on purpose:
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Q: What is the strategy for getting the water out?
As the water level in the lake drops, some of the water in the city will flow back out naturally. In some parts of the city, the water is now above the level of water in the lake. The Corps plans to do several "controlled breaches" (intentionally breaking part of a levee) to let the water drain out. At its peak, the lake was perhaps 20 feet or more above sea level. On Tuesday, the water in the lake was measured to be down to 4.5 feet above sea level. By Wednesday afternoon, it had dropped to 2.8 feet above sea level. The remainder of the water will have to be removed by pumps.
http://www.npr.org/templates/story/story.php?storyId=4826934