Is your pattern making scallops across the bottom of the whole blanket? If it is doing that I'm not sure how it will work when you try to leave a blank area. But if the zigzags are more like you drew (Is there any place I can look at the pattern on line? Or if you give me the exact name of the pattern and maybe who designed it maybe I can find a picture of it somewhere.)
Here is link to the colored wave pattern I found the other day. It is part of the Vogue knitting site. LINK
You could use any part of the chart that you like, you don't have to use all of it. The top part that is in olive and teal was the part I was remembering. (If that didn't take you directly to the chart but to the stitch library, it is in the color section and called "Catch a Wave".
The other idea I had was of just doing the horizontal wave part in a raised stitch but not using another color. For instance the teal part of the chart, do all in reverse stockinette and the olive in stockinette, or something like that.