Landslides

The steep hillsides in Mill Valley may be prone to landslide activity especially during the winter storm season.

A landslide is a body of earth, rock and/or mud that has broken loose from a hillside or mountain and been carried downward by the force of gravity. Rainy weather is the most common cause of landslides. During wet weather, subsurface water pressure can build up and overwhelm the strength of soil in a hillside. When a series of heavy winter storms hits and there is no time for the soil to drain, the probability of a landslide increases.

Landslides are dangerous. People and property can be trapped under fallen debris in a fast-moving landslide. Landslides can undermine roads or fall onto the roadway. Landslides can damage underground infrastructure like gas lines, water lines, sewer lines, and communication lines. Landslides can fall into natural waterways, blocking them temporarily, then release in a destructive flow of mud and debris.

Learn to recognize potential landslide warning signs:

  • Changes occur in your landscape such as patterns of storm-water drainage on slopes
  • Water breaks through the ground surface in new locations
  • Bulging ground appears at the base of a slope
  • New cracks appear in plaster, tile, brick, or foundations
  • Fences, retaining walls, utility poles and trees tilt or move
  • Collapsed pavement, mud or fallen rock along roadways

Personal Safety:

  • Be especially alert when driving. Embankments along roadsides are particularly susceptible to landslides. 
  • If you are in an area susceptible to landslides and debris flows, consider leaving if it is safe to do so.
  • Have an evacuation route and plan. Landslides can cut off highways and roads, so it is a good idea to have multiple evacuation routes. 
  • If you near a creek, 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.

How can you minimize your landslide risks?

  • Regularly inspect your property for land movement, retaining wall failures and blocked drainage routes
  • Meet with your neighbors to discuss and repair mutual drainage problems

What to do if you suspect imminent danger of a landslide: 

  • Contact your local fire, police, or public works department. Local officials are the best persons to assess potential danger.
  • Inform affected neighbors. Your neighbors may not be aware of potential hazards. Advising them of a potential threat may help save lives.
  • 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.

What to do if a landslide or debris flow occurs:

  • Call 9-1-1.
  • 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.
  • Check for injured and trapped persons near the slide, without entering the direct slide area. Direct rescuers to their locations.
  • Help neighbors who may require special assistance.
  • Look for and report broken utility lines and damaged roadways to appropriate authorities. Reporting potential hazards will get the utilities turned off as quickly as possible, preventing further hazard and injury.

Download the Disaster Plan Worksheet and Family Emergency Plan Worksheet.