Thirteen rescue workers killed trying to reach site of Vietnam landslide.

Thirteen rescue workers trying to reach the site of a landslide which hit a construction site in Thua Thien Hue Province have been killed in a second event. The initial landslide hit workers at the Rao Trang-3 Hydropower Plant, and is believed to have killed at least seventeen people on Monday 12 October 2020, and is believed to have killed at least seventeen people, with several dozen more still missing, though details of the event are still unclear as ongoing heavy rains have made much of the area very unstable. 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. It has now emerged that a further thirteen rescue workers, eleven of them army personnel, were killed in a second landslide that hit them while they were resting at a ranger outpost en route to the site on Tuesday 14 October. 

 

Military personnel in Vietnam on the way to the Rao Trang-3 Hydropower Plant construction site to take part in rescue attempts on 13 October 2020. VNExpress.

Thua Thien Hue Province has a wet climate, with mountainous regions typically receiving around 4500 mm of rain per year, of which about 70% falls during the rainy season, between September and December, when flooding and flood-related events such as landslides are relatively common. However, this year the rains have been particularly heavy, with flood events and fatalities recorded across much of Vietnam. This has been made worse in many areas by rapid urban expansion, with concrete and paving rapidly covering many areas where exposed soil previously allowed rainwater to drain away, increasing the risk of flooding. Areas of rapid construction in formerly forested areas are particularly at risk of landslides, due to the presence of exposed soils.

 

Flooding in Quang Tri Province, Vietnam, on 13 October 2020. Ho Cau/VNA/Reuters.

To make matters worse Vietnam has been hit by an unprecedented series of tropical storms this year, with seven Typhoons having hit the country this year, with the most recent, Typhoon Nangka, having hit the coast in Ninh Binh province on 14 October, resulting in widespread flooding in central Vietnam, and at least 36 deaths. A sharp increase in the number of tropical storms hitting Vietnam, combined with the inundation of coastal areas by rising seawaters, has been predicted as a likely outcome of rising global temperatures by the Intergovernmental Panel on Climate Change, with the country considered to be one of the most vulnerable nations to the impact of climate change.

 

Storm surge associated with Typhoon Nangka hitting the coast of Đồ Sơn District in Haiphong, Vietnam, on 14 October 2020. VNExplorer.

 

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.

 

The formation of a tropical cyclone. Natural Disaster Management.

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. 

The formation and impact of a storm surge. eSchoolToday.

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