Saturday 10 August 2019

Landslide feared to have killed more than fifty in Kerala State, India.

Twenty eight people have been confirmed dead and another twenty seven are missing after a landslide swept through the village of Adyanpara in Malappuram District, Kerala, at about 8.00 pm on Thursday 8 August 2019.  Local authorities had warned that part of the Bhoothathan-Muthappan Mountain above the community was unsafe and tried to organise an evacuation, but many people had refused to leave. Around 35 homes are reported to have been destroyed by the landslide.
The aftermath of a landslide that hit the village of Adyanpara in Malappuram District, Kerala, on 8 August 2019. Shaju John/The Hindu.

The incident happened after 240 mm of rain fell within a 24 hour period, associated with the Southwest Monsoon, which lasts from May to September in Kerala.. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. 

Monsoons are tropical sea breezes triggered by heating of the land during the warmer part of the year (summer). Both the land and sea are warmed by the Sun, but the land has a lower ability to absorb heat, radiating it back so that the air above landmasses becomes significantly warmer than that over the sea, causing the air above the land to rise and drawing in water from over the sea; since this has also been warmed it carries a high evaporated water content, and brings with it heavy rainfall. In the tropical dry season the situation is reversed, as the air over the land cools more rapidly with the seasons, leading to warmer air over the sea, and thus breezes moving from the shore to the sea (where air is rising more rapidly) and a drying of the climate.

 Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.

Kerala has a complex seasonal cycle, driven by the presence of the Western Ghats mountain range, which largely block the dry northerly winds which dominate the climate of much of India, and its proximity to the equator, which leads to a double monsoon system. Such a double Monsoon Season is common close to the equator, where the Sun is highest overhead around the equinoxes and lowest on the horizons around the solstices, making the solstices the coolest part of the year and the equinoxes the hottest. In Kerala this results in a Southwest Monsoon, which lasts from May to September, and is driven by winds from the southern Arabian Sea dumping water onto the Western Ghats, followed by a Northwest Monsoon, which lasts from October to December, where winds from the Bay of Bengal do the same. Of the two monsoons, the southwest is the wetter, due to the proximity of the sea, with June typically being the wettest month, with an average of 341 mm of rain falling in the month.

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