Sunday 14 October 2018

Landslides and flash floods kill 36 in North and West Sumatra.

Twenty nine people have died in a series of flash floods and landslides in North Sumatra associated with the onset of the Northeast Monsoon, on Friday 12 October 2018. The worst incident occurred in the village of Muara Saladi in Mandailing Natal District, where a landslide hit an Islamic School sweeping away 29 pupils and several teachers. The majority of these were rescued alive, but twelve of the children died. 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. A further twelve people were killed in flash floods elsewhere in Mandailing Natal District, with waters up to two metres deep that swept away seventeen houses and a number of cars. Flash floods are a particular problem in areas where urban sprawl has resulted in areas where the ground is covered by non-porous concrete of similar materials, preventing rainwater from sinking into the soil. In neighbouring Sibolga District a further four people died as landslides destroyed 29 homes and floods hit about a hundred more. In West Sumatra five people including two children were killed in Tanah Datar District, and four in Padang Pariaman and West Pasaman districts.

The remains of an Islamic School in Mandailang District, North Sumatra, destroyed by a landslide on Friday 12 October 2018. Antara Foto/Reuters.

Sumatra has a wet tropical climate, with a rainy season that lasts from October to April, when rainfall typically reaches 200-300 mm per month and a dry season from May to September, when rainfall is usually below 200 mm per month (though the area is never truly dry. This is driven by the Southeast Asian Monsoon Seasons, with the Northeast Monsoon driven by winds from  the South China Sea fuelling the wetter rainy season and the Southwest Monsoon driven by winds from the southern Indian Ocean the drier dry season. 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.

 The winds that drive the Northeast and Southwest Monsoons in Southeast Asia. Mynewshub.

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.
  
See also...

https://sciencythoughts.blogspot.com/2018/06/eruption-on-mount-kerinci-sumatra.htmlhttps://sciencythoughts.blogspot.com/2018/06/poachers-kill-tame-elephant-in-aceh.html
https://sciencythoughts.blogspot.com/2018/04/explosion-at-illegal-oil-well-kills-21.htmlhttps://sciencythoughts.blogspot.com/2018/03/three-dead-as-flash-flooding-hits.html
https://sciencythoughts.blogspot.com/2018/03/magnitude-52-earthquake-off-west-coast.htmlhttps://sciencythoughts.blogspot.com/2018/02/eruption-on-mount-sinabung-sumatra.html
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