Any form of flood is caused by water running outside of its usual course, such as when a river breaks its banks to overflow onto the surrounding low lying ground, or when an area is inundated as the already saturated ground can’t cope with the water anymore, which accumulates and spreads over the surface. However, a flash flood is a much more severe form of flooding, when an area can be inundated in an instant by a rush of water coming from a number of sources.
Any form of storm can cause a flash flood, no matter where such a storm occurs. A regular thunder storm with torrential rain, if it stays in the same location for any amount of time, or if the same area suffers a number of storms over a prolonged period of time, can cause a flash flood. This is especially true if the land is hilly, and the ground already saturated, as the excess water which falls as rain will flow over the ground rather than be absorbed into it, gathering at the low points such as a valley or a creek. A characteristic of a flash flood is the suddenness of the flood, as in these situations a large amount of water can flow along its given path at great speed, moving like a tidal wave as a flood ten to twenty feet high gushes into the path of least resistance. Anything in the way will as such be swept away from the force of the water, which is often both massive and swift.
The power of the water in such situations also means that what the water sweeps away will be absorbed into the flow, including large boulders, trees or even cars. This adds another element of destruction to the flood waters. Even if the flood waters are very shallow, they will still carry the impetus to sweep away heavy objects.
Even in a desert or extremely dry areas, the same thing can occur. Again, the water will not be absorbed into the ground, which in the case of a desert has often been baked solid, causing the water to flow over the surface. This is why dry lake beds and creeks in the desert are extremely hazardous places during any form of rain storm, as huge torrents of water create lakes and rivers in a matter of hours to days.
More severe storms such as typhoons, cyclones or hurricanes also often cause flash flooding. This is both due to a surge which will often cause the ocean to overflow onto low lying coastlines, and also because of the significant amounts of precipitation dumped in a very short space of time. If land lies beneath sea level or sits in front of a range of hills, a flash flood is likely to overwhelm the coastal plain during such storms. This is exactly what happened when Hurricane Katrina hit New Orleans in 2005. Most of the city lies beneath sea level, so that the Mississippi River, swelled to a point where the levies holding it back weren’t enough of a defence, swiftly became inundated.
When high ranges of hills stand in the way of such storms, such as with the Himalayas preventing the Monsoon clouds from dissipating over northern India or the Andes halting the storm clouds brought to Peru and Chile by El Nino, flash floods are inevitable as the extreme levels of precipitation are stalled in one place, and fall on often rocky of desert-like ground, swiftly swelling the rivers and driving the water back onto the flat lands.
The failure of manmade flood defences or barriers to water can result in even more severe flash floods, like when the levies were overcome in more than twenty spots during Hurricane Katrina. Even more severe due to the suddenness of such events is if a dam fails, channelling a huge amount of water through the old river valleys where such edifices are usually built, sweeping away anything in the flood waters path.
Last amongst the causes of flash floods are rarer but also more devastating natural disasters, such as the 2006 tsunamis which hit numerous islands throughout South East Asia, giving little or no chance of escape to those caught in the path of the waves. Volcanic eruptions will also often lead to flash floods, particularly if the volcano is high enough to be covered in snow. The sudden melting of huge quantities of snow on the cone of the erupting Mount St Helens in 1980 caused a flash flood which swept entire forests of trees, uprooted by the pyroclastic flows or melt water running from the summit of the mountain, down nearby Columbia, Toutle and Cowlitz rivers, sweeping away many of the bridges in the paths of the mud filled flood waters.
So, flash floods are any form of sudden inundation, which inevitably will wreak devastation to anything in their path. Whilst the courses of such floods are many and varied, knowing where such events are likely to occur is the best line of defence, by steering clear of the danger areas before it is too late, although sadly, this isn’t always possible.
References
http://en.wikipedia.org/wiki/Mount_St_Helens
