scour out new channels
- Landslides
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This boulder tumbled down stream and landed next to this tree. Over the years the tree grew and grew around the boulder, making it a permanent part of it. |
Water in flash floods can move extremely fast, and can catch drivers unaware. It is important to never drive through running water that is more than a few inches deep. Why? Water that is 1 foot in depth will exert 500 pounds of lateral force on your car - this is enough to push your car downstream along with the water. Water 2 feet deep not only exerts double the lateral force, but also 1,500 pounds of buoyancy - so now you're not only being pushed by the water but lifted up and floated along. Muddy water of any depth can hide debris which can puncture tires, hide washouts, and more.
What YOU can do:
- Know your flood risk!
- Keep your automobile fueled
- Store drinking water
- Keep a stock of food
- Keep first aid supplies on hand.
- Keep a NOAA Weather Radio, a battery-powered portable radio, emergency cooking equipment, and flashlights in working order.
- Install check valves in building sewer traps to prevent flood water from backing up into the drains of your home.
Flood Styles: Regional Floods
Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.
Flooding can occur downstream from where the rain is falling. If too much rain falls over an area, flooding can occur. The excess water flows into the rivers, swelling them and causing them to flood. This excess water travels downstream and can flood areas downstream from the original area. The image below shows this relationship.
Downstream floods - Result from regional storms of long duration or extended periods of above-normal precipitation. Longer lag times & higher peak discharges.
The Great Midwestern Flood of 1993
Flooding began in the upper Mississippi River Valley. Soon, record flood stage levels recorded. The floods covered nine states and 400,000 square miles.
The cause of the floods began with abnormally high rainfall in many Midwest states (200-300x the "normal" amount). The ground became over saturated and streams soon filled to capacity.
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| Graph of rainfall amounts for the midwest in the summer of 1993. Black bars are average amounts and white are observed. Source: The Great USA Flood of 1993 |
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| Graph of the river stages on the Mississippi River for July and August 1993 Source: The Great USA Flood of 1993 |
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