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<br /> <br />Landslide/Mass Movements <br /> <br />12.6 SCENARIO <br />Major landslides in San Mateo County most typically occur as a result of soil conditions affected by severe <br />storms, groundwater, or human development. The worst-case scenario for landslide hazards in the planning area <br />would generally correspond to a severe storm with heavy rain that caused flooding. Landslides are more likely <br />during the late winter when the water table is high. After heavy rains from November to December, soils become <br />saturated with water. As water seeps downward through upper soils that may consist of permeable sands and <br />gravels and as it accumulates on impermeable silt, it will weaken and destabilize the slope. A short intense storm <br />could cause saturated soil to move, resulting in landslides. As rains continue, the groundwater table rises, adding <br />to the weakening of the slope. Gravity, poor drainage, a rising groundwater table, and poor soil exacerbate <br />hazardous conditions. <br /> <br />Landslides are becoming a greater concern as development moves outside of city centers and into areas with less <br />developed infrastructure. Most landslides would be isolated events affecting specific areas. It is probable that <br />private and public property, including infrastructure, would be affected. Landslides could affect bridges that pass <br />over landslide-prone ravines and knock out rail service through the County. Road obstructions caused by <br />landslides would create isolation problems for community members and businesses in sparsely developed areas. <br />Property owners exposed to steep slopes may suffer damage to property or structures. Landslides carrying <br />vegetation such as shrubs and trees may cause a break in utility lines, cutting off power and communications to <br />community members. <br /> <br />Continued heavy rains and flooding would complicate the problem further. As emergency response resources are <br />applied to problems with flooding, it is possible they will be unavailable to assist with landslides across San <br />Mateo County. <br /> <br />12.7 ISSUES <br />Important issues associated with landslides in the planning area include the following: <br /> The data and science regarding mapping and assessing landslide hazards are constantly evolving. As new <br />data and science become available, assessments of landslide risk should be re-evaluated. <br /> Over 50 percent of the population exposed to the combination of very high and high landslide <br />susceptibility have either “very high” or “relatively high” social vulnerability. <br /> The impact of climate change on landslides is uncertain. If climate change affects atmospheric conditions, <br />the exposure to landslide risks in San Mateo County could increase. <br /> There are existing homes in landslide risk areas throughout the County. The degree of vulnerability of <br />these structures depends on the codes and standards applied in constructing the structures. <br /> Future development could lead to more homes in landslide risk areas. <br /> Landslides may cause negative environmental consequences, including water quality degradation. <br /> The risk associated with the landslide hazard overlaps the risk associated with other hazards, including <br />earthquake, flooding, and wildfire. The County has an opportunity to seek mitigation alternatives with <br />multiple objectives that can reduce risk for multiple hazards. <br /> As the frequency and severity of wildfires increase in the State of California, the probability for post-fire <br />debris flows will increase within the planning area. <br /> <br /> <br /> <br />12-13