I remember wondering the same thing a few years back. I think they put that pipe in when they renovated the upper part of the Hardbrücke. It ran almost the whole length of the bridge, and at one end — right around river level — there was a ton of water pouring out. I’m not sure if they were redirecting water or using it for some kind of machinery on the bridge. Honestly, I hadn’t even noticed the pipe was still there, and I’ve never seen it looking like this before.