I think blue rug juniper will work well too. I have a similar swale where I planted blue rug juniper. It has spread out and completely covered the area. Took almost 5 years to completely fill the area.

Dutch white, or if you want mix in a little color, strawberry remains low and has reddish flowers. Most of the other clovers are forage for livestock and do not stay low.

Thanks. Just what I wanted to know!