Cloudbursts are primarily caused by convective, cumulo nimbus clouds resulting in heavy downpour. Various researches suggest that cloudbursts are manifestations of intense small scale vortices that generate strong convective currents, which lift moisture laden air with sufficient rapidity to create cumulo nimbus clouds that shed the water load with great strength and ferocity. The raindrops form a strong downdraft in an event that appears as if the cloud has burst open, with the larger drops falling with a terminal velocity of around 12 km/h to over 80 km/h. The resulting rainfall is a torrent of water, falling at high speed, over a small area. When the speed of water accumulation on the ground exceeds the surface’s ability to absorb it, localised flooding will occur in low lying terrain. In hilly or mountainous terrain, the runoff of water congregates in stream beds or canyons and cause deadly and damaging flash floods. In India the northwest moving monsoonal systems after recurving over Rajasthan, pass over the hilly regions of Himachal and adjoining areas. Thus these areas are prone to frequent cloudbursts.
There is no satisfactory technique for anticipating the occurrence of cloudbursts because of their scale. A very fine network of radars is required to be able to detect the likelihood of a cloudburst and this would be prohibitively expensive. Only 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. Factors that aggravate the cloudburst devastation are indiscriminate and large scale deforestation, haphazard unplanned urbanisation, construction of roads, dams and large scale mining projects, and increasing human activity along rivers and nullahs. Most debris that arises from human interventions in hilly regions are carelessly dumped along slopes, which eventually finds its way into rivers and smaller streams. Once the level of the bed is raised, the water channel is unable to bear the sudden surge of a larger load that occurs during cloudbursts. Thus sheet flooding occurs along the slopes resulting in widespread devastation.
The Leh Devastation
Monsoon brought with it the disastrous cloudburst of Leh (town in Ladakh division, J&K) on August 6 which has killed more than a hundred people and injured over 400. The slithering mass of mud washed away people and homes. Villagers living in the upper reaches bore the full impact of the sliding pile of mud and rocks. A patch of land 500-metre long and 300-metre wide, about 15 minutes’ drive on the road to Manali was completely overrun by mud that appeared like mortar. This was where most of the Choglumsar village stood. The area is a plateau, with rising hills to its north and west. The wind-swept plain gently slopes towards the Indus that flows in a reverse arc towards its east and south. The road to Manali runs right through the middle of this patch of land, which at many places appears like a desert.
Contrary to popular perception, the unprecedented cloudburst did not trigger floods. Mountains in the dry, cold desert of Ladakh are made of rocks stuck in loose, sandy formations. Concentrated rains, thus, immediately turn them into mortar which then slithers down as a rumbling mass of cold lava and obliterates anything that comes in its way. Once the mass settles after exhausting its momentum, the water locked in the mortar and blocked behind the mass breaks free into gushing streams.
Inputs from: Popular Mechanics: books.google.co.in; newspapers: Times of India, Hindustan Times, deccanherald.com, oneindia.in, 7-10 August 2010.