Friday, December 16, 2011

West bank levee-raising won't take toll on east bank, corps officials claim

Mark Schleifstein in the New Orleans Times-Picayune: The raising of levees along the west bank of the Mississippi River by as much as 2 1/2 feet will not put residents of St. Bernard Parish and the east bank of New Orleans at heightened risk from storm surge in a 100-year hurricane, a team of Army Corps of Engineers officials said Thursday. The briefing was aimed at allaying fears of members of the Southeast Louisiana Flood Protection Authority-East, who have raised concerns that the east bank levees are now too low.

Corps officials said it’s actually the waves atop a hurricane’s surge that threaten the 15.5-mile-long stretch of West Bank levees that have been raised between English Turn and Belle Chasse. Those waves, however, would be created by winds blowing out of the east that accompany the few storms that could push surge up the river, said Max Agnew, a corps hydraulic engineer, and the waves thus would be moving toward the West Bank side of the river.
Concerns about surge traveling upriver were raised in the aftermath of Hurricane Katrina in 2005, when several barges were deposited atop West Bank levees in Algiers.

But the corps didn’t decide higher river levees needed to be incorporated into the area’s system until last year, after complex computer modeling confirmed that some 100-year storms, which have a 1 percent chance of occurring in any year, would push water over the levees.

Corps officials recently announced a second increase in the height of some segments along that 15.5-mile stretch, to be accomplished with either higher earthen levees or installation of concrete walls. Those are supposed to increase what the corps calls “resiliency” — the ability to withstand overtopping from surges caused by a 500-year storm without failing.

The designs also build in expected rises in relative sea level — the combination of sinking soil beneath the levee and higher water levels resulting from climate change — over the next 50 years, which could be between 2 and 3 feet. The average level of water in the river at the Carrollton Gage in New Orleans has been rising by about a third of an inch a year for the past 50 years, according to a corps study. The corps requires levees to be designed to accommodate the effects of sea-level rise over their lifetime, using low, moderate and high estimates of the effects of climate change....

Painting depicting New Orleans, on fire, after Hurricane Katrina; Louisiana Superdome at center. By Joy Garnett, Wikimedia Commons via Flickr, under the Creative Commons Attribution-Share Alike 2.0 Generic license

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