A “French drain” is a perforated pipe surrounded with gravel. It is designed to catch water from a large yard area and channel it away from the house foundation. This is most effective with a layer of exposed decorative gravel covering the top of the trench. Covering the French drain with sod or topsoil defeats the purpose and slows the rate of water infiltration. Sand can be used to cover the trench if the site is level and no risk of washing the sand away.