Flash flooding occurs when too much rain falls too fast and too long for the ground to absorb all that water. Louisianans are used to thunderstorms. But if one of those storm cells stall over one area and drops heavy rain for hours, that can lead to dangerous flash flooding that threaten life and property.
- Most flash flooding is caused by slow-moving or stalled thunderstorms, by thunderstorms repeatedly moving over the same area, or by heavy rains from hurricanes and tropical storms. Heavy sustained rain can create rapid flooding within a few hours or even minutes, and flooding can occur miles away from where the rain fell.
- Several factors contribute to flash flooding. Two key elements are how hard the rain falls (rainfall intensity) and how long the rain lasts (duration).
- Louisiana's land is mostly flat lowlands. This causes drainage problems during heavy summer rainfall, tropical storms, and hurricanes. Roads and parking lots cannot absorb the water, which must drain off into saturated ditches and bayous. Uncleared storm debris in bayous, drainage ditches, and waterways can keep the water from flowing smoothly away.
- Flash floods can sweep away cars, tear out trees, and destroy buildings and bridges. As little as six inches of moving water can knock a person down. Two feet of moving water can sweep a vehicle away. Often there’s no warning that these sudden and deadly floods are coming. Most flood fatalities in the U.S. happen in flash floods.
Flood Warning vs. Flood Watch
A flash flood watch means conditions are favorable for a flash flood. A flash flood warning means a flash flood is taking place or is imminent.
FLASH FLOOD WATCH
When a flash flood WATCH is issued, be alert to signs of flash flooding and be ready to evacuate at a moment's notice.
- Don’t park your vehicle along bayous, canals, streams or other waterways.
- Avoid areas subject to flooding, including dips, low spots and ditches.
- Look out for rapidly rising water.
FLASH FLOOD WARNING
When a flash flood WARNING is issued for your area, or the moment you realize that a flash flood is imminent, act quickly to save yourself.
- Go to higher ground if possible.
- Avoid places that are already flooded and anywhere water is moving rapidly.
- Do not attempt to cross flowing waterways.
- If advised to evacuate, do so immediately.
- Continue monitoring weather forecasts and alerts--on radio, TV, or your phone.
Driving and Flooding: A Dangerous Combination
- Nearly half of all flash flood fatalities are auto related.
- If driving in heavy rain, look out for flooding at highway dips and bridge underpasses.
- Do not attempt to drive across a flooded road. The depth of the water is not always obvious, and the roadbed itself may be washed out under the water. Turn around and go another way.
- If a vehicle stalls in high water, leave it immediately and seek higher ground. Rapidly rising water may engulf the vehicle and sweep it away.