Flash floods remain difficult to predict because they’re a result of rapidly changing conditions. They can be caused not only by sudden cloudbursts but also by quickly melting snow or ice and the collapse of natural or manmade dams. The seeds of disaster can be sown on what seems to be a perfect day.
Floods caused by several days of rainfall or melting snow give plenty of notice, but their effects can be much more damaging and widespread. Many rivers flood on a regular basis and have done so for centuries, but people nonetheless build homes on their banks and floodplains, placing themselves directly in harm’s way. Loss of lives and property is an inevitable outcome—another reason why floods are America’s number one weather-related killers.
When Hurricane Katrina hit the Gulf Coast in August 2005, it smashed many of the levees that protected New Orleans. More than fifty breaches were reported and floodwaters rushed into large parts of the city. Later investigations concluded that the levees might have held (or at least held better) if they had been inspected and repaired regularly. In the years after the storm, many of the levees were rebuilt, this time with materials and designs aimed at withstanding a similar event.
Population growth calls for more housing. With wetlands being drained for developments, rivers being channeled, and levees being built, flooding rivers will crest at a higher elevation. Urban development covers large areas with pavement and concrete, so water that was once able to soak into the ground over time now rushes directly into streams and rivers. When you throw an approaching hurricane or tropical storm into the mix, it’s time to look for higher ground.
One pattern meteorologists look for in a developing flash flood is where storms with heavy rainfall linger. Storms may hover over one area for hours, or they may move in such a way that a continuous line of storm cells passes repeatedly over the same spot in a pattern called training. They also watch out for backbuilding, which occurs when a storm is able to regenerate itself on its back edge as quickly as it moves forward. When this happens, one small area can receive catastrophic amounts of rain in a short time.