A gully trap is a U-shaped plumbing device which is placed within a plumbing network in the ground and receives piped wastewater from your bathroom and laundry before it is emptied into the sewer main or absorption pit.
Because of its shape, the trap retains a small amount of water to prevent foul odors of the sewer reaching the surface. This water in the trap creates a seal that prevents sewer gases from passing from the drain pipes back into the occupied space of the building.
The gully has a water seal. The top or surround is raised above ground level to prevent storm water and other foreign matter entering the sewer.