Flash floods are the most dangerous kind of floods, because they combine the destructive power of a flood with incredible speed and unpredictability. Flash floods occur when excessive water fills normally dry creeks or river beds along with currently flowing creeks and rivers, causing rapid rises of water in a short amount of time. They can happen with little or no warning.

    Densely populated areas are at high risk of flash flooding. The construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. This runoff increases the flash flood potential. Moreover, low spots, such as underpasses, underground parking garages, and basements can become death traps. Similarly, areas near rivers are at risk from flash floods. Embankments, known as levees, are often built along rivers and are used to prevent high water from flooding bordering land. Dam failures can also send a sudden destructive wall of water downstream. Mountains and steep hills produce rapid  runoff, which causes streams to rise quickly. Rocks and clay soils do not allow much water to infiltrate the ground. Saturated soil too can rapidly to flash flooding.

Source: www.nssl.noaa.gov