What type of drainage system is described as not draining by gravity?

The building subdrain is considered a type of drainage system that does not rely on gravity for its function. Instead, subdrains are typically used to manage groundwater and excess stormwater by facilitating the collection and removal of water from areas that may experience saturation or flooding. This system often employs pumps or other mechanical means to move water away, particularly in situations where gravity flow is not sufficient to prevent water accumulation.

In contrast, a building sewer usually drains wastewater from a building to a municipal sewer system or a septic system using gravity flow principles. A cesspool is designed to hold wastewater temporarily until it infiltrates into the surrounding soil, again primarily functioning through gravity. A combination fixture refers to a plumbing fixture that connects multiple functions, such as a sink and dishwasher, and typically does not represent a standalone drainage system like the subdrain does.

Thus, the characteristic of mechanical assistance in the building subdrain makes it distinct from other types of systems mentioned, which generally rely on gravity for drainage.