Allan G. Barrows, Siang S. Tan and Pamela J. Irvine, Geologists
Landslide Hazard Identification Project
Division of Mines and Geology
As has happened several times during recent decades, severe winter storms drenched portions of landslide-prone coastal southern California, with predictable results. Paradoxically, water, like fire, can be both a friend and an enemy. In January and February, when most Californians hailed the end of rigorous water-usage controls, dozens of homes, mostly in Orange and Los Angeles counties, were being destroyed or damaged by disastrous slope failures. It was a case of too much water for many slopes to accommodate in too short a time.
In other disastrous wet years, such as 1969, 1978, and 1980, deadly debris flows smashed into homes killing several dozen people and causing several hundred million dollars of damage. In contrast, landslides associated with the 1993 storms were mostly deep-seated, rotational or translational, slides and slumps in cut slopes. This year’s widespread landsliding caused extensive damage to homes, streets, and utilities, with losses that may ultimately exceed $60 million. Although flooding during the rainstorms caused 20 deaths in California and as many as 34 in nearby Mexico, there were no fatalities or serious injuries from slope failures. One possible exception, if attributable to the storms, is a sea-cliff rockfall near Santa Barbara that killed one man and injured another on March 28.
Whenever severe winter storms strike California, the bulk of the damage and loss of life is caused by manifestations of flooding, such as inundation, erosion of roadways, destruction of bridges, and undercutting of drainage-channel walls. Various kinds of slope failures (collectively referred to as landslides), cause widespread damage as well. Estimating the losses due directly to lansliding is a continual problem because flood and landslide damages are invariably evaluated together by local agencies. With this caution in mind, a preliminary estimate of public and private property damage due directly to landsliding in southern California is $30 million.
Loss estimates are very different if they include disruptions of the economy. For example, the blockage of the Santa Fe Railroad tracks by a landslide in Dana Point forced nearly 6,000 Amtrak passengers traveling between San-Diego and Los Angeles daily to be bused around the closure for 2 weeks. Another example is the loss of business due to blocked access, as along Pacific Coast Highway in Dana Point where the road was closed because of a landslide. Even rare or unique incidents contribute to landslide losses. Such was the case when a landslide ruptured a 26-inch (66-cm) natural gas main near Castaic on February 24. Flames shot up 100 feet (30m) and burned about and acre (4,047m2) of brush.
Twenty-two homes were destroyed and 137 damaged by slope failures in southern California during the storm season. Orange County suffered the bulk of the losses as a result of the prolonged storms that dumped rain intensely on the vulnerable slopes. Geologists with the Division of Mines and Geology’s Landslide Hazard Identification Project observed most of the failures described in the brief accounts that follow.
1a.) Anaheim “Avenida de Santiago” Landslide (Anaheim Hills). The largest and most publicized landslide associated with the 1993 rainstorms is in the elevated eastern part of Anaheim, where homes are valued at $400,000 to $1 million. The landslide, named for a street near the headscarp, covers about 57 acres (23 hectares) on a north-facing dip slope in upper Miocene marine sandstone and siltstone. This translational landslide is about 1,300 feet (396m) wide by 1,900 feet (579m) long. After minor cracks were observed in April, 1992, (following the locally intense February, 1992 rainstorms) the city hired geologic consultants to investigate the slope stability. Before the geologists could complete their studies, January 1993 storms dumped two to three times the normal amount of rainfall of parts of southern California. On January 17, 1993, the slide began to move at the rate of about 1 inch (2 to 3 cm) per day. Cracks and fissures developed across streets and through houses at the head and compressional features damaged houses lower on the slope within the landslide.
The city evacuated residents from 46 homes on January 18 and a massive dewatering program was initiated. More than 100 horizontal and vertical wells were drilled into the slide mass and, by early April, 12 to 15 million gallons (45,420 to 56,775 m3) of water had been removed. By mid-April, it was determined from 10 slope indicators (devices in the ground) the slide was no longer moving. Three homes have since been condemned and nine others have major damage. Most of the remaining homes have been reoccupied, following repair of utilities. Losses are about $4 million.
1b.) Anaheim “Pegasus Street” Landslide (Anaheim Hills). On January 18, 1993, about 2,000 feet (610 m) west of the “Avenida de Santiago” landslide, a slump in a buttress fill at the head of Pegasus Street destroyed the dewatering system that was installed earlier to help stabilize a problematic slope. A retaining wall failed and one home was destroyed. Damage is about $500,000.
Effective and ineffective measures to protect slopes from the effects of heavy rainfall were conspicuous in many places in coastal southern California during the storm season. Especially hard-hit during January was San Clemente, where acres of plastic sheeting were spread to protect hundreds of steep slopes in the hillside residential districts. Adding to the problem is the siltstone and claystone of the underlying Capistrano Formation, which, due to its highly expansive clay content, is the most landslide-prone bedrock unit in Orange County.
The effectiveness of various approaches to slope-stabilization was dramatically demonstrated in San Clemente on streets such as Calle Familia. Here, the approximately 80-foot-high (24-m-high), westfacing slopes had been graded at a slope angle of 1-1/2 to 1 (about 34 degrees). Techniques used to protect the slope materials from erosion or slippage were easy to compare among neighboring properties. Photo 4 shows a system of well-constructed drainage ditches and channelways, combined with a dense ground cover, protecting the center property. In contrast, slopes on both sides planted only with grass were damaged by shallow landsliding and by erosion of the soil mantle. In Photo 5, the slope on the left was planted with grass along with a pattern of drought-resistant bushes with deeper root systems that held the slope intact. The slope to the right, although planted with a few deciduous trees, is mostly grass and it, too, lost much of the soil cover and grass.
During the most intense periods of rain in January and February, news reports were filled with references to “mudslides”, especially affecting Pacific Coast Highway and roads in the Santa Monica Mountains. The term “mudslide”, although vernacular for the media, is a misnomer because mud does not slide, it flows. Terms such as mudflow or debris flow are used by scientists to describe shallow slope-failure phenomena. In recent decades, surficial slope failures, triggered by prolonged and intense rainstorms, were widespread in southern California. This year, despite heavy rain in vulnerable areas, flows rarely developed except, perhaps, in oversteepened road cuts.
One of the places where soil slips and debris flows formed is along the steep south-facing slopes of the Puente Hills in Yorba Linda. Surficial soil slip and debris flow scars are abundant in silty rocks of the Yorba Member of the Puente Formation along the Whittier Fault Zone. The rocks’ composition (predominantly silty sandstone), the uptilted and sheared beds’ unusual steepness along the recently active Whittier Fault Zone, and intense rainstorm activity all contributed to the concentration of surficial slope failures. Expensive homes, built at the base of these hills in recent years, were protected by well-engineered and well-built debris basins and dams because of the nearby slopes’ potential for generating debris (Photo 12). Awareness of the potential for slope instability, and proper planning and engineering, can mitigate the harmful effects of rainstorm-triggered slope failures.
Each natural disaster reminds us that there are steps that can and should be taken to reduce losses associated with it. When disruptions to business and travel are added to actual physical property damage, losses from California landslides average about $100 million a year. In California, property damage most often occurs to older structures built before adoption and enforcement of modern grading codes and ordinances. Nevertheless, landslide damage or destruction of so many recently built structures provokes us to consider what else needs to be done to reduce or eliminate landslide hazards.
One response to the continuing problem of landslide hazards in California was the passage of the Landslide Hazard Identification Act in 1983. It established, via the Public Resources Code, the Landslide Hazard Identification Project within the Division Of Mines and Geology. The Landslide Project prepares landslide hazard maps for California’s urban and urbanizing areas. The aim is to assist cities and counties in fulfilling their responsibilities for protecting public health and safety from landslide and other unstable slope hazards. The maps include inventories of existing landslides and assessments of landslide susceptibility in undeveloped terrain.
Although the Landslide Project maps help local government planners incorporate safeguards into the Safety Element of the General Plan, there are still “weak links” in the chain of public protection from slope failures. Most jurisdictions in California require geoligical or geotechnical studies prior to development on hillsides. Unfortunately, the quality of review of these studies varies greatly. Not all local governments have qualified staff and/or retain professional consulting firms to perform reviews. This variable review of plans for development has probably contributed to some of the slope failures in relatively new developments in areas where “we should have known better.” A remedy, as proposed by the Interagency Hazard Mitigation Team convened by the Federal Emergency Management Agency (FEMA) and Office of Emergency Services (OES) to investigate the California Severe Winter Storms disaster of January-March 1993, may be to establish minimum standards for review of geologic reports, possibly through state legislation.
The catastrophic loss of homes due to landslide during the January-February storms of 1993 provides a sad reminder that, in many cases, the slopes did not have to fail. The severity of failure could have been greatly reduced if mitigating measures had been taken prior to the inevitable rainstorms. Installation and maintenance of dewatering and drainage systems within ancient, but metastable, landslides or in landslide-prone slopes can be effective in reducing, if not eliminating, landslide hazards.
We are grateful for discussions held in the field with several geologists working on various landslides. Mark McLarty with Eberhart and Stone, Inc. provided information about the ongoing dewatering and investigation of the “Avenida de Santiago” landslide in Anaheim Hills. Jim Slosson, of Slosson and Associates, discussed landslide in Agoura Hills. Wes Reeder, San Bernardino County geologist, provided information about the “Rim Forest” landslide.
Copyright © 1973 California Geology
Editor’s Note: As stated in the above article, the Anaheim “Pegasus Street” Landslide, which failed on the same day as its next door neighbor, the “Avenida de Santiago” landslide, was caused by “a slump in a buttress fill at the head of Pegasus Street (which) destroyed the dewatering system that was installed earlier to help stabilize a problematic slope.” When the land in the Pagasus area moved it broke the dewatering wells and pipes. Water was no longer being drained and the homeowners association just assumed that the hill was dry -- while the unseen reality was that it was constantly filling with water. What this proves is that the statement made to me by Mark McLarty, geologist with Eberhart and Stone, Inc., that: “Dewatering is a stopgap measure and not mitigation,” is true and accurate. If all you are doing is dewatering, then you are doomed to failure! The hill will slide again!