At least 12 miles of levees in Sacramento need major repairs to keep their 100-year-flood-protection rating, including virtually all of the city's bowl-like Pocket neighborhood, according to new engineering analyses.
The Sacramento Area Flood Control Agency is in the process of figuring out how much the work will cost, but it is sure to be disruptive. In most cases, the process will require trenching into the center of the levee as much as 120 feet down to build new slurry walls to block seepage.
The problem areas include nearly the entire levee perimeter in the Pocket and Little Pocket neighborhoods along the Sacramento River, as well as other sites along the river near 13th Avenue in Land Park and near Miller Park and the Highway 50 overcrossing.
Most of the levees along Arcade Creek in North Sacramento also need seepage repairs.
Rick Johnson, SAFCA's executive director, said the need for these repairs does not mean the levees are less safe than before. Rather, the work is required by changes in federal levee design and certification criteria that stem from an evolving understanding of levee vulnerability.
No changes in city building rules or flood insurance are expected, at least so far.
A 100-year rating means there is a 1 percent chance of flooding in any given year.
"People aren't any less safe," Johnson said. "It's just that we've got new standards we need to bring them up to. It's just that we're making them better."
The Pocket has been through this before. On three occasions over the past 15 years, SAFCA and the U.S. Army Corps of Engineers have performed similar seepage repairs on levees there.
The largest of those, conducted by the Army Corps a decade ago, included erosion repairs and cost more than $60 million. It put seepage walls consisting of a bentonite, clay and concrete mixture up to 40 feet deep through the center of most Pocket levee segments.
Two subsequent projects fixed smaller problem areas with new slurry walls as much as 110 feet deep.
At the time, this work was instrumental in restoring a 100-year flood rating to the Pocket. This happened only six years ago, ending a federal flood-insurance mandate in the area.
Now, it seems, those repairs didn't go deep enough.
After Hurricane Katrina in New Orleans in 2005, the Army Corps realized deep underseepage poses serious stability problems for levees. So it revised levee-design criteria to take this into account.
The resulting design changes triggered a billion-dollar repair project in Sacramento's vast Natomas basin that is still underway. The flood threat in that basin also resulted in a de facto building moratorium and flood insurance mandate still in force.
Neither of those changes is expected to be triggered by the new work along the Sacramento River and Arcade Creek, said city Planning Director David Kwong. That's because the Federal Emergency Management Agency has no plan at the moment to review the safety of these levees, known as a remapping, he said.
"It's absolutely not another Natomas," Kwong said. "At this point, I think it's fair to say we'll get to develop under the current rules."
Even so, Pocket-area residents are not looking forward to more levee repairs.
The work involves giant excavation machines that dig a trench deep into the center of the levee, which is then filled with a bentonite mixture using pumping equipment. It requires hundreds of truck trips and long work hours.
Rosie Walker, who owns a home along the levee in the Pocket, said the 2003 seepage repairs caused residents a lot of noise and disruption. Some homes were damaged by the vibration of the machines.
"I'm not going to try to stop it. I'm all for safety," said Walker, treasurer of the Sacramento Riverfront Association, a group of Pocket homeowners. "But it is an ordeal. It's a dirty mess."
Others are baffled that more seepage work is still needed and wonder where it will stop.
"It's mind-boggling," said Kathi Windheim. "How do you know when you're finally in compliance and it's going to last? How do we know the criteria won't change again?"
Another factor driving the repairs is a bureaucratic change by the Army Corps of Engineers.
In 2007, it announced it would no longer do the technical work necessary to certify levees to achieve a 100-year flood-safety rating from FEMA, considered the minimum safety rating for urban levees. Instead, communities would have to "self-certify" their levees.
The existing 100-year certification for the Sacramento levees expired Aug. 31. So in February 2012, SAFCA began the process to self-certify the levees, which involves hiring consultants to study engineering and performance relative to Army Corps standards.
SAFCA has paid consultants about $8 million since then to, among other things, analyze soil samples from deep under the levees to look for seepage problems. They found problems 80 to 120 feet deep on 8 miles of levee along the Sacramento River, including the Pocket, and 3.7 miles along Arcade Creek.
Last month, the SAFCA board of directors awarded two more consultant contracts to determine how to fix the seepage problems and how much it will cost. Results are expected early in 2014.
Some of these levees also have erosion problems. In addition, virtually all of the Sacramento levees outside Natomas totaling 68 miles have encroachment problems from trees and intrusive structures. These problems will also be analyzed in the forthcoming studies.
Even that won't be the end of the work.
State law passed in 2007 requires urban levees to meet a 200-year flood-safety rating. This may require taller and wider levees. The Army Corps is working on a study to determine what will be needed to achieve 200-year protection. Called a "general re-evaluation report," it is expected by the end of 2014.
"There may be additional work needed for these levees to get to 200-year (certification)," Johnson said. "But anything we do now, we won't have to redo."
Contact The Bee's Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.