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Satellite survey shows California’s declining coastal hotspots

Satellite survey shows California's declining coastal hotspots

Coastal elevation in California. Coastal zones, which are defined as those with heights less than 10 m, are shown in red. Segments of the coast with heights higher than 10 m are colored by a yellow slope. Credit: USGS DOWN.

A majority of the world population lives on low-lying land near the sea, some of which are predicted to sink in the late 2000s due to rising sea levels.

The most relevant quantity for assessing the effects of changes in sea level on these communities is the relative increase in sea level – the change in altitude between the earth’s surface height and sea level. For an observer standing on the coast, the relative sea level rise is the net change in sea level, which also includes the rise and fall of the land under the observer’s feet.

Now using accurate measurements from state-of-the-art satellite-based interferometric synthetic aperture radar (InSAR) that can detect the country’s surface rise and fall with millimeter accuracy, an Arizona State University research team has for the first time tracked the entire California coastal land movement.

They have identified local hotspots on the declining coast, in the cities of San Diego, Los Angeles, Santa Cruz and San Francisco, with a total population of 4 to 8 million people exposed to rapid rural areas, which will be at a higher flood risk during decades before the planned sea level.

“We have ushered in a new era of coastal mapping with more than 1,000 times more detail and resolution than ever before,” said Manoochehr Shirzaei, the lead investigator of the NASA-funded project. “The unmatched detail and sub-millimeter accuracy solved in our vertical land movement data set can change the understanding of natural and anthropogenic changes in relative sea level and associated hazards.”

The results were published in this week’s issue of Scientific progress.

The research team included doctoral student and lead author Em Blackwell and faculty Manoochehr Shirzaei, Chandrakanta Ojha and Susanna Werth, all from the ASU School of Earth and Space Exploration (Werth has a double meeting at the School of Geography and Urban Planning).

Em Blackwell had a great interest in geology, and when Blackwell started graduate school, the applications from InSAR drew them to continue with this project. InSAR uses radar to measure the change in the distance between the satellite and the ground surface and produces highly accurate deformation maps of the Earth’s surface with a resolution of 10 m over 100 km km spatial extent.

Agriculture can occur due to natural and anthropogenic processes or a combination of them. The natural processes include tectonics, isostatic adjustment of ice, sediment loading and soil compaction. The anthropogenic causes include groundwater abstraction and oil and gas production.

As of 2005, about 40 million people were exposed to a 1 to 100-year coastal flood risk, and by 2070 this number will grow more than three times. The value of properties exposed to floods will increase to about 9% of the estimated global gross domestic product, with the United States, Japan and the Netherlands being the countries with the most exposure. These exposure calculations often rely solely on forecasts of global average sea level rise and do not account for vertical land movement.

The study measured the entire 1,350-kilometer coastline of California from 2007-2018 and compiled thousands of satellite images over time, which were used to make a vertical land motion map with 35 million pixels in a ~ 80 m resolution, including a wide range of coastal uplift and revival numbers. The coastal community’s decision-makers and the general public are free to download the information (link in additional data).

The four metropolitan areas most affected in these areas included San Francisco, Monterey Bay, Los Angeles and San Diego.

“The vast majority of the San Francisco Bay area is undergoing falls at speeds reaching 5.9 mm / year,” Blackwell said. “It’s remarkable that San Francisco International Airport is sinking at speeds faster than 2.0 mm / year. The Monterey Bay area, including the city of Santa Cruz, is sinking fast without any elevation zones. The drop rates for this area reach 8.7 mm / year. Los The Angeles area shows deepening along small coastal zones, but most of it falls inside.

Land uplift areas included north of the San Francisco Bay Area (3 to 5 mm / year) and Central California (same rate).

Going forward, in the coming decades, the coastal population is expected to grow to over 1 billion people by 2050 due to migration to the coast. The future flood risk that these communities will face is mainly governed by the degree of relative sea level rise, namely the combination of sea level rise and vertical land movement. It is important to include soil depths in regional forecasts used to identify areas with potential flooding for the urbanized coast.

In addition to the study, ASU’s research group is hopeful that others in the scientific community can build on their results to measure and identify coastal risks more widely in the United States and around the world.

The study says that seas can rise faster than expected

More information:
“Tracking California’s Falling Coast from Space: Implications for Relative Sea Rise” Scientific progress (2020). DOI: 10.1126 / sciadv.aba4551

Provided by Arizona State University

Quote: Satellite Survey Shows California’s Declining Coastal Hotspots (2020, July 31) Retrieved July 31, 2020 from https://phys.org/news/2020-07-satellite-survey-california-coastal-hotspots.html

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