A $5.8 million underwater barrier completed in mid-September to block the upriver flow of saltwater in the Mississippi River from threatening area water supplies has experienced some erosion but is still doing its job, a spokesman for the Army Corps of Engineers said Monday.
The 1,700-foot-long underwater dam, called a sill, at Alliance in Plaquemines Parish, keeps saltwater flowing upstream from the Gulf of Mexico from reaching the water intake pipes for New Orleans and Jefferson Parish.
On Dec. 12, the leading edge of the saltwater was at river mile 63.8 above Head of Passes, and was expected to retreat southward over the next few days, the result of additional rainfall that has fallen in the Midwest, combined with the release of water from several upriver dams that was required to allow barge traffic to continue moving on the equally low Missouri River.
A survey of the sill a week ago found a small amount of erosion in the center of the river channel, possibly the result of ship traffic above it, said Mike Stack, chief of emergency management for the New Orleans District office of the corps.
“We will survey the sill every two weeks and keep an eye on the erosion,” Stack said. If the erosion gets worse, the sill may have to be replenished with sediment dredged from an area just upstream, he said.
Unusually long drought conditions in the Midwest have resulted in the extremely low river levels in New Orleans this year: since June 1, the level has been at 3 feet or above at the Carrollton Gauge on only 28 days.
Four of those days were before, during and after Hurricane Isaac, when the river reached 9.6 feet in New Orleans because of storm surge moving upriver from the Gulf, before dropping back to near 2 feet.
State Climatologist Barry Keim said the relaxing of La Nina conditions, marked by lower than average water temperatures in the eastern and central Pacific Ocean, should mean more rainfall in the Midwest in coming months. The ocean temperatures are now in a “neutral” pattern, about average, and are expected to stay that way through the Spring, according to a joint forecast of the National Weather Service’s Climate Data Center and the International Research Institute for Climate and Society.