A French drain is one of the most effective and durable solutions for redirecting unwanted groundwater and surface runoff. Named after Henry French, who promoted the design in his 1859 farm drainage guide, the modern French drain combines a perforated pipe, gravel aggregate, and filter fabric to create an efficient, long-lasting drainage system.
When to Use a French Drain
French drains solve several common drainage problems: water pooling in yards after rain, water seeping through basement walls or foundation, soggy spots in lawns that never dry out, and surface water flowing toward structures. They work by intercepting subsurface water flow and redirecting it before it causes damage. Unlike surface channels (swales), French drains handle both surface water and groundwater rising from below.
Sizing with the Rational Method
The Rational Method (Q = C × i × A) is the standard approach for small residential drainage systems. The runoff coefficient C represents how much of the rainfall becomes runoff — paved surfaces (driveways, roofs) have C = 0.9 because nearly all rain runs off, while lawns have C = 0.2–0.35 as grass and soil absorb much of the water. Rainfall intensity i is typically the design storm for your area — 1 inch per hour is a common standard, while 2 inches per hour is used for high-value applications.
The drainage area A must be measured in acres (1 acre = 43,560 sq ft). A 2,500 sq ft yard is 0.057 acres. The resulting Q is in cubic feet per second, which you convert to gallons per minute (multiply by 449) to compare with pipe capacity.
Manning's Equation and Pipe Capacity
Manning's equation gives you the pipe's full-flow capacity: Q = (1/n) × A × R^(2/3) × S^(1/2). The roughness coefficient n is 0.013 for smooth PVC and 0.015 for corrugated HDPE. A is the pipe cross-section area, R is the hydraulic radius (diameter/4 for a full circle), and S is slope as a decimal. At 1% slope, a 4-inch PVC pipe carries about 20 GPM, while a 6-inch PVC handles about 66 GPM.
Always design for full pipe capacity exceeding peak runoff by at least 25%. This safety factor accounts for partial clogging, rainfall intensity variations, and combined surface and groundwater loads.
Installation Best Practices
Slope is critical: minimum 0.5%, recommended 1% (1 inch per foot). Below 0.5%, sediment accumulates and the drain clogs. Above 10%, erosion can dislodge the outlet and the drain may be self-defeating by running too fast to absorb groundwater. Use a laser level or water level for accurate grade control — a 50-foot drain at 1% must drop exactly 6 inches from inlet to outlet.
Excavation should leave clean, stable walls. Lay filter fabric before gravel, making sure it wraps up the sides and over the top after gravel installation. Place the perforated pipe with holes facing down in the bottom third of the gravel, which creates a reservoir that feeds water up to the holes evenly rather than channeling surface water directly.