A French rain or land drain is a ditch filled with gravel, rock that redirects surface and ground water away from an area. French drains are common drainage systems, primarily used to prevent ground and surface water from penetrating or damaging building foundations. Alternatively, the French drain technique may be used to distribute water, such as that which flows from the outlet of a typical septic tank sewage treatment system. French drains are also used behind retaining walls to relieve ground water pressure.