Literary analysis: The image of flooding continues. The "flood" of Assyrians will not only overflow the banks, but be "up to the neck." This is no minor flood, but a major flood of people who will cover the land of Judah; "the stretching out of his wings shall fill the breadth of thy land."
The invocation "O Immanuel" is interesting in this verse. Remembering that Immanuel means "God with us," it should be see as a parallel marker to the "God is with us" that closes verse 10.
Additionally, however, the reference as a name is also significant because it returns to the first prophecy of a child in chapter 17. Isaiah will refer without name to this son of the previously written prophecy in verse 18. The presence of the name in this text both invokes the entire previous prophecy as pertaining to the current text (and indeed, both prophecies refer to the same events and consequences) as well as create the literary parallelism with the meaning of the name and the declaration of God's presence in 10 (also Immanuel, though translated to meaning rather than name in most texts).