Share:
How It Works
The French drain equation Q = K × A × S is derived from Darcy's Law. It calculates how much water a gravel-filled trench can carry based on the soil permeability (K), trench cross-sectional area (A), and hydraulic slope (S). A typical residential drain uses a 6–12 inch wide trench at a minimum 1% slope.
Example Problem
A French drain has gravel fill with K = 50 m/day, a trench cross-section of 0.15 m², and a 2% slope (S = 0.02). What is the seepage rate?
- Q = K × A × S = 50 × 0.15 × 0.02
- Q = 0.15 m³/day per meter of drain length
Frequently Asked Questions
How deep should a French drain be?
Most residential French drains are 18–24 inches deep and 6–12 inches wide. Deeper drains intercept more groundwater but cost more to install. The trench should extend below the problem water level.
What slope does a French drain need?
A minimum slope of 1% (1 inch per 8 feet) is recommended. Steeper slopes improve drainage capacity. On flat terrain, you may need a sump pump at the outlet.
What gravel should I use for a French drain?
Use washed, angular gravel (3/4–1.5 inch). Avoid rounded river rock, which compacts and reduces permeability. Wrap the trench in landscape fabric to prevent soil migration.
Related Calculators
- Darcy's Law Calculator — the general groundwater flow equation behind this calculator.
- Manning Equation Calculator — calculate open-channel flow velocity for drain outlets.
- Gutter Design Calculator — size roadway gutters that feed into drainage systems.
- Hazen-Williams Calculator — estimate pipe flow velocity for drain outlet pipes.
- Hydraulic Radius Calculator — compute the hydraulic radius for trench cross-sections.
- Length Unit Converter — convert between feet, meters, inches, and other length units.