Landscapers: How To Fix Washed Out Boulder Wall?

I have a client I am working with who has a 6 ft tall boulder wall in their backyard. They have seen major washout from the backfill. Now they want to add a gully above the wall to move water away from the rock.

What would you do? The wall is about 40 ft long and 6 ft tall with stones on the base about 5 ft in diameter going up to 3 ft on the top row.