Wednesday, June 25, 2014

The Force of Outdraft

Outdraft is a current that moves across a lock or channel entrance towards a dam's spillway. The higher the flow rate through the dam, the stronger the outdraft current.

My first exposure to  outdraft was the Marseilles Dam accident. Four tow boats were working together to try to get a 14-barge tow out of a flood current and into the safety of a canal as illustrated below.
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The Dale A. Heller towboat was 128 ft, 596 gross tons, and 5,600 hp. The Loyd Murphy was from a 15-barge tow that had already been tied up to a mooring to ride out the flood waters. The Creve Coeur and City of Ottawa towboats were from the Coast Guard.

The Marseilles Dams' gates were almost all of the way open to avoid flooding Marseilles. The four towboats were not successful in fighting the resulting outdraft. And Marseilles ending up getting flooded anyhow.

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And even after the flood subsided, shipping had to evacuate the pools above and below the dam while a rock cofferdam was being built to allow repairs of the damaged trunnions.

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Note that half of the 8 gates could not be used to pass river flow while the repairs were being made.

The second demonstration of the force of an outdraft was when I visited Lock and Dam #18 on the Mississippi River last Fall. Unfortunately, it was BC (before camera), so I don't have any pictures. So I'll use a satellite picture. Below is the upstream guide wall and upstream gate.



The red rectangle represents the leading part of a 15-barge tow. The blue oval highlights a towboat that is normally parked near the upstream gate of the lock. The blue rectangle illustrates where the "blue" tow boat positions itself while the tow approaches the lock entrance. As soon as the front of the tow is past the end of the guide wall, the auxiliary towboat pushes the tow against the wall so that it is not pulled away from the lock entrance by the outdraft. I left when the tow was close to the lock because I had to leave for a meeting. I assume that as soon as the front of the tow entered the lock, it pulled away and went back to its parking spot.


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