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Flood time on the Mississippi – and it’s a big one this year

The action is just south of Natchez where the Red River approaches the Mississippi. The Red is the river that runs the border between Texas and Oklahoma and then heads across Louisiana. It used to get to the Mississippi but now joins the Atchafalaya. The elevation of the Mississippi here can be just 40 feet or so above mean sea level and the Atchafalaya is about 15 to 20 feet lower. It is the site of the only hydroelectric plant in Louisiana.

Where Louisiana and the Gulf of Mexico meet is a big delta. New Orleans is on the east side and the Atchafalaya swamp is on the west. Back almost a hundred years ago, a flood threatened to switch the Mississippi route to the Gulf from the east side of the delta to the west. That would have left the industrial corridor along the river from New Orleans to Baton Rouge high and dry. This would not do.

The Atchafalaya, continuing to grow, had become, by volume of discharge, the second-largest river in the United States. Compared with the Mississippi, it had a three-to-one advantage in slope. Around 1950, geologists predicted that by 1975 the shift would be unstoppable.

The New Yorker has the story about The control of nature, Atchafalaya by John McPhee written in 1987. In 1973 the control structures had a big test. It looks like 2011 will give them another.

Viewed from five or six thousand feet in the air, the structures at Old River inspire less confidence than they do up close. They seem temporary, fragile, vastly outmatched by the natural world—a lesion in the side of the Mississippi butterflied with surgical tape. Under construction nearby is a large hydropower plant that will take advantage of the head between the two rivers and light the city of Vidalia. The channel cut to serve it raises to three the number of artificial outlets opened locally in the side of the Mississippi River, making Old River a complex of canals and artificial islands, and giving it the appearance of a marina.

NOLA has the current story: Morganza Floodway opens to divert Mississippi River away from Baton Rouge, New Orleans

The second-ever opening of the nearly 60-year-old structure 186 miles upriver of New Orleans began at 3 p.m. sharp, when a crane lifted a gate covering one of the spillway structure’s 125 bays, releasing a gusher of about 10,000 cubic feet of water per second into the floodway

The decision was triggered by measuring a flow rate of 1.5 million cubic feet of water per second at Red River Landing. That’s 93,645,000 pounds or almost 47,000 tons of water per second. If the water was 50 feet deep, that’d be a pool 173×173 feet going by every second. At 60lb/square foot, a typical 1800 sq foot house weighs 54 tons. That means a water weight equal to that of a typical house goes by in about a millisecond.

see also Climatology, hydrology, hydraulics and water quality

John Weeks has pictures with his explanation and description.

After Katrina just a few years ago, it looks like the Cajuns are going to get wet again. My prayers are with them. Sean at Our Odyssey indicates the Red Cross has mobilized and stories are starting to appear about the availability of those infamous FEMA travel travels for temporary housing again.

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