From what I gather, the Industrial canal is overtopping - the water is higher than the solid levee, and is running over the top - like if you fill a batchtub past its top. When you turn the water off, the tub is still full of water.
A breach would be poking a hole in the tub, so it drains to someplace below it's designed top.
Also, the overtopping is on the east side of the Industrial Canal, flooding St. Bernard and 9th ward - not the city proper.
The location of the 17th Street canal breach is "downstream" (now, but would be considered upstream if the canal is used for moving water back into the lake) of a steel sheet piling that isolates that part of the canal from the lake.
See the photo in post 2494 - looks like it's turning into a breach.