Peninsula Resilience Project
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San Mateo County

San Mateo County, situated along the Central California coastline, encompasses the major portion of the San Francisco Peninsula. Approximately 55 miles of the County’s western border is Pacific shoreline, and roughly 34 miles of the eastern border is San Francisco Bay shoreline. The County is bounded on the north by the City and County of San Francisco and on the south and southeast by Santa Cruz and Santa Clara Counties. The County’s unincorporated area includes urban pockets east of Route 280 and most of the rural area south and west of Route 280. The unincorporated County consists of approximately 309 square miles (68% of total County area), and there is wide variation in the size, location, and economic and social characteristics of the various unincorporated areas. The County’s population reached approximately 773,000 by 2020.

The topography of the County is extremely varied. Elevation ranges from sea level to 2,572 feet atop the Santa Cruz Mountains. This mountain range, running in a north-south direction, divides the County into two distinct regions, the Bayside and the Coastside. Much of the Bayside consists of mudflats, marshes, artificial fill and broad, flat alluvial plains. This level, low-lying region changes into gently rolling Bayside foothills. On the Coastside, topography ranges from gently sloping foothills abutting the western face of the Santa Cruz Mountains to broad, nearly level coastal terraces, cut by small valleys created by streamflow. Features along the shoreline range from wide, sandy beaches to rocky coves. Where wave action has eroded the coastal terraces, high, steep cliffs rise out of the ocean. Dry, mild summers and moist, cool winters characterize San Mateo County’s overall climate. Fog is prevalent on the Coastside, contributing to a productive agricultural area that grows a number of specialty crops. Bayside climates are generally warm and sunny, particularly in the summer months when hot air moving  east warms the prevailing cool ocean breezes. Temperatures are strongly influenced by the Pacific Ocean and San Francisco Bay. This combination of features has resulted in a variety of microclimates throughout the County with hill and ridgetop areas, valley floors and coastal areas each experiencing different temperatures and precipitation patterns.

The vast majority of unincorporated area within the county is located in rural areas. These areas developed slowly due to limited accessibility and difficult terrain. On the Coastside there are several unincorporated coastal communities north of Half Moon Bay, including Montara, Moss Beach, El Granada, Princeton, and Miramar, which make up the urban Midcoast. In the more rural Midcoast, Southcoast, and Skyline areas, there are small unincorporated communities at La Honda, Pescadero, San Gregorio, and Kings Mountain.

San Mateo County is already experiencing the effects of climate change, including extreme heat and wind events during the summer and fall and flooding in the winter months. Other concerns are extended periods of drought, flooding, wildfire, and sea level rise.

Local Safety and Resilience Highlights

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Seismic and Geologic Hazards

San Mateo County is in a region of high seismicity because of the presence of the San Andreas Fault that bisects the county, the Hayward Fault across the bay to the east, and the San Gregorio Fault to the west. The primary seismic hazard for the county is associated with ground shaking from these three large faults. It is estimated that 38.6 percent of San Mateo County’s population resides in areas considered susceptible to ground shaking from earthquakes.

Landslides have occurred regularly throughout San Mateo County. The southwestern portion of the County has experienced repeated damage from debris flows, including the Tunitas Creek, San Gregorio, and Pescadero watersheds. Debris flows are widespread and have caused several road closures in the past, especially along Highway 1. Landslide risk is heightened following wildfire.

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Inland Flooding

Inland flooding is a result of local drainage issues and heavy rain. Flooding is predominantly shallow along streams on the bayside of San Mateo County. However, flood problems are known to occur along Colma Creek; San Bruno, Crystral Springs, and Lomita Channels; Belmont Creek and Holly Street Channel; San Francisquito Creek; Montara Creek; San Vincente Creek; Denniston Creek; El Granada Creek; Woodhams, La Honda, Alpine, and San Gregorio Creeks; and Pescadero and Butano Creeks.

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Inland Flooding

Flooding has been an ongoing issue for low-lying areas of Pacifica, and likely will continue to be a chal­lenge in the future. There is a risk of flood inundation in several parts of Sharp Park Golf Course, the Rockaway Beach district, and in the Linda Mar neighborhood along San Pedro Creek.

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A tsunami is a series of high-energy waves that radiate outward like pond ripples from an area where a generating event occurs, arriving at shorelines over an extended period. Tsunamis can be induced by earthquakes, landslides, and submarine volcanic explosions. San Mateo County has experienced six known tsunamis since 1859. A number of the County’s major roads, bridges, and utility systems are located within tsunami hazard zones, including Highway 1, State Route 92, and Highway 101.

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Coastal Hazards (Sea Level Rise, Coastal Erosion, Coastal Flooding, and Groundwater Emergence)

The Pacific Ocean is the most likely source of coastal flooding in the County, although flooding from the San Francisco Bay is also a possibility during significant events. Storm surge floods typically result in coastal erosion, salinization of freshwater sources, and contamination of water supplies. These floods are also responsible for significant agricultural losses, loss of life, and damage to public and private structures and infrastructure. San Mateo County has mitigated some of its vulnerability to Bay flooding through a series of levees originally installed for salt evaporation ponds in the southeastern part of the County and for flood protection in the north and central parts of the County. However, these levees were not designed to withstand floods of magnitude greater than the 1-percent-annual-chance flood.

San Mateo County is highly vulnerable to the effects of rising sea levels. If left unmanaged, future flooding and coastal erosion could pose considerable risks to life, safety, critical facilities, the County’s natural and recreational assets, and the economy. Flooding, erosion, and sea level rise directly threaten people and property in the sea level rise hazard areas. They also have indirect effects on all communities in the County, even those on high ground, because assets and infrastructure in the sea level rise areas provide critical services and functions to communities outside these areas.

San Mateo County has more people and property value at risk from sea level rise than any other county in the state. When population projections are considered, the County is one of six counties in the nation (and the only one on the west coast) with over 100,000 people living in an area affected by 3 feet of sea level rise. Vulnerable county assets in potential hazard zones include critical facilities (police stations, hospitals, wastewater treatment plants, and schools), essential regional transportation networks and infrastructure such as the Caltrain tracks and Highway 101, State Route 1), and regional natural and recreational assets such as Pacifica State Beach and the California Coastal Trail.

In coastal areas with underlying groundwater, rising seas may enter aquifers and contribute to rising groundwater levels, or emergent groundwater. Groundwater rise will contribute to flooding in low-lying coastal communities, with impacts often occurring earlier, and farther inland, than coastal flooding from overtopping of the bay shoreline. Rising groundwater can degrade underground infrastructure, mobilize contaminants, and increase liquefaction hazards.

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Dam Inundation

24 dams are in San Mateo County. Twelve of these, plus another nearby in Santa Clara County, have inundation areas that run through the county. The Lower Crystal Springs Dam is the largest dam in San Mateo County, making it a higher priority for regulation and preventative maintenance by county, state, and federal officials. Dam failure events are infrequent and usually coincide with or follow events such as earthquakes, landslides and excessive rainfall and snowmelt. Dam failure may worsen the severity of a drought by releasing water that might have been used as a potable water source. A loss of water supply could exacerbate the wildfire hazard by hindering an impacted area’s ability to fight fire.

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Due to local topography, fuels (forest, chaparral, and grasslands vegetation) and weather conditions, San Mateo County is conducive to periodic large wildfire events. According to the CAL FIRE Fire Resource Assessment Program, areas between Shelter Cove, Moss Beach, Half Moon Bay, Sky Londa, and Crystal Springs Lake, La Honda are within high to very high fire hazard severity zones. Smoke from wildfires can negatively impact air quality and health across the county.

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Extreme Heat

State agencies define extreme heat as temperatures that are hotter than 98 percent of the historical high temperatures for the area, as measured between April and October of 1961 to 1990. Days that reach this level are called extreme heat days. In San Mateo County, extreme heat on average is a daytime temperature above 85.7 degrees Fahrenheit (°F), and a warm night is nighttime low of above 56.1°F, although these numbers vary in different parts of the county. Extreme heat affects community members’ health and increases community costs and energy generation as it continues. These events can also exacerbate wildfires and impact water supplies. High demand for power for air conditioning during extreme heat can stress and overwhelm the electrical grid, leading to brownouts or power loss. Extreme heat events may degrade the quality of roadways and railways, resulting in closures and travel delays.

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Severe Weather

Extreme heat affects community members’ safety and increases community costs and energy generation as it continues. These events can also exacerbate wildfires and impact water supplies. High demand for power for air conditioning during extreme heat can stress and overwhelm the electrical grid, leading to brownouts or power loss. Extreme heat events may degrade the quality of roadways and railways, resulting in closures and travel delays.

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Drought is a significant decrease in water supply relative to what is needed to meet typical demand, leading to a water shortage for some activity, group, or environmental sector. While drought is a normal occurrence for Mediterranean climates such as that of San Mateo County, long and severe droughts have the potential to impact ecosystems and economic activity across the entire community. Potential drought impacts include loss of crops, costs incurred by having to drill new wells, increased costs for water straining household finances and reducing commercial profits, reduced habitat and food supply for plants and animals, and increased risk of wildfire.  

In addition to these potential safety issues, fiber and cellular service is not reliable on the coast. This means that residents of coastal areas of unincorporated San Mateo County may have trouble contacting support and response services during an emergency.

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