Sciency Thoughts: Hiroshima

Showing posts with label Hiroshima. Show all posts

Friday, 16 August 2019

Three dead as Tropical Storm Krosa makes landfall in Japan.

Three people have died and more than fifty more have been injured after Tropical Storm Krosa made landfall on Honshū Islands, Japan, close to Kure City in Hiroshima Prefecture, at about 3.00 pm local time on Thursday 15 August 2019. Neneko Mori, 11, died after being swept out to sea with her sister near Shimoda City; her body was later recovered at Shizuoka on the other side of Suruga Bay, her sister was able to swim back to shore alive. In Hyogo Prefecture fisherman Noriyuki Kumabuchi, 71, died after being swept into a river, and in in Onomichi in Hiroshima Prefecture an 82-year-old man died after being swept into the sea while tyong up a boat.

Large waves associated with Tropical Storm Krosa at Shirahama in Wakayama Prefecture. Tatsuya Onishi/Mainichi.

Wakayama Prefecture

Shirahama

The storm brought with it very high rainfall, with 869.5 mm of rain recorded in three hours at Yanase in Kochi Prefecture. This triggered a number of landslides, which are not thought to have caused any injuries but which have blocked roads in some areas. 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. Other transport networks have also been disrupted, with numerous flights and rail services cancelled due to the storm.

Waves associated with Tropical Storm Krosa in Miyazaki Prefecture. Kyodo News/Reuters.

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.

A tree uprooted by Tropical Storm Kroda in Hiroshima. Kyodo News.

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.

Saturday, 7 July 2018

Fifteen known deaths as floods and landslides hit Japan.

Fifteen people are known to have died around 50 are missing and over 210 000 more have been forced to evacuate their homes as a series of landslides and floods swept across southwestern Japan this week. At least one person has died in a landslide in Okayama Prefecture, on southwestern Honshū Island, while in neighbouring Hiroshima Prefecture two people have died, one due to another landslide and the second a child caught in a flood, while five people are still missing after their hose collapsed. In Hyogo Prefecture three construction workers were swept down a storm drain by flooding, though two of these were pulled out alive. In Kyoto Prefecture a 52-year-old woman is reported to have died, though details of this incident have not yet been released. In Ehime Prefecture on Shikoku Island, a woman has been found dead after her house was hit by a landslide. Evacuation orders are in place for parts of Ibari, Osaka, Kobe, and Gifu prefectures.

A train derailed by a landslide in Saga Prefecture, Japan, earlier this week. STR/AFP/Getty Image.

Many areas of east and south Asia have been hit by exceptional rainfall this year, leading to a string of fatalities in flood and landslide events. Until now Japan has been relatively unaffected, but this week, Typhoon Prapiroon, which passed through the sea of Japan earlier this braining high rainfall with it, with as much as 400 mm of rain falling in a single day in some areas. 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. Parts of Japan are currently considered to be at a higher risk of landslides due to a Magnitude 6.1 Earthquake in Kobe Prefecture last month, which weekend both man-made and geological structures in the region.

Road blocked by a landslide in Gifu Prefecture, Japan, on 4 July 2018. Gifu Prefectural Government.

Gifu Prefectural Government

Tropical storms (called Typhoons in the West Pacific and Indian Oceans) 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 in rushing 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.

Floodwaters in Hiroshima City on Saturday 7 July 2018. Kyodo News.

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.

Saturday, 23 August 2014

At least 49 dead following Hiroshima landslides.

49 people are known to have died and a further 41 are still missing following a series of landslides that hit the Japanese city of Hiroshima on Wednesday 20 August 2014. The landslides occurred following a period of exceptional rainfall, with 215.7 mm of rain falling in just three hours, between 1.30 and 4.30 am local time; more than falls in the city in most months. 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.

One of a series of landslides that hit Hiroshima on Wednesday 20 August 2014. Kyodo News/AP.

The worst affected areas were in the north of the city, in the wards of Asakita-ku and Asaminami-ku. These areas lie on a bedrock of decomposed granite, which can be particularly hazardous when it begins to erode. While granite is generally thought of as being extremely hard, it is made up of large grains of several different minerals, typically plagioclase feldspar, orthoclase feldspar, quartz and mica, and under some circumstances one or more of these minerals can become chemically weathered, breaking up the structure of the rock and allowing the other minerals to crumble as free grains. The most common form of chemical weathering involved is the breakdown of the feldspar minerals in granite to kaolinite clay (the white clay used to make porcelain ceramics), something that happens readily in warm wet climates, such as that of southern Japan.

Rescue workers searching for survivors in Asaminami-ku Ward, Hiroshima, following the 20 AUgust 2014 landslides. Reuters.