Saturday 22 September 2018

Landslides at limestone quarries kill at least 74 in the Philippines.

Landslides at two limestone quarries have killed at least 74 people in the Philippines this week. At Itogon in Benguet Province on Luzon Island the bodies of 45 people have been recovered and 57 more are missing after a landslide struck an accomodation block where people were sheltering from Typhoon Mangkhut (referred to as Typhoon Ompong in the Philippines) on Saturday 15 September 2018. Despite the large number of people still missing, rescue operations have had to be severly cut back following warnings that the site was still unstable by the Mines and Geosciences Bureau, who raised concenrs that the low probability of finding any further survivors were outweighed by the high probability of further landslips at the site, endangering the lives of rescue workers.

Rescue workers remove a body from the scene of a landslide at a limestone quarry at Itogon in Beguet Province, the Philippines, on Tuesday 18 September 2018. Ted Aljibe/AFP.

At Naga on Cebu Island a second landslide, again associated with a limestone quarry, struck two villages on Thursday 20 September, burrying over 20 homes. Twenty nine bodies have now been recovered following this incident, and it is feared that more than fifty further victims could be burried beneath the rubble. 

Rescue workers at the scene of a landside at Naga on Cebu Island, the Philippines, earlier this week. Bullit Marquez/Associated Press.

Both incidents have been linked to heavy rains associated with Typhoon Mangkhut. 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. These events have prompted the Philippine Government to halt mining operations across a large area of the country while safety checks are carried out.

Tropical storms are caused by the warming effect of the Sun over tropical seas. As the air warms it expands, causing a drop in air pressure, and rises, causing air from outside the area to rush in to replace it. If this happens over a sufficiently wide area then the inrushing winds will be affected by centrifugal forces caused by the Earth's rotation (the Coriolis effect). This means that winds will be deflected clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere, eventually creating a large, rotating Tropical Storm. They have different names in different parts of the world, with those in the northwest Pacific being referred to as typhoons.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.

Typhoon Mangkhut is considered to be the most severe tropical storm of 2018, and is attributed with the deaths of 81 people in the Philippines, not counting those at the Naga quarry or any unknown deaths at the Itogon quarry, as well as four in Guangdong Province, China and one in Taiwan. It also caused extensive damage in Maccau and Hong Kong,  where over 240 people were injured, but no fatalities have been reported. 

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