This is a hypothetical question, so that I'm prepared if it happens again.
Sometime ago someone dropped a ball that was almost the same size of the storm drain pipe (10cm/4in) in it. By luck it rolled out without a problem. But if the pipe was dirt or had sand/soil in it the ball would probably be stuck.
How can I solve prevent that?
The pipe section that troubles me is about 20 meters long and has 2 short 45° turns and 2 long 90° turns (the first 90° is vertical, the rest is horizontal).