SR 47(9) (Natural Hazards).Pdf

Natural Hazards A Cloudburst leads to the exact phenomenon one would expect if clouds burst—copious and intense rainfall over a small DangerousDangerous area. It is sometimes also called Rain Gush or Rain Gust. In Hindi it is referred to Badal DelugeDeluge phatna, which is a literal translation of the words Cloud burst. It is easy to see how the name stuck, if one goes by the popular perception that clouds hold large amounts of water that would no doubt, fall to LOUDBURSTS are common clouds that are usually responsible for Earth if they burst. In scientific in the tropics, although they can Cloudbursts. Most Cloudbursts occur in occur anywhere in the world. association with thunderstorms. In such type parlance, cloudbursts are Thunder and Cloudbursts of storms there are strong uprushes of air. described as “devastating usuallyC occur together, although hail may These updrafts are filled with turbulent wind or may not be present. Cloudbursts not only pockets that shove the small raindrops convective phenomena happen abruptly but are also highly around leading to collisions between producing sudden high- unpredictable. The word “burst” has violent raindrops. The collisions lead to overtones so the associated rainfall would conglomerations and large-sized drops are intensity rainfall (<“10 cm per be expected to be of an extreme nature too. formed. The forceful upward rush of air also hour) over a small area.” It is. It has been reported that the intensity prevents the condensing raindrops from of the rainfall may be so great that those falling downwards. So instead of falling amount of rainfall. So a single Cloudburst caught in the onslaught may find it difficult down to Earth the water droplets are pushed can do far more damage than the same to breathe! Cloudburst-associated rainfall upwards till a large amount of water volume of rain falling as a gentle shower. does not usually last very long but it is accumulates at a high level. Eventually all The perilous nature of Cloudbursts is capable of producing overwhelming updrafts become weak and collapse. With therefore because of these large raindrops consequences. nothing to push it up, the entire water falls falling as a torrent, at great speed over a down almost all at once. small area. Culprit Cloud Characteristics The Cumulonimbus is a tall cloud that Consequences contains very high, unpredictable winds. Not only do the larger drops fall with a Cloudbursts cause flash floods. Flash floods Such clouds are associated with terminal velocity of around 12 km/h but in turn, uproot trees, trigger soil erosion, thunderstorms. Typically these are the also they have the added impetus of the landslides and landslips leading to habitat downdraft speed, which can easily exceed destruction, and massive loss of property. 80 km/h. The intensity of rainfall Downstream, the floodwaters slow down Flash Water during a Cloudburst can be as much and deposit large amounts of silt that may as 10.2 cm/h, with 1,220 drops falling choke the mouth of water bodies and/or per square metre at a velocity of up to raise the riverbed. Other things being equal, 7.9 metres/second. Just imagine the the rapidity with which the rain sweeps away impact of raindrops hitting the soil so the soil depends upon the steepness of the hard. The mechanical action of rain is slope. On hillsides, flash floods can be greatly increased by the force and devastating. India is no stranger to this calamity. Perhaps many still Compounding Consequences Cloudbursts are especially common in remember the downpour in mountainous areas. It is believed that this 2005 that completely is because the warm air currents of a paralysed Mumbai. That was a thunderstorm tend to follow the upward slope of a mountain. The consequences of Cloudburst too. a Cloudburst in a mountainous area are 56 SCIENCE REPORTER, September 2010 Natural Hazards

City road or Venice waterway!

House Hill side road washed damaged in away heavy rain

compounded because the falling water described as a “wall of water.” It has peak recorded over eight to ten hours. rushes down narrow gulleys/valleys. The duration of only a few minutes, followed by Newspaper reports of the 2007 enormous amount of rainfall often a rapid subsidence. One of the earliest Cloudburst tragedy at Ghavni, Himachal overwhelms the shallow hilly channels recorded cloudbursts in USA (called a Pradesh, record that a number of apple through which water flows and this leads to waterspout in those days) occurred in orchards, at least 15 houses, a primary flash floods. Golden Gate Gulch, on 14 July 1872. The health centre and a school building were Like the Cloudburst, the flash floods “wall of water” was described as a washed away because of the Cloudburst. too are sudden, severe and short-lived. “perpendicular breast of 10 or 12 feet.” At least 52 people died. When a sudden surge of water flows down The force of the floodwater can be The landslide triggered by a a narrow channel, its destructive powers staggering. For example, a cloudburst flood Cloudburst on 8 August 2009 buried the are unleashed, sweeping away almost on Kiowa Creek (USA) in May 1878 caused two villages of Jhakhla and Lah everything in its path. Areas with dry creeks/ a standard-gauge locomotive to be washed (Uttarakhand) under rubble and claimed 43 gulches etc. are particularly vulnerable as away. Although search was mounted, the lives. This year, Kashmir, parts of these can quickly fill with water that sweep locomotive was never recovered. Uttarakhand and Guwahati have already away people and animals caught unawares. India is no stranger to this calamity. faced the wrath of hard rain and there is no In cities, flooding can transform roads There have been many major Cloudbursts guarantee that Cloudbursts will not strike into the waterways of Venice and make it that have caused untold loss in recent times. again. impossible for people to walk. In 2002, Cloudburst affected four villages And the latest in line was one that led and took the lives of at least 33 people in to untold devastation in Leh recently. Uttarakhand. Perhaps many still remember Casualties Dr Sukanya Datta the downpour in 2005 that completely According to the US Geological Survey’s Scientist NISCAIR posted to Director General's Natural Hazards Gateway, a flood resulting paralysed Mumbai. That was a Cloudburst Technical Cell, CSIR HQ from a Cloudburst rises so quickly that it is too. Reportedly, 950 mm of rainfall was Email: [email protected] SCIENCE REPORTER, September 2010 57