Landslides occur when the soils of a slope, such as a hillside or mountain, become unstable. When this happens, the soil slides down toward the base of the slope, damaging or destroying structures built on the moving soil or in its path. While landslides are often thought of as fast-moving events, some landslides may happen slowly over a long period of time. 

The types of materials that compose a slope and the steepness of the slope help determine the overall risk that a landslide may occur. Soil stability and time also contribute to the risk of rockfall, which is of particular risk along roadways and trails where a path or highway has been cut into a hillside, exaggerating the angle of repose and increasing the likelihood of rockfalls. 

Landslides may be triggered by other hazard events. The shaking of an earthquake or the loss of soil stability as a result of earthquake-induced liquefaction can cause the soil to slide. Alternatively, precipitation can result in saturated soil and a loss of stability, or flowing water may erode the base of a slope. The risk of a landslide is often exacerbated in areas recently burned by wildfire, as the fire burns vegetation that can absorb water and hold back soil. Without the vegetation to stabilize a slope and prevent runoff, sediment and debris are more susceptible to sliding.  

Landslide hazards in Healdsburg are common and generally occur as a result of heavy rainfall. Large earthquakes and improper grading or drainage practices can also cause landslides. 

Within Healdsburg, many of the swales or ravines that occupy the steep hill slopes may be capable of generating debris flows. Debris flows are most likely to originate on slopes underlain by sandstone or Glen Ellen sediments. Areas underlain by mudstone of the Great Valley Sequence are generally characterized by earthflows or slumps and are considered less likely to generate debris flows. 

Landslide Hazard Areas in Healdsburg

Prepare for a Landslide 

• Know the landslide risk in your area. Contact officials for information on local landslide hazards. Ask whether there is a landslide map of your area or how to get your property checked. 
• Learn about the types and signs of landslides common in your area. 
• Sign up for your community’s warning system. The Emergency Alert System (EAS) and National Oceanic and Atmospheric Administration (NOAA) Weather Radio also provide emergency alerts. 
• Know your community’s landslide evacuation plans. Practice driving evacuation routes and identify shelter locations. 
• Gather supplies 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. 
• Avoid building in areas at risk for a landslide, such as steep slopes or property close to cliffs, or near drainage ways or streams. 
• Plant ground cover and build walls to direct the flow around buildings. 
• Keep important documents in a safe place. Create password-protected digital copies. 
• Review insurance coverage. Landslide damage may not be covered.

During a Landslide

• Monitor the area for signs of potential slide activity. These can include cracks or bulges in the ground, street pavement, or sidewalks; soil moving away from foundations; tilting of patios or foundations; broken water lines; or leaning telephone poles, trees, walls, or fences. 
• Evacuate in advance if there are signs of a landslide, especially if a landslide could occur at night. 
• Watch for flooding. Floods sometimes follow landslides because they may be started by the same event. 
• Listen to EAS, NOAA Weather Radio, or local alerting systems for current emergency information and instructions.

After a Landslide

• Listen to authorities to find out if it is safe to return. 
• Save phone calls for emergencies. Phone systems are often down or busy after a disaster. Use text messages or social media to communicate with family and friends. 
• Stay away from downed power lines and report them immediately.