A year after the devastating flood, which claimed 483 lives, damaged around 50,000 houses and landed 14.5 lakh people in relief camps, Kerala is experiencing another bout of floods and landslides, which have ravaged the northern parts of the state.
Express investigates the reasons for the increasing frequency of extreme climatic events.
Reasons for extreme global weather conditions
According to climate experts, the frequency of extreme climatic events, like flood and drought, is set to increase due to global warming.
The amount of water vapour in the atmosphere has increased due to human-caused warming. This extra moisture is carried by storm systems, resulting in heavy rainfalls.
Climate change has also altered the characteristics of the atmosphere that affect weather patterns and storms.
Why does it flood in Kerala?
Kerala’s topography comprising high mountains, deep valleys and coastal plains increases the probability of floods. The state stretches from north to south along the 590 km coastline with an approximate breadth of 35 to 120 km.
Western Ghats plays a key role in maintaining the climate. While rampant deforestation, encroachment on hills, unbridled construction activities on hill slopes and granite quarrying in ecologically sensitive areas has increased the possibility of landslides, the filling of flood plains like paddy fields and wetlands have destroyed the natural mechanism to hold floodwaters.
While Kerala had 8 lakh hectares of paddy fields in the 1970s, around 6 lakh hectares have been filled and converted for non-agricultural use.
Rapid urbanisation has increased the number of concrete houses and tarred roads, which reduced the rate of water percolation, adversely affecting the recharging of groundwater sources.
Why 24 hours of rainfall leads to flood when the big dams are empty?
There is a marked shift in the monsoon pattern in recent years with Kerala recording deficit rainfall in June and July, the two months that accounted for 68 per cent of rainfall till two decades ago.
When it rains at regular intervals, water will seep in and recharge the underground sources. When it rains heavily, the water drains and flows to the sea. While the monsoon was 36 per cent deficit till July 31 this year, the state received an average of 500 mm rain during the first 10 days of August.
How it rains in Kerala?
Troughs along the Western Ghats, low-pressure areas arising in the Bay of Bengal and Somali jet phenomenon play an important role in bringing rains to the Western Ghats.
The depression in the Bay of Bengal pulls the winds in the Arabian Sea. If there is no depression, the wind flows in the North-West direction. The impact of the depression makes the monsoon jet flowing at 1,500 ft height to blow at 90 degrees angle perpendicular to the Western Ghats.
This year, the state did not receive proper rainfall during June and July as Cyclone Vayu pulled the moisture from the Arabian Sea northwards.
There was a marked change in wind pattern. The wind blew at 45 degrees angle, taking the monsoon clouds to Maharashtra which led to the flooding of Central India.
Landslide prone areas in Kerala:
Nedumangad, Munnar, Udumbanchola, Cheruthoni, Thodupuzha, Kanjirappally, Meenachil, Chittoor, Mannarkkad, Eranad Nilambur, Taliparamba.
How Southwest Monsoon takes birth
The Arabian Sea, the Bay of Bengal, and the Indian Ocean help moisture accumulate in the atmosphere during the hot season.
The Southwest Monsoon takes birth in the equatorial region, off the Madagascar coast and after hitting Somalia, it takes a south-westerly direction.
The moisture-laden winds from Madagascar travel north towards Somalia and as it crosses the equator turns right to travel in the southwest direction.
The Western Ghats helps the formation of orographic precipitation. The Western Ghats is the first highland the southwest monsoon winds encounter while entering the subcontinent. The Western Ghats makes effective orographic barriers for the monsoon winds.
In summer, hot conditions prevail in the subcontinent and temperature rises quickly. This creates an extensive low-pressure region, which leads to air over the Indian Ocean to flow towards the land. This air current is called ‘southwest monsoon’.
The winds carrying the wet oceanic air enters the subcontinent through Kerala due to the inverted triangle shape of India and generate precipitation after hitting the Western Ghats.
Expert opinion
S Sudevan, former IMD director.
V P Dinesan, Geomatics Division Head, Centre for Water Resources Development and Management, Kozhikode.
K Satheesan, Associate Professor, Department of Atmospheric Sciences, Cochin University of Science and Technology.