THE SPILLWAY

[ratboy]

High river in New Orleans could prompt May opening of Bonnet Carre Spillway
Published: Monday, April 25, 2011, 1:03 PM Updated: Monday, April 25, 2011, 1:06 PM
Mark Schleifstein, The Times-Picayune By Mark Schleifstein, The Times-Picayune NOLA.com
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The Mississippi River at New Orleans will crest at 17.5 feet on May 17, the highest level in more than a dozen years, and a level that could require the opening of the Bonnet Carre Spillway to reduce the rate of water flowing past the city's levees.
Bonnet Carre TestBrett Duke, The Times-PicayuneA crew of Army Corps of Engineers employees work on top the Bonnet Carre Spillway structure in Norco practicing removing and replacing wooden needles in one of the 350 bays earlier this month.

The spillway, 28 miles above New Orleans, was last opened April 11-29, 2008, with a maximum 160 individual bays having wooden pins removed to allow water to flow into the spillway and then into Lake Pontchartrain. The river that year crested on April 26 at 16.96 feet.

The record water level at the Carrollton Gage was 21.27 feet on April 25, 1922. The river reached 21 feet on the same day during the historic 1927 flood.

The unusually high river has been triggered by heavy rains in the Midwest, including more than 11 inches in Cincinnati and almost 7 inches in tornado-hit St. Louis, during the past three weeks, according to hydrologists with the National Weather Service's Lower Mississippi River Forecast Office.

"A small part of this is snow melt, but the primary driver of this is the ongoing heavy rain stretching from St. Louis to the west and in the Ohio Valley, where they've had seven to 10 inches of rain over the last several days," said David Reed, hydrologist in charge of the river forecast center in Slidell.

Computer models indicate the cold front moving over the Midwest and Ohio Valley could drop another 10 to 15 inches over the next three days, although official forecasts at a number of locations indicate only 2 to 4 inches of rain are expected.

In New Orleans, the official flood stage at the Carrollton Gage is 17 feet, but the city is protected from water rising to at least 20 feet by a combination of levees and floodwalls.

A high river also is likely to require restrictions by the Coast Guard on shipping moving through New Orleans because of the increased speed of river water and its potential to cause or exacerbate steering problems.

Officials with the Army Corps of Engineers, who operate the spillway and regulate water flow in the river, are watching the rising water levels carefully, said Mike Stack, chief of emergency management for the corps' New Orleans District office. Water begins leaking through spaces between the wooden pins in the 350 bays of the concrete spillway when the river reaches about 12 1/5 feet.

Corps officials already have begun inspecting levees along the river, in part because of an earlier high river event about a month ago, he said. The first stage inspections are conducted twice a week by corps officials looking for seepage, sand boils, bank erosion or other threats to the levees, such as ships or debris in the water. Similar inspections also are being conducted by local levee district employees.

When the river reaches 15 feet, the inspections will be conducted once a day.

The decision to open the spillway is based on a combination of concerns, including whether the amount of water flowing south of the spillway is expected to climb above 1.25 million cubic feet per second and how long the upper portions of the levees have been saturated with water.

It takes about 10 days to prepare for an opening, he said. The long, wooden pins that keep the bays closed to water are removed by a crane that moves on a track atop the spillway structure.

"We'll monitor it for the next week and see where we are," Stack said. "If we see that we are likely to reach 1.25 million cubic feet per second, we'll start the process of notifying stakeholders, who will give us their input and then send their recommendations to the Mississippi River Commission."

The commission, chaired by the commander of the corps office overseeing the entire Mississippi River, is based in Vicksburg.

While the southern end of one cold front storm system now dumping rainfall on the Midwest is expected to pass through New Orleans on Wednesday, local rainfall generally does not increase the height of the river, as most of it is drained into area lakes and wetlands, and not over the levees and into the river.

Breaks in levees upstream from Louisiana caused by what's expected to be record high water levels also could result in lowered forecasts during the next three weeks for the river in New Orleans.
Related topics: bonnet carre spillway, corps of engineers, mississippi river

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veefox
veefox April 25, 2011 at 1:25PM
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I am curious and maybe someone can answer this. What is with the Morganza Spillway above Baton Rouge? I don't ever remember that spillway being opened in my lifetime. What triggers that spillway to open? If the river is expected to crest above 17' in NOLA, why not open it for relief of some of the water before it reaches LaPlace?
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[Tanstaafl]

The good news is that the latest forecast (~6:30 PM) has the forecast down to a 17' crest moved out to 5/22. Some places farther upriver (Greenville, Natchez, etc) are predicted to get record crests. Not sure how that's going to translate into high water down here. Hopefully we get lucky and the spillway stays closed.

[TimT]

Ether way its probably wise to secure the bridges that need it. Any one got any re-bar? Bend one end and drive them in at angles. one per corner. Cut any logs (up and overs) flush with the up and overs. I got a torch and a sledge hammer. Trail will be out of commission for at least 5 weeks if it gets flooded.

Tim

[Monkey Boy]

Man the Atchafalaya was way up and mighty angry looking when I went over it on the way home yesterday. Looks like they're already diverting a lot of water out of the Mississippi. Here they're bitchin' and moaning about the droughts. Not even May and they're starting water rationing SW of us.

[ratboy]

tim and i are going out to the trail sunday to stake down some bridges down before the possible opening of the spillway. any and all help will be a big help. thank you.

[TimT]

Yes we will need a few people with sledge hammers. I'll be making the staples this week. Also bring chain saws.

Tim

[Tanstaafl]

According to Nola.com the corps may be opening the spillway sometime between 5/7 and 5/13. Looks like this will be the last weekend for riding.

When and where do you need help on Sunday?

[wddamf]

For anybody who hasn't lived through an opening of the spillway, and who might think that this means the end of the trail as we know it -- fear not! The trail will indeed be completely useless for about a month after the waters recede, but the flood will actually deposit new sand on the trail and it will kill off the weeds. Believe it or not, the trail will come out better!

The real drawback is that anything wooden will indeed float away. If you can't pin it down at least tie it off so you can find it later.

[vtrider]

I am worried that if all Spillway bays are opened and the water current in the spillway is greater than last time, can trail survive that ,and even with the bridges tied down that we could see substantial damage to trail through erosion and damage to berm bridge because of stronger water currents?

Steve

[ratboy]

I to am worried about this. The last time it was opened, only half the bays were opened. This time they are opening all the bays. That is what scares me. The flow rate through there will be much greater than before.

[wddamf]

Nah, it'll be ok. Ya'll will lose some wood - it's amazing how much buoyant force those pilings have - but the trail will be fine.

[Tanstaafl]

I think all the bridge and such should be okay. I think with the trail being off to the side of the spillway I think the speed should be relatively low. Erosion along the canal bank could be a major issue.

Also, according to the news reports the corps will be opening the spillway at 8 AM monday. They are also going to need to open the Morganza spillway which has only been done once since it was built. If they don't open the Morganza spillway the river down here is expected to get a 19.5 elevation and the levees only protect the city of New Orleans to an elevation of 20'.

[That's us]

So does this mean that everyone should ride their asses off this weekend? I'm pretty sure the trail is gonna survive, but will take a beating also. And will be outta commission for a couple of months. I think Annie and I will try to fit a ride out there this weekend.

[LSU_engr]

My son and I will be out there this afternoon around 4:30 or 5. He's only 5, but we should be able to get two laps in pretty easily, maybe 3 if I push him.