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Sea Level Rise and Flooding | SSMC

Sea Level Rise and Flooding

Why is this Important?

Climate change increases the intensity of storms and contributes to sea level rise. San Mateo County faces the greatest risk from sea level rise in the Bay Area due to the high number of valuable assets located in low-lying areas. Climate change also contributes to drought which can lead to excessive groundwater withdrawal,, causing land subsidence and further increasing the risk of flooding. Homes, commercial buildings, streets, highways, railroads, airports, water treatment plants, a power plant, wetlands, beaches, public access areas, and hazardous material sites are all vulnerable. Damage to these sites will affect the entire region and state due to loss of jobs and tax revenue, impaired transportation, and decreased water quality.

What is a Sustainable State?

In a sustainable state, local government agencies will collaborate across jurisdictions on adaptation strategies that serve all residents equally and protect the integrity of environmental and structural assets. Through careful scientific evaluation, communities will make informed decisions about how to safeguard shores. Flood mitigation strategies inspired by nature, such as wetland restoration or floating islands will be implemented along the bay’s shore. Above all, greenhouse gas emissions will be reduced as public awareness of climate change grows.

Key Findings

  • The combination of sea level rise, high tide, and storms pose the greatest risk of flooding and erosion of the coastline.
  • Many analysts believe San Mateo County will experience up to 55 inches of sea level rise by 2100.
  • Tidal marshes disrupt wave energy and therefore decrease flooding from storm surges.
  • 180,000 acres of San Francisco Bay Area wetlands have been lost to development since 1850.
  • According to SPUR, 40 percent of the state’s land drains its water into the San Francisco Bay.
  • According to the 2013-2014 San Mateo County Civil Grand Jury report on sea level rise, the cost to repair buildings and other infrastructure in San Mateo County lost to flooding will exceed $23 billion along the bay, and $910 million on the coast.

Indicators and Trends
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Sea Level Rise and Flooding Projections

Sea level rise is the result of thermal expansion (water temperature increase) and the melting of vast bodies of ice. These phenomena, as well as high intensity storms are a result of climate change. The combination of sea level rise, high tide, and wind caused by storms pose a major threat of flooding in low lying areas along the coast and the bay side of the county.

According to the National Ocean and Atmospheric Administration (NOAA), sea level remained stagnant until the 20th Century. Since 1900, sea level has risen 0.04 to 0.1 inches per year, and an alarming 0.12 inches annually since 1992. Though these numbers may not seem immediately threatening, many analysts believe San Mateo County will experience up to 55 inches of sea level rise by 2100. An estimated 10% of San Mateo County’s population will be directly affected by a sea level rise of 36 inches. According to SPUR, 40 percent of the state’s land drains its water into the San Francisco Bay.

The Bay Institute reports that flooding from storm surges is the greatest threat to developed areas along the bay’s shoreline. Intense waves and sea level rise will result in erosion of the county’s coastal bluffs and beaches. The 2013-2014 San Mateo County Civil Grand Jury report on sea level rise estimates the cost to repair buildings and other infrastructure in San Mateo County will exceed $23 billion along the bay and $910 million on the coast.

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Evaluation and Preparation Projects

  • The San Mateo County Office of Sustainability in collaboration with the Coastal Conservancy is conducting a Sea Level Rise Vulnerability Assessment set for completion in March 2016. The project utilizes sea level rise projections combined with storm scenarios to evaluate the risk to assets including infrastructure, wetlands, and wild life habitat. Learn more at the Sea Change San Mateo County website.
  • In 2013, SFO began its Airport Shoreline Protection Feasibility Study to determine areas of improvement in preparation for sea level rise and a 100-year flood. This resulted in the establishment of the Shoreline Protection Program (SPP) in 2015.
  • The San Francisquito Creek Joint Powers Authority is conducting a feasibility study to restore marshlands, create recreation trails, and protect against flooding and three feet of sea level rise. The project has secured funding from three city governments, a local business, and the US Fish and Wildlife Service. Learn more about the SFJPA‘s projects on their website.
  • In 2014, FEMA revised its flood and wave data for the San Mateo County Flood Insurance Study report and Flood Insurance Rate Map, which are used to determine flood insurance rates.

Flood Risks from Sea Level Rise in San Mateo County



Flood Risks from Storms in San Mateo County


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Flood Management Strategies

Wetland Restoration Projects

  • Wetlands and marshes are natural forms of flood protection that serve to absorb and process excess water.
  • 180,000 acres of wetlands have been lost to development since 1850.
  • The Bair Island Restoration Project in Redwood City has secured 2,634 acres of tidal wetlands and tidal marsh habitat.
  • The South Bay Salt Pond Restoration Project is the largest wetland restoration project in the nation, and is reviving 15,100 acres, including the Ravenswood Pond Complex in Menlo Park.

Horizontal Levees

  • The Bay Institute recommends the large-scale construction of horizontal levees to mitigate sea level rise and flooding.
  • This type of levee mimics natural flood protection systems by shielding a landward levee with marshland.
  • Studies show that this method is half the cost of traditional levees.

Development and Sea Level Rise Protection Strategies

  • Barrier
  • Coastal Armoring
  • Elevated Development
  • Floating Development
  • Floodable Development
  • Living Shorelines
  • Managed Retreat

This list was produced by SPUR. To read the evaluation of each option, visit their website.
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