Scientists: S.F. Bay protection from costly disasters being thrown away
SAN JOSE — For more than 100 years after California’s Gold Rush, developers and city leaders filled in San Francisco Bay, shrinking it by one-third to build farms, freeways, airports and subdivisions.
All that changed in the 1970s with modern environmental laws. But now as sea level rise threatens to cause billions of dollars of flooding in the coming decades, the bay is going to need to be filled again — but this time in a different way, according to a new scientific report out Tuesday.
Twice the amount of sediment excavated for the Panama Canal will be needed to build up the bay’s shoreline, researchers say, to protect communities from disastrous flooding and rising seas that could climb as much as 6 feet by the end of the century.
The best source for that immense volume of fill is the mud and silt scooped up when the bay’s harbors and shipping channels are dredged every year. But currently, that material is being dumped into the ocean 60 miles off the Golden Gate, or sent to the bottom of the bay near Alcatraz Island.
The report calls for a radical change in those disposal practices.
“It’s not a waste product. It’s a valued resource. It should be used for the public good,” said Letitia Grenier, a co-author of the report and a senior scientist at the San Francisco Estuary Institute, a non-profit research organization in Richmond.
The study, called “Sediment for Survival,” was written by San Francisco Estuary Institute scientists, with input from researchers at UC Davis, the U.S. Geological Survey, the Environmental Protection Agency, the San Francisco Bay Conservation and Development Commission, and other bay experts.
The math is fairly simple.
To raise the wetlands and mudflats around the bay by 6.9 feet over the next 80 years, the high end of most estimates of sea level rise, 477 million cubic yards of mud and dirt will need to be added to them. That’s the equivalent load of 48 million dump trucks. Or enough to fill the massive Hangar One at Moffett Field 265 times.
If nothing is done, between 150 million and 170 million cubic yards will come into the bay naturally from streams, rivers and other sources. That leaves a shortfall of about 300 million cubic yards. The good news? About 60% of that deficit can be made up with mud and silt from dredging, Grenier and her colleague Scott Dusterhoff estimate. The rest can be produced by scooping out vast amounts of sediment trapped behind dams, removing dams, rerouting flood-control projects or shifting inland dirt from construction projects over the coming generations.
Using what has been considered a waste product to protect the bay from flooding would be a transformation similar to society realizing that aluminum cans and glass bottles shouldn’t be thrown in landfills, or that wastewater could be cleaned and used again for irrigation, said David Lewis, executive director of Save the Bay, an environmental group in Oakland.