Which type of drainage system is filled with coarse aggregate and may include a perforated pipe?

The French drain is a type of drainage system that consists of a trench filled with coarse aggregate, commonly gravel, which allows water to flow through while filtering out debris. The inclusion of a perforated pipe at the bottom of the trench is a key characteristic that aids in the efficient capture and removal of excess groundwater. This design facilitates the directed flow of water away from areas prone to flooding, ensuring proper drainage and helping to prevent water accumulation that can lead to erosion or damage to structures.

In contrast, gabions consist of wire mesh cages filled with stones, primarily used for erosion control or bank stabilization, which does not align with the drainage function being asked about. A filter strip is a vegetated area designed to treat runoff water, working mainly through biological and physical processes, rather than providing a designed drainage channel like the French drain. Grade finish refers to the final surface contour of a landscape or construction site and does not relate to a specific drainage system. Understanding the distinct purposes and designs of these systems highlights why the French drain is the correct answer.