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Rising Seas And The Delta Part 2: Disaster Looms

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Flooding at the confluence of the American and Sacramento rivers | Photo: Mike Simpson/Flickr/Creative Commons License

An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with kcet.org/baydelta for all the project's stories.

California's Delta is unique in the world. It's an inland delta at sea level with an outlet to the ocean, where the influence of the tides is felt 60 miles from the Pacific. And as we mentioned in Part 1 of this series, the Delta's topography and sea level altitude made it an incredibly rich ecosystem, with biodiversity and rich soils nearly unequalled anywhere in North America.

But that same topography and elevation that have made the Delta so rich have also made it supremely vulnerable. As our carbon-enhanced atmosphere warms the planet, and sea levels rise as a result, the scale of the resulting changes coming to the Delta are truly staggering.

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Climatologists have been famously reluctant to predict just how much sea levels are going to rise in the next century. When they do make predictions, their scientific caution leads them to to understate the magnitude of the changes we face.

The Intergovernmental Panel on Climate Change's 2007 assessment was a case in point, at least as regards the degree to which the climatologists expected sea levels to rise by the year 2100. In that assessment, which looked at a range of scenarios that addressed uncertainties in both climate science and the degree to which human society would get off its collective butt and work to reduce CO2 emissions, the IPCC suggested that the world's global average sea level would rise somewhere between eight and 17 inches.

That suggestion failed to raise much alarm. 17 inches might not seem that big. There are bigger high tides. It seems enough to erode some beaches or carve away a seaside cliff a little, but not enough to endanger our expensive seawalls and coastal infrastructure. At least that's what the public seemed to feel, given the lack of alarm.

Sea level rise that high would in fact be pretty dangerous: that 17 inches would have significant effects like backing up floodwaters well into river basins, contributing to the damage done by tropical storm surges, destroying much of what remains of coastal salt marshes, and the like.

But it doesn't matter: few experts still think sea level rise will be as low as 17 inches by 2100. The 2007 assessment omitted consideration of instability in ice sheets on Greenland and Antarctica, which has recently been increasing, upping the likelihood that those ice sheets will contribute a whole lot more meltwater to the oceans.

Without the ice sheets, sea levels are expected to rise by around a foot due solely to melting of alpine glaciers and thermal expansion of the oceans -- water expands as it warms, and the oceans are so far sucking up about 90 percent of the planet's increase in temperature. Add in the additional meter of rise what we now expect the oceans to receive from Greenland and Antarctica -- 3.28 feet -- and scientists who suggest sea level will rise around four feet by 2100 are not being laughed at by their fellow climatologists.

That is almost certainly conservative. The U.S. government's National Climate Assessment website suggests that emergency planners and decision makers with "a low tolerance for risk" count on a sea level rise of about six feet and seven inches.

What would even a four-foot rise in sea level mean for the Delta? It's hard to find a better way of expressing that than this map from the California Department of Water Resources:

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Land subsidence in the Delta | Map: California Department of Water Resources
 

Most of the Delta is below sea level, the lowlands protected by a 1,100-mile network of levees, many of them privately owned. Decades of farming and draining the Delta's peat soils have caused the organic matter in those soils to oxidize, contributing carbon dioxide to the atmosphere, and reducing the soil's volume. And that means the land subsides. Some places in the Delta, especially on those islands in the west-central part of the Delta, have dropped to more than 15 feet below sea level. (That map from the Department of Water Resources was crafted in 1995: farming and subsidence have continued in the Delta in the 20 years since.)

We'll discuss subsidence in more detail in articles to come, especially the threat it poses to the state's water delivery infrastructure. But for purposes of discussing rising sea levels, all we really need to know right now is this: the Delta has about 1,150 square miles of land that's below sea level. It's kept more or less dry by a network of aging levees, some of them more than a century old, and about two-thirds of them privately owned and maintained.

Those levees fail even without dramatic rises in sea level. A high tide in 2004 took out a levee protecting 12,000-acre Jones Tract west of Stockton, sending 12 feet of water onto farmland a few weeks before harvest. Taxpayer dollars spent to repair the levee ran around $90 million in a few months; the state and federal governments might not have bothered if expensive Highway 4 hadn't been threatened.

In other cases, the state didn't bother with repairs. Levees protecting 1,000-acre Mildred Island, immediately north of Jones Tract, failed in 1983. Its owners abandoned it, and local agencies decided not to cough up the $250,000 the Army Corps of Engineers was asking as a local contribution for levee repair. Now you can fish for striped bass above the former farm fields, though anglers argue whether the fishing's better at Frank's Tract, which flooded and was abandoned by its owners during the Great Depression. One of the most recent abandoned islands, Liberty Island off the Yolo Bypass, suffered a levee failure in 1998, and is now being restored as fish nursery habitat.

A 2010 study by UC Davis researchers examined subsided islands in the Delta, considered both the cost and effectiveness of levee upgrades as compared to the economic value of the land being protected by those levees, and concluded that of the 36 islands they examined, none were worth having their levees upgraded, and between 18 and 24 weren't worth rescuing if the levees breached.

In the wake of Hurricane Katrina, where more than 50 levee failures in New Orleans contributed to perhaps 700 deaths, Californians passed a pair of bond measures to fund levee repairs and upgrades in the Delta. But the repair projects might not do that much to forestall the effects of sea level rise. The current highest standard for levee construction in California, the DWR-200 specification, requires that levees protecting low-lying areas with more than 10,000 residents be three feet taller than the expected levels of a 200-year flood. Very few of the levees in the agricultural Delta will be rehabbed to this standard. Most that do undergo improvements that will be upgraded to lower standards that require a foot to a foot and a half of clearance above a 100-year flood. Most will not be upgraded at all.

And here's the big problem: even a small amount of increase in sea level will dramatically alter flood regimes in the Central Valley and Delta. At present when the Valley floods it drains out at a certain pace through the Delta and Bay. With higher sea levels, those floods will take longer to drain out though the Golden Gate, which means higher floods that are slower to recede.

In 2006, the California Energy Commission released a white paper that conjectured a "modest" increase in sea level, on the order of 12 inches, would make extreme flood events much more common. How much more common? Floods that exceed 99.99 percent of recorded high water levels -- essentially, 200-year floods or worse -- would take place, by 2100, an average of 600 hours per year. That's 25 days on 200-year floods in an average year.

And of course, when you say "in an average year," what you're saying is that half of the years you're considering will be worse.

Adding to the problem is the increased likelihood that our winter precipitation will fall in the mountains as rain instead of snow. Instead of sitting there politely until may or June and then melting quietly and conveniently, January and February storms will swell the Delta's tributary rivers almost immediately, like they did in the winter of 1862 -- a year in which Sacramento was filled with 20 feet of water and the state government had to conduct business in San Francisco. And that was with a fully functioning network of wetlands performing flood control services.

But it won't take a storm like that to overtop the Delta's levees with an extra meter of sea level. A 1997-class winter would suffice. Or perhaps even an unremarkable winter storm that drives winds in precisely the right direction for the right amount of time to create a couple extra feet of storm surge.

Or just the mere presence of an extra meter of water quietly putting unimaginable stress on each levee's weakest points could do it, on a clear day, as happened in 2004 at the Jones Tract.

A few islands might withstand the rising sea, aided by dumb luck or massive government expenditures, though more than likely it would require both. But the amount of sea level rise projected in recent conservative estimates for 2100, a year that kids born today have a reasonable chance of seeing, would almost inevitably submerge a mind-bogglingly large section of the Delta -- from Sacramento's southern suburbs to Tracy, with all the land between them and west of Interstate 5 either underwater or close to it.

And the scenario the state of California is recommending that "risk-averse" risk managers plan for, with two meters of sea level rise, would merely seal the deal. Levee after levee would fail. Water would surge into those subsided "islands, scouring new holes in the levees at the far and and prompting further breaches. A few smaller islands with significant value might merit heroic measures to save them -- Andrus Island, with the town of Isleton on it, comes to mind.

But the state of California has long been short on heroic measures when it comes to fixing the Delta. It seems inevitable that -- if not by 2100, then a few decades after -- the Delta will have become a vast inland sea where once there was a wetland.

That's especially true when we stop to remember that 2100 is just an arbitrary benchmark. Even if we stopped emitting carbon dioxide into the atmosphere yesterday, the planet's warming would continue for some centuries. However much the seas rise by 2100, they'll rise even more between 2100 and 2200.

In the Bay Delta, sea level rise is a looming economic disaster. The Delta isn't just a hub for water diversions and aqueduct deliveries, but for rail, highway traffic, pipelines, and other networks. Rerouting those networks around a new inland sea may well cost trillions.

But could there be an upside to the sea retaking the Delta? If some abandoned islands now flooded now offer habitat for wildlife, couldn't sea level rise be a form of accidental ecological restoration? We'll discuss that in Part 3.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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