Get Ready & Prepare

Landslide

Overview

Landslide is a general term for various types of "slope failure," that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing. Slope failure can range from slow moving rotational slumps and earth flows, which can slowly distress structures but are less threatening to personal safety, to fast-moving rock avalanches and debris flows that are a serious threat to structures and have been responsible for most fatalities during landslide events. Slope failure occurs when the strength of the soils forming the slope is exceeded by pressure, such as weight, gravity, or saturation. This can be initiated by storms, earthquakes, fires, or human modification of the land. Large debris can block transportation routes, dam creeks and drainages, and contaminate water supplies. When these hazards affect transportation routes, they are frequently expensive to clean-up and can have significant economic impacts.

In Monterey County the most common types of landslides, or slope failures, are debris flows, rockslides, and flood flows.

• Debris Flows: Debris flows are common types of fast-moving landslides in Monterey County. These flows generally occur during periods of intense rainfall. They usually start on steep hillsides as shallow landslides that liquefy and accelerate to speeds that are typically about 10 mph, but can exceed 35 mph. The consistency of debris flows ranges from watery mud to thick, rocky mud that can carry large items such as boulders, trees, and cars. They are particularly dangerous to life and property because of their high speeds and the sheer destructive force of their flow. Debris flow risk in the County is primarily associated with burn scars. Vegetation and soil changes after a fire increase the runoff and erosion in a watershed, and significantly increase the likelihood of debris flows. Debris flows can initiate during even moderate rainstorms over burn areas and often occur with little warning.
• Flood Flows: A flood flow differs from a flood in that the amount of suspended sediment is sufficient to significantly change fluid properties and sediment transport mechanisms. Large volumes of sand are transported in dynamic suspension throughout the water column, although maintenance of high sediment loads depends on flow velocity and turbulence. Flows can be highly erosive.
• Rockslides: A rockslide, common along the Big Sur coast, is a slow-moving or “creeping” landslide in weathered, fractured, or otherwise-weakened rock. Movement on deep-seated rockslides typically is only fractions of an inch per year; however, an increase in rainfall or slope instability can accelerate movement to several yards a minute or faster.

Slope Failure Causes

Mass movements are caused by a combination of geological and climate conditions, as well as encroaching urbanization. Vulnerable natural conditions related to the geology, topography, and vegetation are affected by residential, agricultural, commercial, and industrial development and the infrastructure that supports it. The factors that can contribute to slope failure include: Change in slope of the terrain; Increased load on the land; Shocks, vibrations, and earthquakes; Change in water content; Groundwater movement; Frost action; Weathering of rocks and erosion; Removing or changing the type of vegetation covering slopes.

Landslides often occur together with other natural hazards, thereby exacerbating conditions, as described below.

• Shaking due to earthquakes can trigger events ranging from rock falls and topples to massive slides.
• Intense or prolonged precipitation that causes flooding can also saturate slopes and cause failures leading to landslides.
• Landslides into a reservoir can indirectly compromise dam safety, and a landslide can even affect the dam itself.
• Wildfires can remove vegetation from hillsides, significantly increasing runoff and landslide potential.

History

The Big Sur area is one of the most landslide-prone stretches of the California coast, with over 1,500 mapped historic large landslides. Historically, landslide activity has increased during severe El Niño years. During the 1972–1973 El Niño season, a landslide along the Big Sur coast resulted in one death. Throughout the 1997–1998 El Niño season, Highway 1 closed all along the Big Sur Coast due to flooding and mudslides and about 40 different sections of the highway were damaged.

In more recent year’s numerous landslide events have been recorded in Big Sur, damaging structures, and forcing the closure of Highway 1. 2017 saw record damage along Highway 1. In February, a column of the Pfeiffer Canyon Bridge was displaced by an active landslide, which moved eight inches in one night and destabilized the bridge. This section of Highway 1 was closed for eight months and the new bridge cost about $24 million. In March of 2017, Paul’s Slide buried Highway 1 with debris and left only 11 feet of roadway. Around 2 million cubic yards of material came down the hill. In May, at Mud Creek, a deep-rooted debris flow caused by the heavy winter rains, covered a quarter mile of the Highway 1 in 6 million cubic yards of debris, close to 40 feet of debris slid down the hillside. Caltrans rebuilt the highway atop the slide in a $54 million, 14-month project. In 2021, an atmospheric river resulted in a post-fire debris flow washing out a 150-foot-long stretch of Highway 1 near Rat Creek below the Dolan Fire burn scar. Flows also damaged another 60 locations along the highway.

Besides the Big Sur coastline other areas of the County are vulnerable to debris flows. Throughout the 1997–1998 El Niño season, a series of debris slides failed along the northern flank of Saddle Mountain in Carmel Valley and impacted Saddle Mountain Recreation Area. A landslide in Las Lomas in rural north Monterey County caused several homes to be destroyed and resulted in a Hazard Mitigation Grant Program (HMGP) project that involved buying out the affected homes and preserving the land where the slide occurred as perpetual open space.

The 2017 storms caused flooding and debris flows in Prunedale and Spreckels resulting in mud, and rocks covering various sections of roadway, including Crazy Horse Canyon Road, Live Oak Road, Garin Road, and Elkhorn Road. Highway 101 was blocked at Crazy Horse Canyon Road and Highway 68 was blocked with over 1 foot of mud at River Road. One heavy slide caused mud, about 3 feet deep, to block Moro Road. In 2019, heavy rainfall caused large rocks to block Carmel Valley Road at Laureles Grade. In 2021 an annual expected storm caused a large debris flow in the River Fire burn scar. The flow damaged 25 homes and caused one injury.

Landslide Risk

Landslides are often triggered by other natural hazards such as earthquakes, heavy rain, floods, or wildfires, so landslide frequency is often related to the frequency of these other hazards. In Monterey County, landslides typically occur during and after severe storms, so the potential for landslides largely coincides with the potential for sequential severe storms that saturate steep, vulnerable soils.

Slope failures destroy property and infrastructure and can take the lives of people. When landslides occur — in response to such changes as increased water content, earthquake shaking, addition of load, or removal of downslope support — they deform and tilt the ground surface. The result can be destruction of foundations, offset of roads, breaking of underground pipes, or overriding of downslope property and structures.

Slope failure can pose a serious hazard to properties on or below hillsides. They can block access to roads, which can isolate residents and businesses and delay commercial, public, and private transportation. This can result in economic losses for businesses. Vegetation or poles on slopes can be knocked over, resulting in possible losses to power and communication lines. Slope failures can also damage rivers or streams, potentially harming water quality, fisheries, and spawning habitat.

In Monterey County, landslides and mudslides are a common occurrence and have caused damage to homes, public facilities, roads, parks, and sewer lines. Landslides along the county’s coastline, in conjunction with wave action, have resulted in road failure, severe erosion, cliff failure, and loss of bluff top area that threatens development. In Big Sur, landslides have caused millions of dollars in damages and have led to closures of Highway 1, which can range from a few days to over a year.

Prepare for Landslides

The following are things you can do to protect yourself, your family and your property from the effects of a landslide or debris flow:

• Make sure to Sign up for Alerts. Landslides and debris flows can sometimes occur with little warning time. Signing up for alerts is important to ensure to receive quick and accurate information.
• Make a Plan for your household, including your pets, so that you and your family know what to do, where to go, and what you will need to protect yourselves from landslides.
• Create an emergency kit in case you have to leave immediately or if services are cut off. Keep in mind each person’s specific needs, including medication. Do not forget the needs of pets.
• Prepare for landslides by following proper land use procedures - avoid building near steep slopes, close to mountain edges, near drainage ways or along natural erosion valleys.
• Become familiar with the land around you. Learn whether landslides have occurred in your area by contacting local officials. However, don’t assume that what happened last time will happen next time. Debris flows can start in places they’ve never been and return to slopes where they’ve already been.
• Get an assessment of your property by a qualified geotechnical professional. Protect your property based on of recommendations from the qualified geotechnical professional and/or local guidance on protection from debris flow and flooding. You can't stop or change the path of a debris flow. However, you may be able to protect your property from floodwaters or mud by use of sandbags, retaining walls or k-rails.
• In mud and debris flow areas, consider building channels or deflection walls to try to direct the flow around buildings. Be aware, however, that when a flow is big enough, it goes where it pleases. Also, if you divert a flow and it flows on a neighbor's property, you may be liable for damages.
• If you are at risk from a landslide talk to your insurance agent. Debris flow may be covered by flood insurance policies from the National Flood Insurance Program (NFIP).

Learn to Recognize Warning Signs

Watch for debris flows and other fast moving landslides that pose threats to life:

• If you are near a wildfire burn area, sign up for emergency alerts and pay attention to weather forecasts for the burn area. The weather in the burn area could be very different from where you are.
• Listen and watch for rushing water, mud, unusual sounds.
• Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
• A faint rumbling sound that increases in volume is noticeable as the landslide nears.
• Fences, retaining walls, utility poles, k-rails, boulders, or trees move.
• Huge boulders in the landscape can be signs of past debris flows.

Watch for slow-moving landslides that pose threats to property:

• Changes occur in your landscape such as patterns of storm-water drainage on slopes (especially the places where runoff water converges) land movement, small slides, flows, or progressively leaning trees.
• Doors or windows stick or jam for the first time.
• New cracks appear in plaster, tile, brick, or foundations.
• Outside walls, walks, or stairs begin pulling away from the building.
• Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
• Underground utility lines break.
• Bulging ground appears at the base of a slope.
• Water breaks through the ground surface in new locations.
• Fences, retaining walls, utility poles, or trees tilt or move.
• The ground slopes downward in one direction and may begin shifting in that direction under your feet.

Stay Safe During a Landslide

• Listen to local news stations on a battery-powered radio for warnings.
• Heed all warnings and evacuation notices.
• During a storm that could cause a landslide, stay alert and awake. Many deaths from landslides occur while people are sleeping.
• Be aware that by the time you are sure a debris flow is coming, that will be too late to get away safely. Never cross a road with water or mud flowing. Never cross a bridge if you see a flow approaching. It can grow faster and larger too quickly for you to escape.
• If you do get stuck in the path of a landslide move uphill as quickly as possible.
• Avoid river valleys and low-lying areas during times of danger.
• If you are near a stream or channel, be alert for any sudden increase or decrease in water flow or water that changes from clear to muddy. These can be signs that a landslide is coming.

Stay Safe After a Landslide

• Stay away from the slide area. There may be danger of additional slides.
• Listen to local radio or television stations for the latest emergency information.
• Watch for flooding. Floods sometimes follow landslides and debris flows because they may both be started by the same conditions.
• Check for injured and trapped persons near the slide, without entering the direct slide area. Direct rescuers to their locations.
• Report broken utility lines and damaged roadways and railways to appropriate authorities. Reporting potential hazards will get the utilities turned off as quickly as possible, preventing further hazard and injury.
• Allow trained professionals to check the building foundation, chimney, and surrounding land for damage.
• Replant damaged ground as soon as possible since erosion caused by loss of ground cover can lead to flash flooding and additional landslides in the near future.
• Seek advice from a geotechnical expert for evaluating landslide hazards or designing corrective techniques to reduce landslide risk. A professional will be able to advise you of the best ways to prevent or reduce landslide risk, without creating further hazard.

Additional Resources

• Landslide Information Sheet (PDF)
• U.S. Geological Survey Landslide Hazard Program
• Protective Actions Research for Landslide
• American Red Cross