Seepage is the third basic category of drainage. Most seepage systems use perforated pipe packed in gravel to accept water from the surrounding soil. Although many new and effective products are available for this type of drainage, the time-tested French drain is still the most popular. A French drain uses a trench deep and wide enough to completely surround a perforated pipe with clean 3/8- to 3/4-inch gravel. If you cannot locate perforated pipe, you can make your own by drilling 0.375- to 0.5-inch holes at 2-inch intervals in four straight rows along one side of solid drainage pipe.
Line the bottom of the trench with a bed of gravel a minimum of 2 inches deep. Place the pipe in the trench with its perforated side down (if they do not go all the way around the pipe) and place a minimum of 4 inches of gravel cover over the pipe. Also surround the sides of the pipe with at least 1-inch of gravel so that the pipe is completely embedded.
Some installations are "open" with gravel all the way to the top of the trench. If the path of flowing surface water runs across an open French drain, it will intercept it and channel it away. If an open-gravel trench at the surface is inappropriate, you can cover it, but with no more than 4 inches of topsoil. Use a silt fence or other geotextile to prevent soil from contaminating the gravel.
For low-lying, extremely flat or large areas, you may have no alternative but to use extensive seepage-drainage networks. A common technique is to install the drainage in a herringbone pattern, which has proven over many years to be an effective design for channeling subsurface water. The mainline of the herringbone should be slightly lower (deeper) than the branches that angle out from it. If a recognizable low spot exists within the drainage area, the mainline should run through it.
The herringbone pattern is also the traditional method for sub-grade drainage in putting greens. Here, following the initial pipe installation, a 4-inch surface blanket of gravel covers the entire drainage area (the whole green), with a 12-inch layer of root-zone mix on top of that.
Some alternative seepage-drainage methods are gaining recognition because they create far less ground disturbance than traditional French drains. Professional sports fields often have a complex subsurface seepage network to keep playing conditions at peak levels even in heavy rain. The basic principles employed in such high-profile jobs are the same as those described here, however. If you have a good working knowledge of the basics of drainage, you can become proficient with some of the more complex drainage technology available.
Gary S. Kaye is a certified irrigation designer and founder of Golf Engineering Asscociates Inc. (Phoenix).