Flash floods are one of the most dangerous natural disasters. As their name suggests, they happen suddenly, and often people don’t know they’re coming.

Heavy rainfall in a mountainous region can result in a devastating flash flood. The flood can happen within about two hours of the rainfall. When a lot of water collects suddenly at high ground, the earth cannot absorb it quickly enough. The water has nowhere to go but downward.

Eventually, water surges down the mountain with a great deal of force. Sweeping through the valley below, the deadly torrent destroys homes and buildings, sets vehicles afloat, and submerges farmland. Underground storm drains can become overwhelmed, causing low areas such as subways, parking garages, and basements to fill with water.

If the water moves through a city, it can be even more devastating to communities. The construction of buildings, roads, and parking garages means that the ground is unable to absorb much water, so the flood continues to rise. People may be forced to sit on roofs or cling to trees as they wait for rescue.

Fast-moving water can also weaken soil on hills and mountains, causing mudslides. As a mudslide rolls downward, it collects rocks, debris, and even trees. The cascading mass of mud can push buildings off their foundations and make roads and bridges impassable.