A cloudburst is a sudden downpour within a radius of few kilometres. It usually lasts no longer than few minutes but is capable of flooding the area. Rainfall from a cloudburst is usually over 100 mm per hour. This leads to flash floods/ landslides, house collapse, dislocation of traffic and human casualties on large scale.

Cause

• The Cumulonimbus is a tall cloud that contains very high, unpredictable winds. Such clouds are associated with thunderstorms. Typically these are the clouds that are usually responsible for Cloudbursts. Most Cloudbursts occur in association with thunderstorms. In such type of storms there are strong uprushes of air. These updrafts are filled with turbulent wind pockets that shove the small raindrops around leading to collisions between raindrops. The collisions lead to conglomerations and largesized drops are formed. The forceful upward rush of air also prevents the condensing raindrops from falling downwards. So instead of falling down to Earth the water droplets are pushed upwards till a large amount of water accumulates at a high level.
• Eventually all updrafts become weak and collapse. With nothing to push it up, the entire water falls down almost all at once.
• Hilly areas are more prone to cloud burst. The topographical conditions like steep hills favour the formation of these clouds. And also the devastations, as water flowing down the steep slopes bring debris, boulders and uprooted trees with great velocity damaging any structure that comes in their way.
• In one of India's worst cloudbursts on 26 July 2005, Mumbai was completely paralysed. Approximately 950 mm of rainfall was recorded in India's financial capital over a span of eight to ten hours. Leh recorded over 12 mm of rainfall in just few minutes on August 6, 2010 cloudburst, that at least 1000 dead, and hundreds injured.
• There is no satisfactory technique for anticipating the occurrence of cloud bursts because of their small scale. A very fine net work of radars is required to be able to detect the likelihood of a cloud burst and this would be prohibitively expensive. Only the areas likely to receive heavy rainfall can be identified on a short range scale. Much of the damage can be avoided by way of identifying the areas and the meteorological situations that favour the occurrence of cloud bursts.

URBAN FLOODS

• Urban flooding is significantly different from rural flooding as urbanization leads to developed catchments, which increases the flood peaks from 1.8 to 8 times and flood volumes by up to 6 times. Consequently, flooding occurs very quickly due to faster flow times (in a matter of minutes). Urban areas are densely populated and people living in vulnerable areas suffer due to flooding, sometimes resulting in loss of life. It is not only the event of flooding but the
• secondary effect of exposure to infection also has its toll in terms of human suffering, loss of livelihood and, in extreme cases, loss of life.
• Urban areas are also centres of economic activities with vital infrastructure which needs to be protected 24x7. In most of the cities, damage to vital infrastructure has a bearing not only for the state and the country but it could even have global implications. Major cities in India have witnessed loss of life and property, disruption in transport and power and incidence of epidemics. Most city areas around rivers are flooded during monsoon period. Mumbai floods in 2006 caused havoc in the whole city. Therefore, management of urban flooding has to be accorded top priority. Increasing trend of urban flooding is a universal phenomenon and poses a great challenge to urban planners the world over. Problems associated with urban floods range from relatively localized incidents to major incidents, resulting in cities being inundated from hours to several days. Therefore, the impact can also be widespread, including temporary relocation of people, damage to civic amenities, deterioration of water quality and risk of epidemics.

Dos and don’ts before, during and after the flood

Before floods

• Do not litter waste, plastic bags, plastic bottles in drains
• Try to be at home if high tide and heavy rains occur simultaneously
• Listen to weather forecast.
• Evacuate low line areas and shift to safer places.
• Make sure that each person has lantern, torch, some edibles, drinking water, dry clothes and necessary documents while evacuating or shifting.

In the Flood Situation

• Shift to a safer place.
• Be at safe place and they try to collect correct information.
• Switch of electrical supply and don’t touch open wires.
• Don’t get carried away by rumors and don not spread rumors.

Don'ts

• Don't walk through flowing water - currents can be deceptive, and shallow, fast moving water can knock you off your feet.
• Don't swim through fast flowing water.
• Don't drive through a flooded area.
• Don't eat any food that has come into contact with flood water.
• Don't scrub or brush mud and other deposits from materials.
• Never turn on ceiling fixtures if ceiling is wet.
• Don't remove standing water in a basement too fast. If the pressure is relieved too quickly it may put undue stress on the walls.