Flashfloods are instantaneous floods that cover roads and swell streams and rivers above their banks, or even create rivers where there were none to start with. These floods have many different potential causes and all are unpredictable and possibly deadly if underestimated. There are no geographical land areas that are safe from these floods.
Often striking where a ground is saturated with water or too dry, large downpours triggered by thunderstorms, tropical storms, and hurricanes drop large amounts of water on an area and the water collects rapidly. These waters are unable to be absorbed into the ground and simply flow on top of it downhill, over dry river beds, or down streets. Additionally, these floods can occur great distances away from where the rain falls, bringing a flood to an area that held no indication of storm or flood to start with.
Other causes of flashfloods can be due to melting ice and rain, ice dam breakage across river ways, human dam breakage, tsunamis, and volcanic activities. Typically only lasting a few hours, flashfloods can strike in an instant and maintain a non-threatening visual, but actually are capable of washing away cars and SUVs that underestimate their threat potential. Even roadways that are covered with a minor amount (1 foot or 30 cm) of water are hazardous to cross and should be avoided altogether. Most drivers that underestimate the waters and attempt to cross will be trapped in it, even if they are not carried away by it.
Currently there are no low elevations that are immune to the threat of flashfloods, with even mountainside locations at risk from glacier melt or heavy rains and mudslides. Easiest to form in desserts and areas where the soil contains a lot of clay, water will collect fast and flow unrestricted at quick speeds. In regions where there are large river intersections, water flow from far away can increase the volume to a point that the outlet can’t handle it, and cause the banks to overflow as it backs up. Storm surge can cause a similar event and force water up a shore intersecting river, causing waters to spill into neighborhoods. Even tsunami carry this effect and more, as ocean water sweeps across land in a sudden occurrence, but more than deadly enough for its unsuspecting victims.
Although none of these floods last long, it is the initial surprise and sudden onset that does more than its fair of damage.