As this story from the Sacramento Bee reports, the levees protecting Sacramento from flooding are not much, barely a 100 year protection level, and in constant need of repair and strengthening.
Building Auburn Dam would give Sacramento a 400 year flood protection level, and a recent regional forum on water by the Auburn Dam Council provides information on the issue.
An excerpt from the Bee article.
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.