Flash floods are sudden flooding of water in which low lying areas are completely submerged or washed. Flash flood occurs due to natural phenomena like heavy rain, storm, hurricane etc. Sometimes it may happen due to dam break. However flash floods are extremely dangerous.
Here is an amazing video footage of east creek near Chalk Drive / Chalk Lane rising and washing away lots of cars during Flash Flood in Toowoomba (Australia) on Monday 10 January 2011.