It's hard to say. Typically, the two do not like the same sort of soil conditions. Indeed, American Beach Grass doesn't like soil at all, doing best in pure sand. Sea Oats, on the other hand, prefers moist and fertile soil.

If you have effectively established Beach Grass on this embankment, I think it's unlikely Sea oats will be vigorous enough there to displace it.

I looked back at your other thread on this embankment. It sounds like you have patches of clay and patches of sand/gravel. The ABG won't grow in the clay. Sea Oats should work there, and it won't self-seed or spread into the sand/gravel.

Wow, thorough research, donn_. Thanks!!!!

The bottom (Near the Wall) is sand with a mix of grass including American Beach Grass. Then you have bare sand and gravel with a scattering of Rugosa Rose, Crab Apple, and Eastern Red Ceder. Then you have a dense mix of Bayberry, Japanese Honeysuckle, poison ivy, and Eastern Red Cedar with a scattering of bare clay patches near the top.

Useful to know that I should plant the Sea Oats in the clay spots. The possible catches are they hard to get to (because it is steep and because of the poison ivy) and the risk of reseeding into the lawn...

My original concern was that I would plant Northern Sea Oats, it would outcompete and shade out the American Beach Grass, then the first time a big hurricane scattered salt spray all over the place the Northern Sea Oats would die off.

My Sea Oats were under 31" of saltwater during Sandy, and were unharmed.

Donn,
Are your northern sea oats different from my northern sea oats (chasmanthium latifolium)? Up here it is, at best, rather a whimpy grass and certainly nothing that would outcompete another grass. I find it grows in any kind of soil and even in part shade.
curious .....
A.

Same critter, A, but my climate is more suitable for it than yours. Mine get well over 4' tall and are extremely vigorous.