I was working on an office building once, it had a down pipe that was blocked at the bottom. The building was about 8floors and the pipe was a large diameter.
I could see that it was clearly blocked by leaves, so I set about pulling them out. The stuff was so compacted that it was hard to get it out by hand so I started using a screw driver. After a bit of digging i finally started to get a bit of water weeping out. Next came a really fine jet of water, "that's funny" thought I. Then it went off, the blockage cleared and eight floors of down pipe erupted much to the delight of the office workers who had been watching.