Stop 2: French Drain

A French drain is a subsurface drainage system that intercepts and redirects groundwater and surface water away from problem areas. Using a perforated pipe surrounded by gravel in a lined trench, it collects water and channels it to a designated discharge point—preventing pooling, erosion, and foundation damage.

Practice Type: Subsurface water interception and conveyance

Many residential and commercial landscapes have areas where water collects after rain events—soggy patches in the lawn, puddles near walkways, or persistent dampness near foundations. Surface grading alone cannot always solve these problems, especially where subsurface water movement contributes to the issue.

French drains work below the surface to intercept water before it causes visible problems. By creating a preferential flow path using gravel and perforated pipe, the system draws water away from saturated areas and redirects it to an appropriate discharge location. At the demonstration site, the French drain collects subsurface outflow from the pervious paver area and channels it toward the bioswale.

French drains are one of the most versatile residential stormwater solutions. Common applications include directing downspout runoff away from foundations, draining soggy lawn areas, intercepting slope runoff before it reaches patios or walkways, and connecting multiple stormwater practices. Most homeowners with basic excavation skills can install a French drain as a weekend project.

French drains are addressed in the Georgia Stormwater Management Manual as a conveyance practice that can be integrated with other stormwater controls. For compliance applications, consult the manual for sizing requirements and discharge specifications.

A French drain should drop at least 1 percent in depth for every 100 feet of length (or 1/8 inch per foot) to deliver collected stormwater to another control measure or drainage area in the lawn. The maximum slope recommended for a French drain installation site is 6%, and if it is any steeper, the flow from the drain pipe could cause erosion upon exit or back up at the exit.

A small French drain with a 4-inch pipe, a 1% slope, and a 12 inch × 12 inch gravel trench can handle approximately 5 GPM or 300 gallons per hour. For reference, an 800 square foot roof surface will produce about 500 gallons of rainwater under a 1-inch rainfall. Depending on the rainfall rate, a larger pipe and trench size might be necessary.