I think this is what is meant.

Mike :)

I have actually already answered this in your previous question, when I said that I thought the corona was the top width.

The applicant proposes to excavate a channel approximately 5,300 linear feet in length to 15 feet deep with a 30-foot bottom width, and a 75-foot top width

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Note added at 4 hrs 54 mins (2003-11-14 16:11:42 GMT)
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Completed in May 1996, the Channel, through Vermillion Bay, and intersecting the Intracoastal Waterway, has 150\' bottom width and a minimum of 200\' of top width. The Channel is under navigational supervision of the U.S. Coast Guard.
http://www.geoshipyard.com/shipyard/repair_yard.htm

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Note added at 5 hrs 16 mins (2003-11-14 16:33:50 GMT)
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In doing this the new bottom width would increase, depending on justified depth and location, from 12 to 42 feet less than the bottom width of the existing channel, while the top width would remain the same
http://planning.saj.usace.army.mil/envdocs/JaxHbr/FEIS.htm

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Note added at 5 hrs 19 mins (2003-11-14 16:37:00 GMT)
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The channel was dug 36 feet deep by 500 feet across (bottom width) and 600 feet across (top width).
http://www.saveourlake.org/area2.html