What could happen in worst-case scenarios?

Published 12:10 am Sunday, May 8, 2011

NOAA National weather Service collection courtesy of the Family of Captain Jack Sammons, C&GS In 1927, the Mississippi River flooded the entire town of Vidalia. The federally controlled levee system was begun the next year. The mainline Mississippi River levee is marked in red at left. The ring levee that nearly wraps Concordia Parish is shown in purple.

VIDALIA — Officials don’t really like to play the “what if” game, but for thousands of area residents who feel like sitting ducks in the path of a natural disaster, it’s natural to wonder.

The worst-case scenario for residents of Concordia Parish and the collective Miss-Lou would certainly be water overtopping the levees or a levee break.

Neither is going to happen — U.S. Corps of Engineers and Fifth District Levee Board officials insist — but even if it did, it’s likely not as bad as what many may fear.

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Before you panic over a levee failure, it’s best to first understand what could cause it.

Only two reasonable options for levee failure exist, said Reynold Minsky, president of the levee board.

First, the height of the Mississippi River could rise higher than the top of the levees.

Second, a break could form in a levee caused by a weakness or the pressure of the water rushing by.

Once you understand the possibilities, it’s pretty easy to prepare for them, Minsky said.

At its current predicted crest — 64 feet on May 22 at Natchez — the Mississippi River will not overtop any levee in the area, he said.

“The Corps has measurements of how much it can withstand,” Minsky said. “It’s a numbers game.”

The Corps will continue to watch the predicted water levels, and if the river is predicted to reach a height that would overtop the levees, an evacuation order would be issued.

Since the National Weather Service and the Corps can look up river to know what is coming down, river level predictions can be made weeks in advance.

The current record-breaking crest was predicted three weeks ahead of time.

So, in the case of an over-topping event, residents would have plenty of time to get out.

The second problem, weaknesses in the levees that could lead to a breaking point, is also easily predicted, Minsky said.

Weak points in levees are formed when water begins seeping underground, taking with it dirt, sand and clay from the base of the levee.

Not all seepage is a worry, but inspectors do watch any patches of seep water.

When water seeps through and brings dirt with it a sand boil is formed.

Sand boils are common and containable, but they are the first cause for concern.

A sand boil left unattended can be the site of a continual washing away of the levee, which would eventually lead to a break or collapse.

“As bad as they are and as bad as they sound, they are manageable,” Minsky said. “We can take the necessary steps to manage those.”

When a sand boil is identified, levee board officials build a sand bag wall around it, equalizing the pressure and stopping the damage.

The boil is continually watched, and as long as the water bubbles up clear, it’s fine. If dirt or sand particles come up, the levee board knows it has a problem.

If the board is unable to stop the displacement of sand and dirt, a levee collapse is a real possibility.

But, from the first moment the board realizes it can’t contain the boil, it would likely be at least seven to eight days before enough damage could occur to breach the levee, Minsky said.

When the levee board knew there was a problem, an evacuation order would be issued.

“We are not going to sit here and think we could control it if we can’t,” he said. “If there were to be any threat of that happening, we would advise the public soon enough that they could evacuate.”

The plan does, of course, rely on the fact that the levee board knows where all the sand boils are on miles and miles of levee.

Minsky’s not worried about that either. With 28 years of personal sand boil-spotting experience, he trusts himself, not to mention the dozens of other eyes that are also on the levees.

Crews working with the levee board man 24-hour patrols from the ground and air.

Minsky spent all day Friday in a helicopter riding over levees in the fifth district.

A sand boil is easy to spot from above, he said.

“You are looking for the color of the (seep) water — a discoloration of water — to know if there is a boil,” he said. “In a clear ditch you can see the boil if you know what you are looking for. I taught the pilot today.”

The Louisiana Department of Transportation, Wildlife and Fisheries and the National Guard are supplying personnel to aid in the search for sand boils.

What happens if a levee does break? Well, no one really knows the answer to that.

The size and location of the break would determine everything, said Kavanaugh Breazeale, U.S. Corps of Engineers public affairs and communications officer for the Vicksburg District.

Water would flood into the parish, but where and how much is unknown.

It won’t be a 40-foot wall, Minsky said, but it could be enough to drown someone.

“The Corps is not excited about this, nor or we excited about it,” Minksy said. “But unless something absolutely unforeseen happens … we are managing what we’ve got to manage.”

Vidalia water wells

However, it won’t take a levee break to cause problems in the City of Vidalia.

The city’s two water wells sit closer to the river than most every house or building, and floodwaters could paralyze the systems.

One water well is on the riverside of the riverfront’s sidewalk, another is on land on the other side. Both may be surrounded by river water soon.

“The worst-case scenario would be if water gets into them and shorts them,” Mayor Hyram Copeland said.

But Copeland doesn’t intend to let that happen.

City workers and inmates began earlier this week stacking Hesco Bastion instant levees around the wells. The wall of sand will be eight-feet tall and sandbags will fill in the top.

The wells provide water to two tanks — one of which holds 250,000 gallons of water, the other holding 500,000 gallons. Both tanks would be emptied in one day with normal usage volumes, Copeland said.

If a short were to occur, the city would be without water indefinitely.

Borrowing water from their closest neighbor — Ferriday — isn’t a great option since the town has a weak water system regardless of disaster. In addition, Ferriday’s water could be affected by flooding.

Water buffaloes — portable water tanks — could be brought in, Copeland said.

But again, the mayor doesn’t think any such action will be necessary.

In addition to the sandbag barriers, the city has sump pumps at the ready to begin pumping out any water that gets inside the brick-encased wells.

At the first sign of trouble, Copeland would order the wells shut off to prevent a short.

The city could be without water for several weeks, until floodwaters receded, but it would be several days less than the alternative if a short occurred.

What about Natchez?

In Natchez, nearly every heavily populated area is completely safe, City Engineer David Gardner said.

The city sits on such a high hill that even a heavy downpour of rain when the river is at it’s highest point won’t overwhelm the drainage system, which does ultimately empty in to the Mississippi River, Gardner said.

The city’s two main drainpipes will be plugged to prevent river water from backing into them. But rainfall from above will still have plenty of places to run downhill, he said.

Only areas that sometimes see standing water after a heavy rain anyway should have any temporary flooding in the case of a major rainfall event in the coming weeks.

But the worst-case scenario for Natchez on top of the hill concerns your toilets, not your storm drains.

The City of Natchez’s Wastewater Treatment plant does sit in one of the area’s lowest points, Gardner said.

That’s by design, of course, because you want all of the city’s sewage to easily drain down to the plant.

But if the plant — which sits just under 2 miles as the crow flies from the Mississippi River and sits much closer to the backwaters of St. Catherine Creek — were to take on water from the flood the system could shutdown.

“The plant elevations are lower than the river,” Gardner said. “It would be absolutely under water. You wouldn’t be able to flush your toilet.”

But multiple layers of preparation are in place to ensure that doesn’t happen, he said.

“What we have done is gone in and analyzed every condition,” Gardner said. “We know at every (river) level what will happen. We have prepared ourselves.”

Crews worked much of last week to sandbag the area, providing a first line of defense if the floodwaters do reach it.

But if the sandbags don’t keep the water out, two pumps stand at the ready to handle the situation.

“We discharge 1,750 gallons per minute. The two six-inch pumps are capable of doing 3,000 gallons per minute,” Gardner said.

The pumps would be more than adequate to clear the plant of water, he said.

The only other problem would occur if the pumps lost power, a problem for which there are also multiple solutions.

First, the pumps and the plant feed electricity from an Entergy substation nearby. If it takes on floodwaters, it will be shutdown, but Entergy has already said it is no problem to simply supply power to the Wastewater Treatment Plant from another substation in the area.

So as long as some portion of Southwest Mississippi has power, the power to the plant will stay on, Gardner said.

But, just in case, the plant has a generator en route from Hinds County that is strong enough to operate the entire plant, pumps and all.

“I feel like we have the sewer taken care of,” Gardner said.