Slippin' and Slidin' in Cali

Chances are that you have either read about the recent mudslides in California or you have heard about them in previous instances. Portions of California are unfortunately located in a perfect storm of environmental conditions for mudslide development.

So what are the basic ingredients of a good mudslide, besides Kahlua? Mud, most obviously, which forms when very fine sediment called clay is surrounded by water molecules. Clay particles are often electrically charged, so one clay particle is usually electrically attracted to another, and so on, creating a viscous, sticky mass of sediment. When water molecules fill the voids surrounding clay particles, the resulting mud becomes saturated, begins to lose cohesion and slides downslope. This is especially true if the area has been denuded of vegetation, either intentionally or as a result of fire. The root systems of plants act as powerful anchors to soil and sediment, but when these systems are impaired, mass wasting events (when materials slide downslope) and erosion occur with greater frequency. Similarly, if an area is prone to earthquakes, sediment can vibrate at such a frequency that again, sediment particles are no longer touching, and can behave like a fluid, a process called liquifaction. Finally, rock and sediment are more likely to slide downhill in rugged, steep terrain.

Given these variables: steep terrain, water, fire, earthquakes, it is easy to see why much of California is prone to this sort of geologic hazard. Much of California enjoys warm, dry summers, often creating conditions suitable for wildfire. You can probably think back to previous summers, reading about tens or hundreds of homes destroyed by wildfires in California, which has been experiencing drought conditions for nearly a decade. Winter months are often wet for portions of California, and those same canyons that experienced fire in the summer are likely to experience mudslides in the winter. California is a seismically active, rugged region, which only increases the likelihood of mudslide.

While only 100 people in California have died as a result of mudslide in the past 25 years, that is largely due to evacuation, sound scientific advice, and in some cases, engineering efforts. In Colombia, the eruption of the Nevada del Ruiz volcano melted its snowcap, creating a volcanic mudslide that killed 23000 people in a single day in 1985.

The California Geological Survey advises that people in mudslide-prone regions to not sleep in bedrooms on lower levels during rainy periods, especially those living near rivers and canyons, where mudslides are generally confined. They recommend avoiding building on steep slopes or adjacent to rivers, which is true in all areas of the world. In existing structures, installation of a rain gauge is recommended to be more aware of possible soil conditions, as mudslides are more likely to happen after 3-4" of rain have saturated soils. In some rare instances, retaining walls, as seen in the figure above, can be built around the property, deflecting mudslides to adjacent areas.

As people continue to develop in unsustainable or unsuitable terrains, expect to hear about these events with increased frequency!