A gully trap is a basin in the ground which receives piped wastewater from inside your home before it is emptied into the wastewater network.
The basin has a water seal to prevent foul odours of the sewer reaching the surface, and a vent pipe that allows fresh air in.
The visible grates are raised above ground level to prevent stormwater and other foreign matter entering the wastewater network, but are low enough to prevent overflow of wastewater into the home if there is a blocked pipe downstream.
If a stormwater pipe is incorrectly connected to a gully trap (this is known as a cross-connection), it can cause the wastewater system to overload in heavy rainfall, and a wastewater overflow can occur.
The authority on sustainable building, Level, has some more information about the compliance requirements for drainage on properties, click here for more information.