In drainage systems, the term "gravity flow" refers to:

Gravity flow in drainage systems is defined by the principle that water or fluids move from a higher elevation to a lower elevation solely due to the force of gravity. This is a fundamental concept in civil engineering and fluid mechanics, as it allows for efficient drainage without the need for mechanical assistance. By utilizing the natural slope of the terrain, systems can effectively transport water away from areas prone to flooding, such as basements or low-lying regions.

Relying on gravitational force is cost-effective and environmentally friendly, as it avoids the energy consumption associated with mechanical pumps. This method is commonly found in various applications, including sewer systems, stormwater management, and irrigation designs.

The other choices present methods and techniques that do not pertain to the natural movement of fluids under the influence of gravity. Instead, they refer to mechanical applications or manual processes, which are not characteristic of true gravity flow systems.