FEMA and the US Army Corps of Engineers had to work together for the Advisory Base Flood Elevation guidance levels to be released. What the USACE planned to do with the levees was an important, if not the most important, variable that would determine where FEMA would set the Base Flood Elevations. Ultimately, FEMA did not change the 1984 BSE levels for the ABFE levels, but rather required adherence and tacked on the “three feet above ground” requirement.
That means FEMA thinks the levees will provide 100-year protection at the ABFE levels. I am sure the USACE assured FEMA that their plans for the levee system would do just that. But an assurance by the USACE isn’t always assured (copy and paste to avoid login, I think):
On a cloudy February day in 1998, the Army Corps of Engineers wrote Sacramento flood officials to tell them that Natomas, a deep floodplain with wide stretches of undeveloped land, had been fortified against 100-year floods.The southeastern Louisiana levee system is much bigger project than the Natomas levees, and much more is at stake. And there is much more that can go wrong:
It was what everyone had been waiting for. It would lead to cheaper flood insurance, an end to mandatory insurance and the complete lifting of a building moratorium. It would encourage acres of new homes to spread through Natomas, nearly tripling the area's population to 67,000.
Yet that assurance of 100-year protection from rivers and canals was based on illusion.
New studies have concluded the area would not be safe in a 100-year flood, the kind spawned by storms with a 1 percent chance of sweeping through in any year. It will cost $140 million to $200 million and take until roughly 2011 to provide that protection
In retrospect, it's clear the corps missed basic features of the ground beneath Natomas levees because of those limited explorations, said David Ricketts, a former corps section chief who reviewed old and new Natomas findings at The Bee's request.This is not to say that the USACE can’t build good levees. This is not to say that the USACE is sloppy. This is not to say that the USACE lied when they said Natomas was protected.
The misstep was fairly simple, resting in how many holes engineers drilled into the levees to bring up fat cylinders of soil.
This simply shows that guaranteeing 100-year protection is complicated:
Lessons learned in Natomas illustrate the elaborate - and sometimes shaky - foundation on which assurances of 100-year flood protection are built, and the countless ways those assurances can be undermined for levees throughout the state.The ABFEs that FEMA released depend on the USACE building levees that work by 2010. When the levees are finished, the governor may get a call from the USACE assuring that southeastern Louisiana is protected. But they might be wrong – not negligently wrong – just wrong. The levees might not be good enough. And that will throw all the plans out the window.
"It gets to be extremely complicated because of assumptions," said Herb Hereth, a retired Army Corps hydrologist. "You lay out for me the assumptions that you're going to analyze, and I'll give you an answer. You change the assumptions and I'm going to give you a different answer."
"No matter what kind of levee you're behind, you should assume there's a risk," said John Hess, chief of the geotechnical and environmental engineering branch at the Sacramento corps.That’s good advice for those of us who are sticking around.