Planted attic cover

Hello. We have planned a terrace on our garage. This is surrounded by a 1m high parapet as a boundary. Half of the terrace should be covered with patio slabs and the other half with gravel and greenery. As a cover for the parapet, I would like a kind of U-shaped troughs which I could then plant, for example, with boxwood shrubs. However, I have to drain the rain or watering water here. Do you have any idea how I can drain the water? Possibly with a drainage? And 2 - 3 outlets. Thank you very much for your ideas.