Rainfall and Landslides in Southern California

Science Center Objects

A summary of recent and past landslides and debris flows caused by rainfall in Southern California.

Bluebird Canyon landslide, near Laguna Beach CA

Bluebird Canyon landslide, June 1, 2005, near Laguna Beach, California that caused 350 homes to be evacuated, of which 15 were damaged or destroyed. The movement was most likely caused by heavy rains in January and February 2005.

(Credit: Jim Bowers, USGS. Public domain.)

Like the northern part of the state, southern California is well known to be susceptible to landslides (see Preliminary soil-slip susceptibility maps, southwestern California - Open-File Report 2003-17). Some are triggered by earthquakes, but more frequently landslides are caused by intense and/or prolonged rainfall. Some, but not all, of the major winter storms that have caused landslide fatalities and property damage in southern California have occurred during El Niño (1997-98 info) conditions.

The USGS has a long history of research to identify landslide hazards in southern California (see Southern California Landslides—An Overview - Fact Sheet 2005-3107). Below is a summary of reports and maps to identify hazards associated with different types of landslides in this part of the state.

These reports and maps can be used as examples of what may occur during the upcoming 2015-2016 El Niño season if heavy precipitation occurs.

A large landslide that transformed into debris flow and recent smaller landslides west of Santa Paula, CA.

A large landslide that transformed into debris flow and recent smaller landslides west of Santa Paula, CA. These landslides were triggered by strong winter storms in 2005.

(Credit: Mark Reid and Jonathan Godt, USGS. Public domain.)

Shallow Landslides and Debris Flows

Shallow landsides are generally less than (3-5 m) (10-15 ft) in depth and can transform into rapidly moving debris flows. Previous work at the USGS has identified both the areas of southwestern California most susceptible to shallow landslides and the rainfall conditions required to trigger slope failures. Maps displaying where shallow landslides are most likely to occur are based on observation of previous landslide activity, topographic slope, and information on the bedrock material (see Preliminary soil-slip susceptibility maps, southwestern California - Open-File Report 2003-17).

Shallow landslides can occur at any time during the winter, but are more likely happen when the ground is nearly saturated. In southern California, at least 25 cm (10 in) of rainfall during the winter is needed to nearly saturate the ground. After this point, a rain burst of 5-6 mm (0.2 to 0.25 in) in one hour has been observed to trigger abundant shallow landslides (see Landslides in Santa Monica Mountains and Vicinity - Prof Paper 851).

Deep-seated Landslides

View of the La Conchita landslide taken January 14, 2005.

View of the La Conchita landslide taken January 14, 2005. The light-colored, exposed rock in the upper part of the photograph is the main scarp of the 1995 slide. The southeast part of the 1995 deposit (right side of photograph) remobilized in 2005.

(Credit: Randy Jibson, USGS. Public domain.)

Deep-seated landslides are generally greater than 3-5 m (10-15 ft) deep. Deep-seated landslides can be triggered by deep infiltration of rainfall, which can take weeks or months to occur. Some move slowly, while others can move rapidly with little notice. The La Conchita landslide in Ventura County is an example of a deep-seated landslide that has experienced both styles of movement (see Landslide Hazards at La Conchita, California - Open-File Report 2015-1067). In 1995, after an exceptionally wet winter, the landslide moved tens of meters (tens of yards) damaging nine houses. In 2005, after a 15-day period of near-record rainfall, a larger area failed rapidly, remobilizing part of the 1995 slide. The catastrophic movement of the 2005 landslide damaged or destroyed 36 houses and killed 10 people.

Recent Burned Areas

2014 Colby Fire above Glendora, CA.

2014 Colby Fire above Glendora, CA. This same area experienced widespread post-fire debris flows following a major winter storm in 1969.

(Credit: Dennis Staley, USGS. Public domain.)

Steep, recently burned areas in southern California are especially susceptible to debris flows (see Southern California–Wildfires and Debris Flows - Fact Sheet 2001-3106). Even modest rain storms during normal, non-El Niño years can trigger post-wildfire debris flows. The USGS has conducted hazard assessments for post-wildfire debris flows for four recent fires in southern CA, as well as numerous fires across the Western U.S. including central and northern California.

In southern CA, the USGS has also identified the rainfall conditions required to trigger post-wildfire debris flows. NOAA uses this information to provide early warning for debris flows in areas affected by the fire.

Early Warning System for Southern California

Coastal Cliff Erosion

Coastal cliffs are subject to wave action as well as precipitation induced seepage.

Coastal cliffs are subject to wave action as well as precipitation-induced seepage. These examples from both northern and southern California showcase several different styles of failure.

(Credit: Brian Collins, USGS. Public domain.)

Many areas of coastal California are subject to cliff erosion and coastal landslides (see new research on El Niño coastal hazards in California). Hazards from these types of landslides can occur both at the bottom of cliffs (from burial) and at the tops of cliffs (from falling over). During the winter season in California, beaches typically erode thereby allowing waves to reach further inland and to inundate the bottoms of coastal cliffs. Wave energy is also typically higher during the winter, and particularly during El Niño events, thereby exacerbating the potential for coastal erosion. Coastal cliff failures may also occur simply as a result of heightened precipitation as well – wave action makes cliffs inherently unstable, and rainfall may be the ultimate trigger for failure, even during times with little to no wave action.

During and just after storms, existing coastal landslides may become reactivated and seemingly stable coastal cliffs may erode and fail rapidly. Background rates of coastal cliff erosion are variable along the California coast (see National Assessment of Shoreline Change Part 4: Historical Coastal Cliff Retreat along the California Coast - Open File Report 2007-1133) and tied to the rock or soil strength of the cliffs among other factors, but these measurements of historic coastal cliff retreat provide indications of places most susceptible to coastal landslides.