A French drain, sometimes called a dry well, allows surface water to drain into the ground to alleviate water problems. The drain is rather simple in design and construction and consists of a trench filled with rock or gravel. The size of the rock or gravel does play a factor in the efficiency of the French drain, although there is no set size requirement. In many cases construction of the drain is done with the best available materials.
French drain design
A French drain is a trench in the ground filled with rock or gravel to the ground surface. The voids or open spaces between the rocks or gravel allow water to flow down from the surface. The bottom of the trench can include a perforated PVC pipe that accumulates the water. This pipe is then sloped to carry the water to a natural drainage such as a pond or stream.
Gravel or rock
Gravel is made up of crushed rock, which may include sand. In some cases the stones are 2.5 to 5 cm (1 to 2 inches) in size. Rock is available in various sizes but in some cases is available in baseball to softball size.
Big rock advantages and disadvantages
The larger the rocks are, the larger the voids between them. This allows water to flow through the rocks quicker and drain away from the surface. The larger voids also increase the total void volume in the French drain. This allows more water to be stored in the drain if the drainage into the ground is slow. Generally, larger the rocks make for a more efficient French drain. Also, large rocks can give the surface of the drain a rock garden look.
Coarse sand is also used in French drains. This can pose a problem if the drain includes a perforated pipe as part of the system. To prevent sand from entering the pipe, wrap it in a pipe filter that allows the flow of water but restricts sand or dirt from entering the pipe.