A curtain drain is an underground trench that slopes downward. It usually begins around two feet below ground level and is one and a half feet wide. Gravel fills the hole to give the water somewhere to go. A French drain sits at ground-level. You can see the gravel even with the ground around it. These are frequently installed in yards or on lots that already have some slope to them. French drains can take advantage of the existing terrain and move water accordingly.