At least fifty people, including a number of children, have died, with several hundred still missing ans fifty nine being treated in hospitals for a variety of serious injuries, with several thousands having been made homeless, following a series of landslips and floods around in the Sentani Subdistrict of Papua Province, Indonesia, on Saturday 16 March 2019. The worst of the flooding appears to have occurred after a landslip partially blocked the path of an already swollen river, causing it to burst its banks.
The flooding and landslips have been linked to heavy rains in the area, which has been experiencing a particularly severe rainy season this year. 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. This problem has been made worse by the clearing of forests for palm oil production, the wholesale clearing of trees for timber (often illegally, due to the high value of tropical hardwoods on the international market), both of which lead to the loss of the trees which often stabilise soil on slopes, as well as an expanding population, which has led to people farming higher on hillslopes, in an area where soils tend to be volcanic in action and poorly consolidated (i.e. lack much cohesion), making them more prone to landslides.
Papua has a tropical climate, with a Rainy Season related to the Northeast Monsoon that lasts from December to March, with each of those months typically receiving more than 250 mm of rain. Indonesia has two distinct Monsoon Seasons, with a Northeast Monsoon driven by winds from the South China Sea that lasts from November to February and a Southwest Monsoon driven by winds from the southern Indian Ocean from March to October (this Southern Monsoon does not typically reach Flores, resulting in a dry season over these months). 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.
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