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Landslide/Mass Movements  Stream or wave activity, which has caused erosion, undercut a bank or cut into a bank to cause the surrounding land to be unstable  The presence or potential for snow avalanches  The presence of an alluvial fan, indicating vulnerability to the flow of debris or sediments  The presence of impermeable soils, such as silt or clay, mixed with granular soils, such as sand or gravel. The best predictor of where slides might occur is the location of past movements. Past landslides can be recognized by their distinctive topographic shapes, which can remain in place for thousands of years. Most landslides recognizable in this fashion range from a few acres to several square miles. Most show no evidence of recent movement and are not currently active. A small proportion of them may become active in any given year, with movements concentrated within all or part of the landslide masses or around their edges. The recognition of ancient dormant landslide sites is important in the identification of areas susceptible to flows and slides because they can be reactivated by earthquakes or by exceptionally wet weather. Also, because they consist of broken materials and frequently involve disruption of groundwater flow, these dormant sites are vulnerable to construction-triggered sliding. 12.1.4 Secondary Hazards Landslides and mass movements that block rivers or streams can contribute to flooding. 12.2 HAZARD PROFILE 12.2.1 Past Events Landslides have occurred regularly within San Mateo County; one such event led to the deaths of three children in 1982, and several events have required apartment evacuations along coastal bluffs. Table 12-1 lists known landslide events that affected San Mateo County between 1980 and 2020. Table 12-1. Landslide Events in San Mateo County Date Event Type FEMA Declaration Number Location December 19, 1981 – January 8, 1983 Severe Storms, Flood, Mudslides, High Tide DR-651 San Mateo County Impacts: Not reported January 4, 1982 Landslides, Severe Storm N/A San Mateo County (Pacifica and Various) Impacts: After an intense storm, many small to major landslides occurred in steep sections of the western and northern County, mostly in low population areas. Three children died after a strip of hillside slid hundreds of feet and destroyed two homes in Pacifica. The County recorded millions of dollars in property damage from the landslides. January 21 – March 30, 1983 Coastal Storms, Floods, Slides, Tornadoes DR-677 San Mateo County Impacts: Not reported January 3 – February 10, 1995 Severe Winter Storms, Flooding, Landslides, Mud Flows DR-1044 San Mateo County Impacts: Not reported 12-3