What is a drain gully?

This Catalyst video shows how a gully drain, or gully trap, works.

A drain gully is a ground-level drainage fixture that collects surface water or wastewater and channels it into the underground drainage system. Its primary functions are to prevent solid debris, such as leaves, from entering belowground drains and sewers.

A u-bend built into gullies also maintains a water seal that blocks foul sewer gases, prevent pests from entering a property, or collects silt to prevent blockages further along the pipe. Surface water gullies and foul water gullies are usually kept separate.