Sunday 18 October 2015

Flash flooding brings chaos to South California.

Many areas of southern California are recovering after a series of thunderstorms caused flash flooding across parts of Los Angeles, San Luis Obispo, Santa Barbara, and Ventura counties on Thursday 15 and Friday 16 October 2015. The flooding triggered mudslides and landslips in many areas, blocking roads and trapping people in vehicles as well as causing damage to homes and other properties. 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. No loss of Human life has been reported following the incident, but farms in many areas are reporting losses of Cattle and other livestock.

A pickup truck trapped by a mudslide on State Route 58 close to the Mojave Desert. CBS.

Flash floods are a common problem in southern California (and other areas with a dry climate) as protracted periods of dry weather can cause topsoil to dry out completely, making it vulnerable to being blown away by the wind. When rain does arrive it then falls on exposed bedrock, which is much less absorbent, triggering flash flooding as the water escapes over the surface of the ground rather than sinking into it. These floods wash away more topsoil, making the problem progressively worse over time. In Los Angeles this is reflected in the concrete channel which contains the Los Angeles River; a spectacular concrete structure which contains for much of the year only a tiny trickle of water, but which was built after a series of devastating floods in the early twentieth century.

Los Angeles River and flood control channel. US Army Corps of Engineers.

The floods come after several months of extreme drought in southern California, that has killed of plantlife in many normally non-arid areas, weakening the network of roots that tends to protect hillsides against collapse, and hardening the soil surface, making flash flooding more likely. It has been suggested by some observers that this may be the beginning of a much wetter climate phase associated with this years El Niño weather system over the southern Pacific Ocean, a phenomenon that typically brings high levels of rainfall to the southwest United States, though if this is the case then the weather has arrived dramatically early, as the increase in storm activity associated with El Niño does not usually begin in California till December.

Vehicle trapped in mud in Green Valley, California. KABC-TV/AP.

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.

Vehicles trapped by a mudslide on California Highway 58 in the Mojave Desert. Mark Ralston/AFP/Getty Images.

Hopes that the onset of high rainfall may bring an end to California's drought are also likely to be thwarted by the low state of reserves in the state's aquifers (underground water resources) which have been heavily over-exploited for industry and agriculture for some years. Such aquifers are slow to refill, with the effect that even a particularly long and wet Californian winter is unlikely to have much of an effect without a change to water-use policies in the state.

See also...

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