Tuesday 3 November 2015

Multiple deaths after landslide pushes bus into ravine in Rasuwa District, Nepal.

Thirty-four people are known to have died, twenty five people are being treated in hospitals and many more people are missing after a landslide swept a bus into ravine in Rasuwa District, Nepal, on Tuesday 3 November 2015. The incident happened at about 12.45 pm local time, hitting the bus which was en route from Kathmandu to Chilime, pushing it off the road and causing it to tumble about 500 m down the hillside. At least one child is known to be among the dead. The bus is reported to have been heavily overloaded at the time of the incident, with about 100 people on board, many of whih were traveling on the roof.

Rescue workers searching for survivors following the November 2015 Rusawa District bus crash. Kathmandu Post.

The cause of the incident is not clear at the current time. Landslides are common during the monsoon season in Nepal, which lasts from May to September, with  the highest rainfall occurring in July. 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. 

The approximate location of the November 2015 Rusawa District bus crash. Google Maps.

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. This situation is particularly intense in South Asia, due to the presence of the Himalayas. High mountain ranges tend to force winds hitting them upwards, which amplifies the South Asian Summer Monsoon, with higher winds leading to more upward air movement, thus drawing in further air from the sea. 

Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.

However this year many areas of Nepal have suffered a severe droughts during what should be the wettest months of the year, leading to widespread crop failure. This has been linked to the El Niño weather system that has disrupted weather patterns across much of South Asia this year.

The El Niño is the warm phase of a long-term climatic oscillation affecting the southern Pacific, which can influence the climate around the world. The onset of El Niño conditions is marked by a sharp rise in temperature and pressure over the southern Indian Ocean, which then moves eastward over the southern Pacific. This pulls rainfall with it, leading to higher rainfall over the Pacific and lower rainfall over South Asia. This reduced rainfall during the already hot and dry summer leads to soaring temperatures in southern Asia, followed by a rise in rainfall that often causes flooding in the Americas and sometimes Africa. Worryingly climatic predictions for the next century suggest that global warming could lead to more frequent and severe El Niño conditions, extreme weather conditions a common occurrence.

However late October and Early November have seen unseasonably heavy rainfall and early snows across much of Nepal, which may be related to the activity of Cyclone Chapala, which has moved across the northern Arabian Sea during the past week and is currently battering the coast of Yemen, and has brought unusually wet weather and even flooding to many areas of southwest Asia and northeast Africa.

The passage of Cyclone Chapala across the northern Arabian Sea. Tropical Storm Risk.

Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides.

See also...

Three people have been confirmed dead after a landslide hit a bus traveling from Palpa to Butwal on the Siddhartha Highway near Siddhababa...

Over 120 people are feared to have died after Cyclone Komen made landfall in Bangladesh on 1 August 2015. The storm brought widespread flooding to coastal...

One pupil has been confirmed dead and at least 13 more students and teachers have sustained injuries after a landslide struck a secondary school in the Chhorepatan area of Pokhara on Thursday 16 July...

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