Flash Floods: Effects of Human Activities

                         Flash flood is a rapid flooding of geomorphic low-lying areas—washes, rivers, dry lakes andbasins. It may be caused by heavy rain associated with a storm, hurricane, or tropical storm ormeltwater from ice or snow flowing over ice sheets or snowfields. Flash floods may occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam, as occurred before the Johnstown Flood of 1889. Flash floods are distinguished from a regular flood by a timescale less than six hours. The temporary availability of water is often utilized by foliage with rapid germination and short growth cycle, and by specially adapted animal life.

                     Flash flooding occurs when precipitation falls too quickly on saturated soil or dry soil that has poor absorption ability. The runoff collects in low-lying areas and rapidly flows downhill. Flash floods most often occur in normally dry areas that have recently received precipitation, but may be seen anywhere downstream from the source of the precipitation, even many miles from the source. In areas on or near volcanoes, flash floods have also occurred after eruptions, when glaciers have been melted by the intense heat.

                     Flash floods occurs when large rivers that run through flat terrain overflow their banks, they usually do so gradually. The slowly rising water provides warning to communities that a flood is developing. An intense rainstorm, however, may overwhelm smaller rivers and streams, especially those carrying water a short distance or down steep slopes. The sudden increase in the volume of water flowing into these smaller waterways can cause dangerous overflows known as flash floods. In dry regions, heavy downpours can suddenly fill dry streambeds, known as washes, with torrents of water.

                      Flash floods usually occur when storms drop large amounts of rain within a brief period. They can occur with little or no warning, sometimes reaching their peak in only a few minutes. Waters move very fast during flash floods. The force can roll boulders, uproot trees, destroy buildings, and wash out bridges. Walls of water can reach as high as 10 feet (3 m) to 20 feet (6 m), carrying debris that can kill people and destroy property.