On January 10, 2005, residents of La Conchita worst nightmares became true. Heavy rain triggered a massive landslide that swallowed 13 houses and damaged 23 other homes. By the time rescuers pulled out the last bodies, ten were confirmed dead.
California’s natural landscapes are filled with rock cliffs and verdant rolling hills. However, a deep, dark secret is lurking underneath the veneer if beauty. The sad truth is that many California slopes have the potential to catastrophically move.
Landslides are not new or uniquely Californian, but the do threaten many places where people live, work, and play.
Site inventories should identify if landslides are a possible threat on your site. There may be a question or two on the California Supplemental Exam (CSE) for landscape architects on identifying and inventorying slope stability issues on the test.
A landslide is defined as the movement of rock, soil, or debris down a slope. There are several different types of landslides and many reasons why slopes fail.
Types of Landslides
Several different landslide events are lumped into the generic category of landslide:
- Rock slides
- Debris flows & mud flows
A rockslide is a landslide that consists of a large mass or rock collapsing on a slope. Rockslides occur on steep slopes when rain or seismic activity weaken the cohesive forces on the slope.
Debris flows and mud flows are surface landslide events. These events are triggered by heavy rainfall that causes surface material to become saturated and flow down a slope. Debris flows are particularly common in foothills and mountain areas after fires rage through the landscape. Mudflows are also triggered by exceptionally heavy rain and carry soil and superficial rock downhill. Deep-rooted vegetation can help reduce the risk of mudflows by anchoring the soil. While mud and debris flows can cause a great deal of damage, these events are uncommon and somewhat predictable because the weather greatly influences these events. If the site has bee recently burned, use sand bags or other geotechnical engineering materials to stabilize sensitive areas and protect structures. Long-term solutions involve mitigating slide risk through a combination of engineering and vegetation restoration.
Landslides involve widespread movement of the slope. Landslides can be characterized as shallow or deep.
Shallow landslides are located in the soil profile and move downslope in a block. Shallow landslides usually occur when a permeable surface layer becomes saturated above a less permeable soil or bedrock layer. The extra weight of the water combined with reduced internal friction cause the top layer to slide over the bottom layer.
Deep landslides occur below the rooting depth of vegetation and generally include failure or the bedrock. Some landslides are very slow moving while others can move rapidly with catastrophic consequences. When you inventory a site, you may be able to observe evidence of past landslides. The upper part of the slope will be a steep scarp while the toe will be steep due to the deposition of material during the movement.
Why Slopes Fail
There are many reasons why landslides occur and slopes fail. Many are natural conditions that occur only during the right combination of weather events. Other causes may be man-made.
Natural Causes of Landslides
Groundwater pressure – Excessive rain can fill the soil’s pores and reduce internal friction that helps hold the slope in place.
Loss of vegetation – Roots fail to hold the soil together and anchored in place. This is common after wildfires reduce vegetation.
Erosion at the toe of a slope – This is a frequent occurrence on coastal slopes where wave action has eroded the base of the slope.
Saturation by rainwater or snow melt is another cause of landslides in California. Most landslides occur during the winter rainy season. Above average rainfall may create conditions that trigger landslides.
Earthquakes can also trigger landslides due to vibration that reduces the slope’s ability to resist sliding. Soils prone to liquefaction may be at higher risk of sliding during an earthquake.
Man-made Causes of Slope Failure
Deforestation and construction may create conditions favorable for landslides by altering the natural equilibrium of the slope and changing conditions that permitted slope stability.
Earthwork that alters the shape of the slope could trigger a landslide.
Saturation of the soil through man-made means. For example broken pipes or leaking swimming pools could trigger slope movement the way that excessive rain can. Extra soil water causes the slope to lose it’s natural friction and slide.
These are some reasons why slopes fail and landslides occur.
How to Identify Landslide Conditions
Many areas in California are susceptible to landslides. This map is based off of USGS and CGS data.
Here are some resources to consult during a site inventory.
Landslide Hazard Identification Maps
The California Geological Survey (CGS) and United States Geological Survey (USGS) have many maps that show the potential for landslide activity for a given area. You can consult these maps (most of which are available online) during the site inventory process.
If a map indicates that landslides are a possible threat for your site, consult a geologist, a geotechnical engineer, or a professional engineer for further analysis.
Core drilling, also known as coring, involves removing a sample of the site’s soil and geology to identify the site’s underlying statiography. A trained engineer can identify potentially hazardous conditions and suggest mitigation solutions.
Famous California Landslides
California has its share of landslides. Here are a few well-known disasters.
- Devil’s Slide, San Mateo County
- La Conchita, Ventura County
- Laguna Beach, Orange County
Watch California landslides in action. See the devastation that moving earth can cause to the built and natural environment.