Unfortunately, they’re beneath 300 feet of water outside the Golden Gate

Glen Martin, Chronicle Environment Writer

Thursday, July 20, 2006

San Francisco long has been renowned for its hills, bay and bridges - but not for expanses of sand dunes. That’s liable to change.

It turns out there are more than 2 square miles of dunes right next to the city, and world-class dunes at that: Only a few sites around the globe have larger dunes of this sort.

Access, however, will remain difficult unless you’re a sand dab or Dungeness crab. The dunes are just west of the Golden Gate, submerged in 100 to 350 feet of sea water.

What you are looking at: Sediments, carried out the Golden Gate by powerful tides, have created some of the largest underwater sand dunes on the planet. U.S. Geological Survey scientists used powerful sonar equipment to measure the ocean bottom and mapped the dunes in different colors to emphasize their shape. Photo courtesy of U.S. Geological Survey

Here is a digital rendering of sand waves in the bay — when under water they’re not called dunes. Photo courtesy of U.S. Geological Survey

Here is a digital rendering of sand waves beneath the surface of San Francisco Bay. Photo courtesy of U.S. Geological Survey

 

Scientists grasped the extent and size of the underwater dunes - technically known as “sand waves” - only recently, aided by sophisticated, multiple-beam sonar that provides stunningly detailed images of the submarine topography.

“These are some of the largest sand waves in the world,” said Patrick Barnard, a coastal geologist with the Santa Cruz office of the U.S. Geological Survey. “They’re certainly in the upper 10 percent.”

The sand waves range up to 700 feet long and reach heights of more than 30 feet, Barnard said. It is a dynamic system, he said, with the configuration of the individual dunes changing significantly with each tidal cycle. But overall and over time, the net change to the entire field is slight.

“From 2004 to 2005, the field only moved 7 meters (about 23 feet),” Barnard said.

Dan Hanes, a USGS oceanographer, said researchers first mapped the sand waves while doing water-depth studies in and around the bay with advanced sonar devices in 2004 and 2005.

“We knew they were there, but we didn’t know their extent or how large they were,” Hanes said. “Our work gave us the first 3-D map of the area.”

Hanes said he and Barnard were surprised by size of the sand waves “and also by the pattern. Frankly, it was beautiful.”

Barnard and Hanes posted their study of San Francisco’s sand waves this week on a restricted Web site run by the American Geophysical Union.

Gary Greene, a professor emeritus in geology at the Moss Landing Marine Laboratory and a senior scientist at the Monterey Bay Aquarium Marine Research Institute, said he wasn’t necessarily surprised at the size of the dunes, given San Francisco Bay’s robust tidal dynamics.

“We know some of the sand waves off the San Juan Islands in Puget Sound reach heights of 90 feet, but that system up there moves such a tremendous amount of water,” Greene said. “What I find especially significant (about the USGS study) is its predicting of the tidal currents off the Golden Gate, and the way they affect” the sand waves’ shape.

Hanes said scientific interest in sand waves has been growing around the world because sonar technology has improved to the point that high resolution, three-dimensional maps can now be made of the ocean’s floor. Sand waves are important components of traveling marine sediment and can help researches understand processes involved in beach erosion.

San Francisco’s sand waves may have some peers in length and height, but they could be unique in one respect: the size of the water system that created them.

Most extensive sand wave fields, explained Barnard, occur in large inlets, such as Puget Sound, the Bay of Fundy in eastern Canada or Cook Inlet in Alaska. These huge waterways have tremendous tidal ranges that move great amounts of water and sediment over vast areas, allowing underwater dunes to form.

In comparison, the San Francisco Bay system is relatively small, and its tidal range is narrow. But its geology nevertheless permits the deposition of big sand waves.

“In San Francisco, you have a large bay funneling through a very small opening that is constricted by rocky headlands,” Barnard said.

That flow is powerful. About 500 billion gallons — or enough water to fill 660,000 Olympic-size swimming pools — move through the mile-wide Golden Gate over each six-hour period, Barnard said.

Squeezing so much water through such a narrow opening creates extremely strong currents.

“The Golden Gate has one of the strongest tidal currents in the world, averaging about 5.6 miles an hour,” Barnard said. “Water moving at that speed can move a great deal of sediment.”

Plus, there is — or at least, has been — a lot of sediment to move in the San Francisco Bay system. The drainages of the Sacramento River, the San Joaquin River and Alameda Creek historically contributed huge volumes of gravel and sand to the estuary.

The sediment loads of the Sacramento and San Joaquin Rivers increased significantly after the Gold Rush, Barnard said. A major concomitant of the hydraulic mining that ravaged the Sierra was the reduction of entire mountainsides into sand and gravel, which were then sluiced downstream into the bay and out the Golden Gate.

Human activity may therefore have augmented San Francisco’s sand wave field. But an opposite process could now be at work, he said.

“Part of the purpose of our (sand wave) survey was to assess changes in San Francisco Bay in the last 50 years,” Barnard said. “We’ve found the bay has lost a huge amount of sediment since 1956 — approximately 137 million cubic yards.”

A variety of factors could be contributing to the reduced sediment loads. Among them, the stabilization of the watersheds of the Sacramento and San Joaquin rivers, and the reduction of tidal current due to development in the bay.

“We also know that dredging has removed a lot of sediment - about 50 million cubic yards,” Barnard said.

Greene said dams on the Central Valley’s big rivers have impeded sediment flow, and “the great slug of sediment from the Gold Rush has worked its way through the bay. There’s just less available now.”

That reduced sediment supply could ultimately affect the size and configuration of the sand waves. Other consequences also are likely.

“Less available sediment affects all the local coastal processes,” Barnard said, “including Ocean Beach. We know Ocean Beach has been experiencing accelerated erosion, and it increasingly appears linked to reduced sediment transport from the bay.”

The city councilman objects to that color for the next MTA light rail line, saying it doesn’t ‘resonate.’ Others see it differently.

By Jean Guccione, Times Staff Writer
July 20, 2006

Some public transit advocates are seeing red about rose.

That’s the color that City Councilman Bernard C. Parks wants to use on maps to trace the route of a light rail line being built west from downtown.

The Los Angeles County Metropolitan Transportation Authority has suggested calling the light rail along Exposition Boulevard the Aqua Line, and using that color to mark its path. It already uses aqua on planning maps, and transit buffs seem to like it.

“It’s just ludicrous,” Roger Christensen, a member of Friends 4 Expo Transit, said of Parks’ proposal. “Metro itself has been using the color aqua for ages.”

Parks has further riled some in the transit world by ignoring the MTA’s long-standing practice of designating rail lines by colors — Red, Blue, Green and Gold. He wants the route called the Expo Line.

The hue and cry, which began in March, has reached such proportions that some MTA board members would not talk about it.

The MTA board is set to pick a color for the line as early as next week.

“I’m looking forward to this color controversy being put behind us so we can focus on the construction and funding of the entire line to Santa Monica,” said Ken Alpern, president of the Transit Coalition and a Mar Vista Neighborhood Council member.

For four years, MTA staff has drawn an aqua line on maps to indicate the first phase of construction, an 8.5-mile railway between downtown Los Angeles and Culver City commonly referred to as the Exposition Line.

During that time, the MTA has received more than 2,000 comments on the proposed rail line, mostly about safety, sound barriers and landscaping.

“We never had any comments on color designation whatsoever,” said Maya Emsden, the agency’s deputy executive officer for creative services.

Aqua fit the MTA’s map color criteria. “It has to be bright, legible and easy to pronounce,” Emsden said.

Advocates say they like the way it echoes the hues of several landmarks along the proposed route, including a band painted around the Los Angeles Memorial Coliseum, the Los Angeles Convention Center, Al-Khattab Mosque, Ballona Creek and Dorsey High School.

Not to mention the train’s proposed final destination: the ocean.

But Parks, whose City Council district will be bisected by the new rail line, doesn’t like the Aqua Line, or another MTA suggestion: the Purple Line. Nor, for that matter, does he like the selection process — which failed to ask residents for their color choices.

So, what’s wrong with aqua or purple?

“Those are colors that don’t resonate,” Parks explained.

He also said the MTA doesn’t listen to his constituents in South Los Angeles as much as it does to those on the line’s proposed Westside end.

“It seems to me that the folks who supported aqua seemed more affiliated with Phase II, not Phase I,” Parks conceded.

Construction has begun on the first phase of the light rail line.

A second phase of construction, from Culver City to Santa Monica, is on the drawing board.

In recent months, Parks said, he has sought the opinions of his constituents and four other members of the Los Angeles City Council, which he said will contribute $40 million to the first phase of the $640-million transit project.

“We just asked people to give us some ideas of colors,” Parks said. Their suggestions could fill a paint store: amber, aqua, copper, olive, plum, rose, sienna, gray, lemon, lime, pink, purple, salmon, sky, tan, teal and violet.

Rose represents Exposition Park’s rose garden, the city councilman said. It was his second choice.

Parks and others decided his first recommendation — gray — was too similar to the San Gabriel Valley’s new Silver Streak, a rapid 40-mile bus line from Claremont to downtown Los Angeles.

Alpern, who lives in Sherman Oaks, took issue with Parks’ characterization that Westside residents were dominating the debate. “Regrettably, Bernard Parks is being an army of one on this issue,” he said, noting that the color aqua has widespread support from people living all along the proposed rail line.

The Friends 4 Expo Steering Committee and the Mar Vista Community Council Board of Directors endorsed the color aqua.

Darrell Clarke, co-chairman of Friends 4 Expo, offered several reasons for rejecting Parks’ color choice, including that it had not been vetted by the public.

The color rose also “is most identified with Pasadena — the Rose Bowl and Rose Parade — on the other side of town,” Clarke, who lives in Santa Monica, wrote in an e-mail to members of his group.

 

The crack is 8m-wide in places

The largest tear in the Earth’s crust seen in decades, if not centuries, could carve out a new ocean in Africa, according to satellite data.

Geologists say a crack that opened up last year may eventually reach the Red Sea, isolating much of Ethiopia and Eritrea from the rest of Africa.

The 60km-long rift was initially sparked by an earthquake in September.

Follow-up observations reported in the journal Nature suggest the split is growing at an unprecedented rate.

 

We think if these processes continue, a new ocean will eventually form Dr Tim Wright, University of Oxford

It betrays events deep beneath the ground, where some of the tectonic plates that form Africa are gradually moving apart from the Arabian plate, causing the crust to stretch and thin.

As rifts appear, molten rock bubbles up from beneath the surface, hardening to form a new strip of ocean floor.

Dr Tim Wright from the University of Oxford, UK, said if the ripping of the crust continued, the horn of Africa would eventually split off from the rest of the continent, in about a million years.

“We think if these processes continue, a new ocean will eventually form,” he told the BBC News website. “It will connect to the Red Sea and the ocean will flow in.”

Fundamental processes

Dr Wright is a member of a team from the UK and Ethiopia that has been monitoring the creation of the new ocean basin; a rare event on dry land.

They used sensitive seismic instruments, field measurements and satellite images from the European Space Agency’s Envisat spacecraft to study what is happening beneath the ground.

“We’ve been able to work up all the satellite data and get a very precise map,” said Dr Wright.

“It’s the biggest rifting episode at least since the 1970s and possibly in hundreds of years.

“It’s the first time we’ve been able to use satellite images to investigate the fundamental processes behind rifting.”

The shift in the Earth’s plates has been happening gradually over the course of two million years but every now and again earthquakes and volcanic eruptions herald sudden break-ups.

Space techniques

One such event took place in September last year, opening up a 60km-long (37 mile) stretch of a fault-line that runs from Ethiopia to the southern edge of the Red Sea.

 

An international team is monitoring two million years of rock history

“It’s amazing,” said Cindy Ebinger, from Royal Holloway, University of London.

“It’s the first large event we have seen like this in a rift zone since the advent of some of the space-based techniques we’re now using.

“These techniques give us a resolution and a detail to see what’s really going on and how the Earth processes work.”

Scientists have calculated that 2.5 cubic km (0.6 cubic mile) of magma has flowed up through the crack in the Earth’s crust.

It is enough to fill London’s Wembley stadium 2,000 times or smother the area within the capital’s M25 orbital motorway with molten rock to a depth of 1m (1 yard).