Monday, May 9, 2011

The Great Flood of 2011 (Part III)

Mississippi River Flooding: Bonnet Carre Spillway Opened

By Alex Sosnowski, Expert Senior Meteorologist
May 9, 2011; 11:00 AM ET
Army Corps of Engineers personnel work to open the Bonnet Carre Spillway in Norco, Friday, April 11, 2008, 30 miles above New Orleans. Heavy rain farther north, in the Mississippi Valley, prompted the decision to open the spillway for only the ninth time since 1937 to reduce pressure on the levees. (AP Photo/Bill Haber)

In attempts to mitigate river levels in the Mississippi Delta region, the Bonnet Carre spillway was opened Monday north of New Orleans.

While the Mississippi River is forecast to crest around Memphis by early Tuesday, areas downstream continue to brace for record or near-record flooding during the middle and latter part of May.

Even with the opening of the spillway, which directs water into Lake Pontchartrain, the Mississippi River is forecast by National Weather Service hydrologists to crest within 0.5 of a foot of the top of the levees in New Orleans. Levees protect the Big Easy from Old Man River up to 20 feet.

While considered to be very strong, there is no guarantee that the levees protecting New Orleans and other communities will hold.

The opening of the Bonnet Carre Spillway will have no effect on river levels in Memphis, Tenn.

Water seeping under a levee (a boil) is always a concern. While there is nothing that can be done beforehand, crews may react quickly to contain the boil, or risk levee failure.

The fact that much of this part of the Mississippi has seen little rain in recent weeks may help slightly in the short term from a levee standpoint, in that the ground is firm from the drought conditions.

Unlike river flooding that occurs in parts of the Northeast, the event lasts weeks on the Mississippi, as opposed to days say on the Delaware, Connecticut and Susquehanna rivers.

Since its completion in 1931, the Bonnet Carre spillway has only been opened 10 times and most recently in 2008 for a period of approximately three weeks.

The U.S. Army Corps of Engineers has requested to open the Morganza Spillway northwest ofBaton Rouge for the first time since 1973 in attempts to relieve pressure on levees nearby and downstream further.

This spillway directs water into the Atchafalaya River. The spillway was constructed to prevent the natural migration of the Mississippi River farther west to the Atchafalaya Basin.

Such a shift in the Mississippi's course would have had tremendous negative impact on industry and commerce on communities in the region.

Similar to the intentional breach of levees at Birds Point, Mo., to save Cario, Ill., and other towns nearby, the opening of the spillway would direct flood waters into prime agricultural areas and other communities.

Part of Lafourche and much of Terrebonne parishes could be under 5 feet of water. Other parishes to be affected by flooding from opening the Morganza Spillway included Pointe Coupee, St. Landry, St. Martin, Iberia, Iberville and St. Mary.

Bayous in the Delta Region were also rising as the Mississippi continued to swell, causing water to back up.

Full reservoirs upstream, mainly in the Ohio Valley, will continue to raise concerns for a slow recession of the Mississippi after it crests and could play a role later. A full reservoir cannot absorb runoff.

Officials will likely attempt to drain down these reservoirs as the Mississippi level declines. However, if more heavy rain hits the Ohio Valley, then correspondingly more water will be released into the Ohio, which drains into the Mississippi at Cario, Ill.

A period of heavy rain, via thunderstorms, will affect the Ohio Valley late this week into this weekend, but it would take an substantial amount of rain to impact river levels downstream.

While the rain late this week will be more substantial than this past weekend's rainfall, it is not likely to be enough to lead to new rises on the Mississippi. However, it is possible enough rain will fall to slow the recession of waters.

1 comment:

mike carson said...

My prayers are with you guys over there.

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