What type of secondary drainage utilizes gravity to remove fluid?

Gravity flow is the correct answer because it is a method that relies on the force of gravity to move fluids from higher elevations to lower ones. This natural process occurs without the need for external energy sources or mechanical assistance, relying purely on the gravitational pull acting on the fluid.

In contrast, the other options involve external forces or mechanisms: forced flow incorporates pressure differences generated by pumps or compressors to move fluid; vacuum flow employs suction to draw fluid through a system; and pumped flow explicitly uses mechanical pumps to transport fluids. Each of these methods contrasts with gravity flow's inherent, passive mechanics, which is fundamental in various engineering applications where efficient drainage is necessary.