A water trap, often known as a U-bend in domestic situations is a u-shaped dip in a plumbing pipe which retains a small amount of water. Specifically, there is enough water retained so that the top wall of the pipe is submerged, and there is no air-connection between the two parts of the pipe.
The purpose of the water trap is to prevent anything nasty returning back up the pipe to nuisance you. Such things can include vermin and odours.
The kink in the pipe caused by a water trap can encourage blockages to occur. On an indoor system, the trap will usually be a plastic pipe which may be unscrewed and unblocked by hand. A trap in a sewarage system must be unblocked with drain rods.
Since the drain rods will not travel around the complex corner formed by the trap, a rodding eye is provided at ground level above the trap. A sewerage water trap includes a pipe leading down diagonally from the ground to the parts of the sewer beyond the trap. This pipe has an oval airtight lid to prevent vermin and odours.