Six houses have been evacuated and another 23 have been left without water following a landslide in the Oakland Hills of Alameda County, California, on Thursday 6 April 2017. The incident happened at about 6.00 pm local time, following over a day of heavy rains. 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 scene of the 6 April 2017 Oakland landslide. Gary Reyes/Bay Area News Group.
California has been battered by a succession of Pacific storms this winter, bringing widespread flooding and triggering a number of landslip and
subsidence events. This is due to exceptionally high temperatures over
the Pacific Ocean off the coast of the state this year due to a La Niña weather system over the southern Pacific; ocean
storms form due to heating of air over the sea, as
the air is heated the the air pressure drops and the air rises, causing
new air to rush in from outside the forming storm zone. If this zone is
sufficiently large, then it will be influenced by the Coriolis Effect,
which loosely speaking means the winds closer to the equator will be
faster than those further away, causing the storm to rotate, clockwise
in the northern hemisphere and anticlockwise in the southern hemisphere.
Whilst
the high winds associated these storms is extremely dangerous, the real
danger from such storms is often the flooding. Each millibar drop in
air pressure can lead to a 1 cm rise in sea level, and large storms can
be accompanied by storm surges several meters high. This tends to be
accompanied by high levels of rainfall, caused by water picked up by the
storm while still at sea, which can lead to flooding, swollen rivers
and landslides; which occur when waterlogged soils on hill slopes lose
their cohesion and slump downwards, over whatever happens to be in their
path.
The La Niña weather system is the opposite of the El Niño weather
system, in which unusually cold surface temperatures spread across the
equatorial Pacific from the upwelling zone on the South American coast.
This traps warm water from the western Pacific, preventing it from
spreading east and warming the central Pacific. This leads to lower
evaporation over the (cooler) east Pacific, leading to low rainfall on
the west coast of South America, and higher evaporation over the
(warmer) west Pacific, leading to higher rainfall over East and
Southeast Asia and northern Australia.
The effect of the La Niña weather system on the weather of North America. NOAA.
This also leads to a breakdown in surface circulation in the North
Pacific, which generally rotates clockwise, so that the same body of
water stays off the coast of California, where it is constantly warmed
by the Sun, leading to high levels of evaporation and onshore winds that
bring high rainfall and flood events to the state.
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