Congress will finish fixing Natomas’ levees, experts argue that’s still not enough to lift building moratorium

As climate-change predictions make low-lying areas increasingly vulnerable to flooding, some say many Sacramento residents are living on borrowed time.

But the clock is soon going to be reset in the community of Natomas, where a longstanding development moratorium will likely be lifted as a levee-improvement project underway since 2007 advances toward the finish line.

Already, 18 of the 42 miles of levees surrounding the Natomas Basin have been replaced by the Sacramento Area Flood Control Agency.

Now, a bill passed by Congress last week would authorize the U.S. Army Corps of Engineers to use federal money to finish repairing the remainder of the levees, which keep river water from swamping the homes of 100,000 people and roughly $7 billion in infrastructure. The president still must sign the Water Resources Reform and Development Act for it to take effect.

The goal, according to local officials, is to make the levee system protecting Natomas capable of withstanding the largest flood likely to strike the area in the next 200 years. Once the project begins in earnest, the Federal Emergency Management Agency would lift the building ban.

While most city officials and community advocates have cheered the progress, others remain skeptical. Jeffrey F. Mount, a professor emeritus of geology at UC Davis, questions the wisdom of continuing to build in flood-prone regions at all.

“Is it a good idea to build on a floodplain? That’s hard to say,” Mount said. “For anyone living there now, it’s a great idea to protect the area with new levees.”

But the area is likely to be flooded again, Mount said, in spite of planned fortification efforts. He says climate models are predicting greater frequency and magnitude of winter storms in the coming decades.

“It’s hard to say if it makes economic sense to improve levees if it spurs development,” he said. “Because when the levees do eventually fail, the economic damage is very, very high.”

The Natomas levees lost their certification in 2006 after national safety standards were increased following the Hurricane Katrina disaster in New Orleans. Here, and in Sutter County in 1997, levees failed due to water seeping under the barriers, creating cavities, and leading to their collapse.

“The Corps withdrew its previous certification of Natomas’ levees based on a better understanding of how vulnerable those levees are to seepage, under-seepage and stability problems,” explained Chris Gray-Garcia, a spokesman with the Army Corps of Engineers, in an email.

The development moratorium followed the decertification when FEMA required any new projects to be elevated above the levees. It wasn’t exactly a ban, but construction projects have been on hold ever since.

Now, 18 miles of levees have been rebuilt. They are significantly wider than before and with a key defense feature of a 3-foot-wide cement barrier, called a slurry wall, that plunges about 100 feet down and prevents incursion of water from below.

The repair work has been expensive. The project has so far cost $410 million, funded by the state and by local property-assessment fees, and the remaining 24 miles will probably cost more than $600 million in federal money and take until 2020 to complete. In addition, 25 miles of levees along the American River have also been fortified with slurry walls in recent years, according to Rick Johnson, the executive director of the Sacramento Area Flood Control Agency. The Sutter Butte Flood Control Agency is also fortifying its own 41-mile levee system.

Another reason that levees need to be repaired is for insurance reasons. Basically, homeowners in any region buffered by levees rated at a 100-year-flood protection level and deemed at low or moderate risk of inundation do not need to buy flood insurance.

Mount believes this standard is too lax.

“Imagine you have a 100-sided [die], and you throw it once per year,” he said. “Eventually, that bad number is going to come up. The 100-year standard isn’t enough. There’s a one-in-four chance that in the next 25 years every home [in Natomas] will be flooded. That’s a much, much higher risk than that of your house burning.”

John Cain, a flood-management specialist with the group American Rivers, agrees that a 100-year insurance limit puts too many people at high risk.

“Hypothetically, it only takes a 101-year event to breach the levee, and then you’re 20 feet underwater,” he said.

Building a stronger, or just taller, levee can also have the unintended consequence of increasing the likelihood of a levee breach somewhere else.

Cain said if only one community bolsters its levees, floodwaters may be pushed over the levee tops of an adjacent community. This concept was the underlying current beneath the so-called levee wars of last century, in which adjacent communities found themselves at odds with one another as they built up their levees independently of the other. Today, levee-improvement projects are generally conducted through a larger systemwide approach that has no negative impacts on other nearby communities.

Mike Inamine, the executive director of the Sutter Butte Flood Control Agency, said his district is currently pouring slurry walls into 41 miles of levees protecting Yuba City and agricultural lands to the south. The plan is to meet both the 100-year-flood insurance standard and the 200-year-flood protection rating required by the state for urban areas.

But UC Davis’ Mount doesn’t quite trust the national levee-rating system to begin with. It is based on historical norms for storm and flood occurrence. In theory, a levee with a 200-year rating will survive the biggest flood event statistically feasible in a two-century period. The levees in Tacoma, Wash.; St. Louis; and Kansas City, Mo., are much stronger, rated to withstand a 500-year event.

But the problem with this system, he explained, is that climate patterns are changing, making it harder, even for computer programs, to make accurate storm predictions.

“Experts think the frequency and magnitude of storms will increase, so the rating system doesn’t work, because the past is no longer a predictor of the future,” he said.

Sea-level rise will not significantly affect most of the Sacramento area upstream of the Delta, Mount said. But flooding caused by heavy rains in the Sacramento Valley almost certainly will.

Since 1950, the American River has set five flood records. In other words, floods have been getting more severe for 60 years, and many experts expect this trend to accelerate.

Mount feels that the city and state have essentially consigned themselves forever to protecting areas like Natomas.

“The state took a gamble to build a city on a floodplain they knew was prone to flooding,” Mount said. “Every new house out there increased the economic risk, and to this day, I don’t know why they allowed it. It would have made much more sense to leave Natomas a farming area, but we’re not going to move people out of there. The best we can do is protect it.”