Knowing Flashfloods

Flashfloods are the flood events where the water rises rapidly within few hours of the rainfall. These floods are very dangerous, as the respose time is very small. People aren’t getting much time for the response to the floods. Flashfloods occur in small catchment areas. These causes large losses of life and property.Lately, flashfloods have become very common. Every year flashfloods are causing destruction in terms of loss of life and property. One of the negative impact of climate change is the increase in the frequency of the floods, storms, cyclones etc. The flashfloods are occurring frequently due to climate change. Global warming is dumping large amount of heat into the occeans and other water bodies. Which results in enhanced evaporation. This results in more formation of the excessive clouds. These clouds are sometimes bringing large amount of precipitation over a small area. The amount of precipitation is such that the small streams overwhelm within few hours of rainfall resulting in flashfloods. Many hydrological factors have relevance for the occurence of flashfloods; terrain gradients, soil type, vegetative cover, torrential rains etc. In steep rocky terrains or heavily urbanized areas, even small amounts of precipitation could trigger flashfloods. Flashfloods depends both on the meteorological and hydrological factors. Mostly the flashfloods associated with rainfall are produced by the thunderstorms. Thunderstorms are meteorological events which bring heavy rain, strong winds, hails etc. A single thunderstorm can’t trigger a flashflood. Sometimes successive thunder storms occur over the same area for extended periods of time. Such thunderstorms are called training thunderstorms. These training thunderstorms trigger flashfloods.
The moisture laden winds strike the mountains and rise upwards in the process resulting in the formation of the clouds. These clouds bring the precipitation called ornographic precipitation. These are not favourable conditions for the formation of the thunderstorms. Yet this orographic precipitation could bring large amount of rainfall which can trigger flashfloods. One of the important factor which leads to flashfloods are cloudbursts. The cloudbursts triggered a flashflood keeps coming on news channels. It mostly occurs in mountainous areas. The large amount of water vapours is moving upwards forming the updraft. The intensity of the updraft is so much that it prevents the condensed vapours from falling. When the updraft weakens, all of this water accumulated at higher levels falls down as cloudbursts. So cloudbursts cause sudden heavy rain usually of local nature and of brief duration. A characteristic of flashfloods is localised heavy rainfall over a small area over extended periods of time. The sandy soils have good infiltration rate could prevent the flashfloods to some extent. The dumping of polythenes and cementation of the soil coupled with heavy rainfall would amplify the flashfloods. Sometimes the areas suffering most from flashfloods would be the areas having recieved little or no rainfall. In this case, the heaviest rainfall has occurred in the up-stream areas. So it may be a bit confusing, as there might be no rainfall in areas, where flashflood warnings are issued. At times, it could be raining heavily during the nights and it would be difficult to warn the sleeping residents of flashfloods which could occur while they are asleep in midnight and the casualties could increase many times. The rapidity of the flashfloods makes it more damaging and dangerous. As the waters are fast rising, fast moving, it could caught people unaware and unprepared. The cities are more prone to flashfloods as compared to the rural neighbourhoods. As cities promote less absorption of rainwater as compared to villages, little amount of precipitation would be sufficient to trigger a flashflood in a city, which might be too less to trigger similar flashflood in a village. As villages promote more and more absorption of rainwater. It is after the saturation of the soil, the chances of flashfloods are high. Debris carried in flashfloods can form temporary debris dams. These debris dams would fail as more and more water gets accumulated behind it which results in the formation of large wall of water surging downwards. These debris dam failures could occur repeatedly over the course of flashfloods. All flashfloods are not characterised by wall of water, but rapidly rising water occurs in all flashfloods.
(The author is a lecturer at Govt Degree College Kupwara. Views are his own)

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