MB4 224a: Mavoi Open at Both Ends (363:29)

What if a mavoi has only two walls (מבוי מפולש) with one side open to a body of water and the other is open to a public garbage area (higher than 10 tefachim). Do we have to be concerned that silt will accumulate and cause the body of water to rise to within ten tefachim of the mavoi?

If a bridge crosses the body of water at the entrance to the mavoi.

A ditch that surrounds a city (10 tefach deep and 4 tefach wide) is considered walled vis-a-vis carrying. What if there is a bridge over the ditch?