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Malibu Disasters & Hazards: Slides
The steep topography of Malibu, combined with heavy rainfall, leads to earth movement. These slides can take the form of individual rocks loosened from a hillside (photo above) or mud and debris flows that can bury an area under many feet of material. A compounding factor is the effect of fires that denude the hillsides as well as produce chemical changes that augment the flow of water and soil.
Some of the slides in recent year include:
Most rockfalls and slides are quickly cleared by highway department crews and few result in closures of more than a few days. However, some unstable areas result in much longer inconvenience for residents, such as the three month closure of PCH in 1979, the slide blocking lower Rambla Pacifica since 1984, and the slide that partially blocked Rambla Vista in 2005.
By one estimate, forty percent of Malibu's residential lots are on land steep enough to be subject to slides. Since most flat plots were developed long ago, new construction is often targeted to very steep and risky property requiring very expensive site preparation to avoid the pitfalls of slides under or above the proposed home. Some of these will be the subject of future news stories when the inevitable slide damages or destroys them.
Fire and Rain Produce Mudslides
Malibu's periodic fires strip vegetation from the hillsides and make them more prone to erosion, flooding and slides. Less obviously, in Malibu's chaparral-covered mountains erosion is amplified by the chemistry of the creosote laden plants, which leave an oily residue just beneath the soil's surface. In a fire, the heat vaporizes the hydrocarbons, some of which are driven into the ground to condense and form a subsurface waterproof layer that is ripe for erosion or the formation of a mudslide in the next heavy rain event. Sometimes all the soil above the layer will just slide off the hill, burying whatever is below.
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