I grew Vinca (Periwinkle) on a similar sized bank in my last garden and it spread well and smothered almost everything else after a time. You'd need a few starter plants for a bank this size and I don't know how it would cope with the possible flooding.
I'm sure others will come up with further suggestions, though.

Mimulus grows well at the edge of water but I don't know how fast it spreads. Very pretty though.Yellow iris - the wild ones- would spread and put up with occasional flooding as long as the current isn't strong. Greater spearwort likes its feet in the water. Meadowsweet would be happy. Sorry, getting carried away, these aren't ground cover plants are they, but what a lovely opportunity to make a patchwork of wild flowers.