The general principle behind landslide mitigation is stabilization. Slopes that are too steep are shallowed or strengthened. Loose rocks can be bolted or cemented into place. Erosion that could over-steepen a slope is controlled. Saturation is prevented with drainage systems. In this section, we will look at examples of landslide mitigation.

Landslide mitigation measures include the following:

1) slope reduction

2) surface and subsurface drainage

3) terracing & rock walls

4) plants with deep root systems

5) buttressing – putting a heavy rock blanket on a soft slope ((TVA.gov)

Landslide Mitigation (BYUI)

Slope Stabilization The diagram at right shows some ways that slopes can be stabilized to reduce risk from landsliding.

Increasing the Resisting Weight Another soft slope (left) this one stabilized with a blanket of large rocks. The weight of the rocks holds the soft soils in place, while allowing water to drain from the slope (City of Seattle).

Mitigation Strategies

Before After

Before After

Terrace and Rock Wall Stabilization

A soft slope before (top) it was stabilized with terraces and rock walls. The rock walls are permeable, allowing water to flow out of the hill without allowing the mud beneath to flow (City of Seattle).

(state of California)

Mitigation Strategies

Slope Stabilization Using Vegetation

Before and after landslide mitigation (City of Seattle).

Before After

Slope Stabilization Using Vegetation Using vegetation to stabilize slopes is aesthetically pleasing and economic. The photos below show an effective – and attractive – application of slope reduction and vegetation planting

Illustration of Buttressing (State of California)

Mitigation Strategies

Grading and Buttressing Steep slopes must be graded to be no steeper than 2:1 – for every two feet horizontally, the slope rises only 1 foot. In addition, a buttress or blanket of heavy, loose rock can hold soft materials in place and prevent landsliding. The rock blanket must be permeable to allow water to drain out of the slope – a saturated slope is an unstable slope!

Concrete Slope

Highway 1 along the California coast is famous for high maintenance costs, primarily because its builders decades ago steepened slopes by excavation, making them more prone to landslides. This concrete slope in San Francisco is an effort to hold the hillside in place (BYUI).

Mitigation Strategies

Landslide Preparedness

Keep in mind that homeowner’s insurance typically doesn’t cover landslide damage!

(USGS)

Ground crack. Damaged sidewalk.

Landslide Warning Signs

Landslide warning signs involving water:

• Springs, seeps, or saturated ground in areas that have not typically been wet before. • Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content). • Sudden decrease in creek water levels though rain is still falling or just recently stopped. • Broken water lines and other underground utilities.

Landslide warning signs you can see in your community:

• New cracks or unusual bulges in the ground, street pavements or sidewalks. • Leaning telephone poles, trees, retaining walls or fences. • Sunken or down-dropped road beds. • Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.

Landslide Warning Signs

Landslide warning signs involving your home:

• Soil moving away from foundations. • Ancillary structures such as decks and patios tilting and/or moving

relative to the main house. • Tilting or cracking of concrete floors and foundations. • Offset fence lines. • Sticking doors and windows, and visible open spaces indicating

jambs and frames out of plumb.

The final sign is the most dangerous of all:

A faint rumbling sound that increases in volume is noticeable as the landslide nears.

Foundation damage.

Door frame out of plumb.

What a homeowner can do to mitigate landslides:

1) Avoid destabilizing of slopes:

• Avoid removing material from the base of slopes or adding weight at the top.

• New construction on landslide-prone land can be improved through proper engineering (for example, grading, excavating) by first identifying the site’s susceptibility to slope failures and by creating appropriate landslide zoning. -Plant ground cover on slopes and build retaining walls.

2) Avoid water-related problems:

• Drain water from surface runoff, downspouts, and driveways well away from unstable slopes and landslides.

• Minimize landscape irrigation - overwatering on bluff-tops is a common cause of landslides.

• Make sure gas , water, and sewer lines do not leak. Have flexible pipe fittings installed id as flexible fittings are more resistant to breakage (only the gas company or professionals should install gas fittings).

3) Get educated:

• Contact local officials, state geological surveys or departments of natural resources, and university departments of geology. Landslides occur where they have before, and in identifiable hazard locations. Ask for information on landslides in your area, specific information on areas vulnerable to landslides, and request a professional referral for a very detailed site analysis of your property, and corrective measures you can take, if necessary.

• Inform affected neighbors. Your neighbors may not be aware of potential hazards. Advising them of a potential threat may help save lives. Help neighbors who may need assistance to evacuate.

• In mudflow areas, build channels or deflection walls to direct the flow around buildings.

If you build walls to divert debris flow and the flow lands on a neighbor's property, you may be liable for damages.

What To Do During a Landslide

• Stay alert and awake. Many debris-flow fatalities occur when people are sleeping. Listen to a NOAA

Weather Radio or portable, battery-powered radio or television for warnings of intense rainfall. Be aware that intense, short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.

• Consider evacuating. If you are in areas susceptible to landslides and debris flows, consider leaving if it is

safe to do so. Remember that driving during an intense storm can be hazardous. If you remain at home, move to a second story if possible. Staying out of the path of a landslide or debris flow saves lives.

• Listen carefully. Listen for any unusual sounds that might indicate moving debris, such as trees cracking or

boulders knocking together. A trickle of flowing or falling mud or debris may precede larger landslides. Moving debris can flow quickly and sometimes without warning.

• Be especially careful around rivers. If you are near a stream or channel, be alert for any sudden

increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate landslide activity upstream, so be prepared to move quickly. Don't delay! Save yourself, not your belongings.

• Turn Around, Don't Drown®!. Be especially alert when driving. Bridges may be washed out, and

culverts overtopped. Do not cross flooding streams!! Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of possible debris flows.

• Aftershock awareness. Be aware that strong shaking from earthquakes can induce or intensify the effects

of landslides.

(USGS)

(USGS)

What to do if you suspect a landslide is eminent:

• Evacuate. Getting out of the path of a landslide or debris flow is your best protection.

• Curl into a tight ball and protect your head if escape is not possible.

(USGS)

California Landslide

These images are from a 2005 landslide in Bluebird Canyon, Laguna Beach California. 350 houses were evacuated and 15 homes were damaged or destroyed. (Photo by Jim Bowers, USGS).

What To Do After a Landslide

Stay Away

• Stay away from the slide area. There may be danger of additional slides.

Be Aware

• Check the building foundation, chimney, and surrounding land for damage. Damage to foundations, chimneys, or surrounding land may help you assess the safety of the area.

• Listen to local radio or television stations for the latest emergency information.

• Watch for flooding, which may occur after a landslide or debris flow. Floods sometimes follow landslides and debris flows because they may both be started by the same event.

(USGS)

(USGS)

What To Do After a Landslide (continued)

Help

• Check for injured and trapped persons near the slide, without entering the direct slide area. Direct rescuers to their locations.

• Help a neighbor who may require special assistance - infants, elderly people, and people with disabilities. Elderly people and people with disabilities may require additional assistance. People who care for them or who have large families may need additional assistance in emergency situations.

• Look for and 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.

Prevent

• 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

• 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.