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Landslide/Mass Movements slopes west of Skyline Ridge. They have been observed in Alpine Road, Crystal Springs, San Bruno Mountain, and Point San Pedro, as well as the County’s coastal sea cliffs.  Highway 1 has been closed by landslides multiple times at Devils Slide. In 1995 and 2006, landslides led to extended closures. The new Tom Lantos Tunnel, opened in March 2013, allows the highway to bypass Devils Slide and reduce vulnerability. Post-Fire Debris Flows Wildfire can significantly alter the hydrologic response of a watershed to the extent that even modest rainstorms can produce dangerous flash floods and debris flows. California’s first major rainfall event of the winter after the historic 2020 wildfire season prompted evacuation orders and flood watches and warnings for several recent burn areas in the state. The biggest debris-flow impacts were in Monterey County and include major damage along the Big Sur Coast closing Highway 1 indefinitely (Dolan Fire) and damage to numerous homes causing at least one injury (River Fire). Minor home damage occurred in the Bond Fire in Orange County, and small non-destructive debris flows were observed in the CZU Lightning Complex burn area in Santa Cruz and San Mateo Counties. The USGS has extent maps available for each of these events (USGS, 2021b). 12.2.2 Location In 2011, the California Geological Survey used a combination of regional rock strength and slope data to create classes of susceptibility to deep-seated landslides statewide. The analysis assumed that susceptibility to deep- seated landslides is low on very low slopes in all rock materials and increases with slope and in weak rocks. The analysis also factored in locations of past landslides. Figure 12-2 shows the mapped susceptibility classes (none, low, moderate, high, and very high) for San Mateo County. 12.2.3 Frequency In San Mateo 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. Most weather-induced landslides in the county occur in the winter after the water table has risen. Landslides that result from earthquakes can occur at any time. The probability of a landslide in the county in any given year is high. Table 12-1 lists 10 federal disaster declarations related to landslides in the County between 1981 and 2017, an average of one such major event every three or four years. 12.2.4 Severity Landslides destroy property and infrastructure and can claim human lives. They have the potential of destabilizing the foundation of structures, which may result in monetary loss for community members. Slope failures in the United States result in an average of 25 to 50 lives lost per year (USGS, 2020a). Slides can pose a serious hazard to properties on or below hillsides. They can cause block access to roads, which can isolate community members 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. Landslides also can damage rivers or streams, potentially harming water quality, fisheries, and spawning habitat. Historically, landslides in San Mateo County have proven to be very severe, with landslide activity being responsible for at least 14 deaths since 1982 (Bay Area News Group, 2016). 12-5