Sunday 22 July 2018

Landslides kill four children in the Philippines.

Four children have been killed in two separate landslides in the Philippines over the past three days. On Friday 20 July 2018 a landslip in Agoo in La Union Province, on northern Luzon Island, crushed a house and burried it under more than a metre of mud, resulting in the deaths of John Jonard Galleros, 12, and his brother Joseph Galleros, 11. In a separate, but similar, incident early on Sunday 22 July, a second house was destroyed in Barbaza in Antique Province, on Panay Island in the Western Visayas group, resulting in the deaths of Rosalia Agapito, 6 and her brother Andrew Agapito, 3, as well as injuring their mother Tessie Agapito, 48.

The aftermath of a landslide that killed two children in Barbaza on Panay Island, the Philippines, on 22 July 2018. Johnny Juanites/Bong Erfe/GMA News.

The events are thought to have been triggered by high rainfall associated with Tropical Storm Josie, which swept past the Philippines this weekend, bringing with it high rainfall and widespread flooding, as well as a storm surge that resulted in thousands of people being evacuated from coastal areas on Luzon Island. 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.

Flooding in Pangasinan Province on Luzon Island. Pangasinan Provincial Disaster Risk Reduction and Management Council Rescue Operations.

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.

Floodwaters in Manila on Friday 20 July 2018. Bullit Marquez/AP.

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.

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