A French Drain has been described simply as a ditch full of rocks. Many engineers call a French drain “lowering the water table.”
French drains are intended most always for subsurface water problems only, whereas as the water table rises, it will fall into this “ditch full of rocks”, and then into a perforated pipe at the bottom of the rocks that will discharge the collected water downhill or to a pump.
French drains typically run horizontally around the perimeter of the affected area. Any structure underground, as in a basement, garage, or room, must be accompanied by a perimeter French drain, and sometimes other means, to prevent certain and sure water problems.
Note: even though many call all drainage corrections a French drain, really subsurface moisture problems are the only place for a French drain. A surface drain system is usually employed, with proper grading to correct most ‘surface’ drainage problems.