Synthetic rocks are used as a substitute for natural gravel to fill in a water drainage trench. The synthetic rocks may be used around perforated drain tiles in a basement water drainage system, or they may be used without such tile. The synthetic rocks may be of a size and shape that mimics the natural rock they replace, but they are preferably significantly lighter in weight. The synthetic rocks may alternatively be tubular or cubic shaped, for example, and may include one or more lumens or other passageways to facilitate the flow of water through and/or around the rocks. The synthetic rocks may be provided in mesh bags to facilitate placement in a water drainage trench.