Whether you live on a beach or atop an inland mountain, life everywhere in San Mateo County may change in the coming decades due to sea level rise.

Ocean intrusion will likely affect beaches and highways, wastewater treatment facilities and groundwater supplies, mass transit and public utilities. The county is hoping to prepare and plan for those changes in advance and avoid adverse impacts on local communities and economies.

This week, the county released a draft assessment of its vulnerabilities to sea level rise in order to address flooding and erosion that will cause long-term damage before it occurs. The draft details the risks and costs to local cities in the coming decades based on available modeling of ocean levels. A final version of the report is expected this summer.  

The assessment is part of the county’s Sea Level Rise Initiative, which aims to bolster the county’s economy and environment in the face of rising ocean surface levels. The initiative will be used to assist cities and businesses in planning for such climate change results in the coming years.

The 291-page report focuses on where rising ocean levels pose the greatest and most immediate risk, and what effect they would have on local communities.

And although sea level rise has received recent attention, it’s been around for more than a century. From 1905 to 2005, California experienced around 7 inches of sea level rise, and the rate of increase is expected to grow for the rest of this century, according to the report. Surfer’s Beach has lost around 140 feet of beach due to erosion since 1964, according to the report.

Those increases are expected to escalate. By 2100, portions of the California coast will see an increased sea level rise of 17 to 66 inches, according to the National Research Council. Based on such predictions, the assessment considers three sea level rise scenarios, ranging from baseline to high-end, as well as one erosion scenario based on 4.6 feet of sea level rise.

But the effects of rising oceans can’t be measured by looking exclusively at water levels. Daily tides, king tides, storm surges, and El Niño years can swell water levels for days or months.

“Each of these factors has some impact alone, but in combination and in conjunction with sea level rise, their impacts become more significant and are potentially compounded,” the report read. And San Mateo County is one of the most vulnerable to flooding in the state, according to recent reports.

Despite the widespread risk, the report doesn’t present a one-size-fits-all solution. Instead, it recommends localized solutions developed by each city. Adapting to sea level rise requires “very specific knowledge about the physical and socio-political characteristics of each community, ecosystem, and asset in question,” the report reads. “Even locations that appear to be similar or are in close proximity can face very different climate change impacts and thus require unique adaptation approaches.”

In the city of Half Moon Bay, 32 acres of land would be inundated by sea level rise in a baseline scenario and 103 acres in the high-end scenario. If the former happens, $6 million in parcels would be at risk, while the economic cost would rise to $30 million in the latter scenario. The assessment predicts that 263 Half Moon Bay acres will be affected in the erosion scenario, totaling $181 million in value. And all of the 4.2 miles of beaches within the city’s limits would be affected in every scenario, the report says.

Other unincorporated communities like Miramar stand at risk, especially in the Mirada Road area, where a portion of the road is being repaired due to erosions. And Montara has 150 parcels that would be affected in the erosion scenario. In Moss Beach, a limited portion of wetland would be inundated in any sea level rise scenario. In Princeton and the Pillar Point Harbor area, more than a third of the land area would be inundated if the high-end sea level rise scenario transpires.

The report did not assess the vulnerability of the coast south of Half Moon Bay, due to “limitations in the available sea level rise inundation data.” Once modeling data is available for the South Coast, the county plans to release similar maps and assessments for that region including Pescadero.

If the county takes no action, parcels totaling up to $39 billion in value could be affected by long-term flooding.

Each of the cities included in the assessment will have opportunities to provide their feedback before the report is finalized this summer.

To read the full report and submit comments, visit http://seachangesmc.com/current-efforts/vulnerability-assessment/. The deadline to submit public comments is May 5.

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