I think the turf will grow tall and shade out the wintercreeper. The best answer is to research lasgna bed on this site. The best thing is to get rid of the grass first.

"I think the turf will grow tall and shade out the wintercreeper."

That can be controlled. This is not a large area. I'm more worried about the root system. The reason I don't want to smother the grass is because we've had some bad floods in the valley here, enough to wash out any ground that isn't tied down. A lasagna bed will be cost more than it's worth unless I find a miraculous source of freebies (and I've looked), and probably is just as likely to get washed away on me.

If you Round-up the grass, the roots will hold the soil for quite some time, yet not take up water and nutrients from the new groundcover.Just a thought.