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Local Research with Global Effects: Coastal Scientists Study El Niño in Northern California
Coastal research can be hard work. That’s what U.S. Geological Survey (USGS) oceanographer Dan Hoover thought one cold and foggy night as he trudged a couple of miles north on San Francisco’s Ocean Beach. A specially equipped off-road vehicle (ORV) had broken down. While his beach survey partner waited in the gloom, Hoover retrieved their four-wheel-drive SUV to tow the busted ORV back to the shop. Then the SUV got stuck on a partly buried timber, forcing the scientists to dig through shifting sand to free both vehicles.
“It was miserable,” said Hoover. “Every time you go out, especially at Ocean Beach, there’s always something.” Hoover and his colleagues have surveyed Ocean Beach over 200 times as part of a series of research projects. This year, they’ll find out what happens during a major El Niño winter.
Researchers with the USGS Coastal and Marine Geology Program study coastal hazards like beach erosion and cliff collapse at many locations over years and decades. By comparing conditions before, during, and after this year’s very strong El Niño, these scientists hope to improve forecasts of coastal changes during future events.
El Niño is the common name for warm ocean temperatures that appear around Christmas off the west coast of South America. The weather pattern warms or cools other parts of the Pacific Ocean, and changes weather around the world. El Niño seasons typically last about one year and return at irregular intervals every two to seven years. Every winter is different, and so is every El Niño. The most recent very strong seasons occurred in 1982–1983 and 1997–1998. Sea surface temperature differences measured during this year’s El Niño have set new records.
For the mainland United States, El Niño winters typically bring more rain to southern states from California to Florida, and stronger storms with bigger waves to the West Coast. In some other years, La Niña brings colder waters offshore of South America and drought to the southern U.S. Scientists refer to the irregular patterns of El Niño, La Niña, and neutral years as the El Niño-Southern Oscillation, or ENSO.
This year’s very strong El Niño should show up in USGS coastal studies from Guam to Florida, and Panama to Washington State, including Northern California, home to the USGS Pacific Coastal and Marine Science Center.
San Francisco’s Main Beach Disappears and Reappears
Ocean Beach is San Francisco’s 5.6-kilometer (3.5 mile)-long “front yard” on the Pacific Ocean, part of the Golden Gate National Recreation Area. The sky is often overcast or foggy, the water is too cold for most swimmers, and the waves challenge even expert surfers. The sandy beach expands in the summer and autumn thanks to gentle waves and currents that carry sand onshore. During unusually stormy winters, up to 150 meters (490 feet) of beach washes away, and large waves can damage the Great Highway running beside the beach. San Francisco’s Oceanside sewage treatment plant sits underground, just inland from the south end of Ocean Beach, protected from waves and erosion by a rock wall and a constantly shifting sandy shore.
That’s one of the reasons USGS scientists keep going back—the beach helps protect critical infrastructure. Every month since 2004, teams survey the sand using precision GPS receivers in backpacks or mounted on ORVs. A few times each year, and if the surf’s not too rough, they use personal watercraft to measure the ocean floor just beneath the waves. Intensive beach surveys over many years like this are rare. “There’s a curse in doing time-series work, because once you start you can’t stop,” said Hoover. “The longer it is, the more valuable it is, and then it’s harder and harder to let it go.”
Using the precision surveying data, USGS researchers calculate how much sand moves on and off Ocean Beach. Combined with data on weather, swells, tides, and currents, they can link ocean conditions to beach erosion. Scientists use the long-term Ocean Beach data to improve computer forecasts of coastal hazards due to El Niño and climate change. The National Park Service wants to know what could happen to Ocean Beach and other areas they manage. The city of San Francisco can use those forecasts to help protect the sewage treatment plant from extreme storms.
The residents of cliff-top homes in Pacifica worry about the future, too.
Crumbling Cliffs in Pacifica
Unfortunately, Pacifica, California, has become the poster child for coastal cliff erosion. The small seaside town is on a section of the coast with the highest erosion rates in the state. Since the 1980s, shoreline erosion along that stretch has increased 50 percent. Even during calm years, big chunks of bluffs fall into the ocean, endangering or destroying roads, homes, and other buildings on top of those cliffs.
USGS research geologist and Mendenhall post-doctoral fellow Patrick Limber knows Pacifica’s cliffs well. He studied the rocks that form those bluffs (and others) for his thesis, and he’s continuing his research at USGS to forecast cliff erosion due to climate change and El Niño. The erosion process is a lot more complicated than waves bashing weak rocks and triggering spectacular collapses. The beach plays a critical role.
“For example, if you have a really wide beach in front of the cliff, the cliff is pretty well protected because the waves break farther offshore,” said Limber. “If you have no beach in front of the cliff, the waves can really get in there and, when they break, they break very close to the cliffs. There’s a lot of energy at the toe of the cliff, so cliffs tend to erode faster.” During especially stormy seasons, like this year’s El Niño, waves and currents can wash all the sand away.
It gets worse. As the last of the sand is washing away, the mixture of sand and ocean water beating against the cliffs acts like a belt sander. “There’s a sweet spot between having too much beach and having no beach, where the cliffs actually erode the fastest,” said Limber.
Some years, cliffs collapse even without big storms. Many factors affect the longevity of these coastal precipices. “Bluffs are complicated geologically,” oceanographer Hoover told local TV station KRON4 (Watch the video: “Powerful El Nino storms could lead to erosion.”) “They’re hard to measure, and there’s a lot of them. And there’s a lot of differences in local geology and hydrology that affect how bluffs are going to fail.”
Measuring the strength of cliff rocks is one of Limber’s specialties. Some cliffs are little more than slightly compacted sand dunes, porous and easily crumbled in your hand. Others are made of much tougher stuff, like granite. Limber measures hardness using a small spring-loaded device called a Schmidt hammer, then figures out how likely the rock unit will crumble while under attack by waves, sand, and debris. Using these data and long-term location measurements, Limber plans to add cliff erosion to the Coastal Storm Modeling System (CoSMoS) computer forecasts of coastal hazards. Like all cutting-edge research, it won’t be easy.
“There’s no handbook for it,” Limber said. “We’re developing our own methods.” He also plans to include well-accepted forecasts of rising sea levels and altered weather patterns due to climate change.
Scientists often measure and forecast cliff erosion as a long-term average, such as 30 centimeters (1 foot) per year. However, most cliffs don’t erode that much every single year. “It’s a sort of stop-start process,” said Limber. A cliff-side home could be fine for 99 years and then, boom. That’s what we see in Pacifica, and that’s why USGS research on coastal bluff erosion is so important.
Ocean Beach and Pacifica are not the only places with eroding beaches and cliffs.
Taking Advantage of El Niño near Santa Cruz
You might remember that weather forecasters called for an El Niño in 2014–2015. USGS researchers in the Pacific Coastal and Marine Science Center wanted to take advantage of that uncommon event, and began monitoring beaches and cliffs in their own backyard—Santa Cruz County, California. “We got it going, and then El Niño didn’t happen,” said Patrick Barnard, USGS coastal geologist and project chief. “It was a nice baseline data collection, so now we’re ready for this El Niño. We can understand how these shorelines vary seasonally due to normal summer-winter oscillations and then during El Niño.”
Besides using GPS receivers mounted on backpacks, ORVs, and personal watercraft, the scientists also measure beaches and bluffs with ground-based and airborne lidar. Lidar stands for light detection and ranging. It’s like radar, but instead of radio signals, lidar uses invisible (and harmless) laser beams to precisely determine the distance to millions of points. A tripod-mounted ground-based lidar rotates rapidly to measure buildings and roads, beaches and cliffs, both vertically and horizontally out to 1,400 meters—more than three-quarters of a mile. If scientists repeat those measurements before, during, and after a series of El Niño enhanced storms, they can measure how the beaches and cliffs have changed.
These devices are expensive, though, and take a long time to set up at each location. For the best results, you need scans from several spots along each stretch of beach. To quickly scan broad areas, USGS scientists use lidar mounted in airplanes flown by contractors. Airborne lidar can’t measure beneath overhanging cliffs, but it can cover far more ground in far less time than its ground-based counterparts. Researchers get a “bird’s eye” view of the coast that’s impossible to get from the beach.
Hilary Stockdon is a USGS research oceanographer stationed in St Petersburg, Florida, who’s used airborne lidar for years. She’s helping her colleagues in Santa Cruz collect data along the entire the West Coast later this year. “We’ll compare it to a survey that was collected in 2014 and start to document the changes along the coastline,” she said. “El Niño is giving us a great opportunity to look at coastal change on the West Coast and test [computer forecasts] that we’ve been developing.”
Many USGS scientists are excited about a new technique for precisely measuring landscapes, called Structure from Motion. Jonathan Warrick, a USGS research geologist, is spearheading that effort along the coast. “I’m a data geek,” said Warrick. “A geo-geek nerd who loves science.”
In some ways, Structure from Motion is a throwback to mapping techniques from the middle of the 20th century. Cartographers made maps of remote locations using air photos. By carefully examining pairs of images using special lenses, they could trace the contours of the land. “One of the ideas behind photogrammetry and Structure from Motion is that you get a three-dimensional perspective from having multiple views of an object,” said Warrick. “It’s just like your eyes. You have two eyes, and if they both work well, you can understand the three-dimensional environment.”
Structure from Motion combines air photos from digital cameras and positions from GPS receivers, analyzing them with powerful computers instead of a brain. Feeding precisely located digital images into special software can produce landscape measurements as good as, and sometimes better than, lidar or GPS—with an angled view of the entire cliff face and far more measurement points.
“Aerial lidar typically gets a few points per meter,” said Warrick. “We’re getting ten, 100 times more data density.” Data collection is fast, too. “From Año Nuevo to Monterey, [we] took 1,800 photos and it took us an hour and a half total.” Gathering so much data comes with a cost: processing all that information took several weeks.
Scientists can almost travel back in time to make Structure from Motion measurements. “One of the exciting things that we’re finding is that historic imagery can be used quite well, if the imagery has enough overlap,” said Warrick. “We’re finding, for example, the California Coastal Records Project images have plenty of overlap in most places.” Those images go back to 1928 along some stretches of coast.
Preparing for a Changing World
All these measurements from Ocean Beach, Pacifica, and Santa Cruz should help researchers tune their computer forecasts, and help people concerned about coastal hazards caused by climate change and El Niño to make better plans for the future, everywhere.“There are many reasons to work in your backyard,” said Warrick. “Often you can do fantastic science that is nationally or internationally recognized, and yet locally important at the same time.”
in this issue:
Local Research with Global Effects: Coastal Scientists Study El Niño in California
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