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.
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.