Thursday 9 August 2018

Landslides and floods kill at least 22 in Kerala State, India.

Twenty two people are known to have died and several more are missing following a series of landslides and flash floods across Kerala State in India on Thursday 9 August 2018. Ten people are reported to have died in Idukki District, five in Malappuram District, two in Kannur District and one in Wayanad District, with details of the other fatalities not yet released. The incidents have been caused by heavy rains 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.  

 The aftermath of a landslide in Kozhikode, Kerala, on 8 August 2018. AP.

Many of the worst ht areas were along the banks of the river Idamalayar, where water was released from the Idamalayar Dam at an increased rate after floodwaters threatened to overstress the dam. Waters on the Ikkudi Dam are also reported to be dangerously high, and water will probably also be released from this dam in the near future.

Water being released from the Ikkudi Dam in Kerala on 8 August 2018. Etemaad.

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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