A French drain collects diffuse seepage with a perforated pipe in a trench 0.5-1 m deep dug across the slope, surrounded by 10-20 mm gravel, wrapped in geotextile or graded gravel so fines cannot clog it, capped with clay, and laid on a steady fall to a sealed chamber. The pipe is never the limit, since a 50 mm pipe on a 0.5% fall carries about 34 L/min, far more than a seep gives; what limits the drain is whether the trench reaches the water-bearing layer and whether the gravel stays unclogged.
Part of Spring Capture
A French drain is a gravel-filled trench holding a perforated pipe, here used in reverse of its usual job: not to drain a field, but to intercept the groundwater of a seep and deliver it to a chamber.
The Trench Must Cut The Flow, And The Fabric Must Stay Open
Groundwater in a seep moves downslope through a thin water-bearing layer. A trench across the slope intercepts it: the water flows into the gravel, along the pipe, and out at the chamber. Two things decide whether it works.
Depth. The trench must reach the base of the water-bearing layer, where the water runs on the less permeable ground. A trench that stops above it intercepts only the upper part. The Spring Capture guide gives 0.5-1 m; test pits tell you where the layer lies, and if it is deeper than 1.2 m the trench needs shoring.
Clogging. Fine soil carried into the gravel fills the voids and stops the drain. The fabric or the graded gravel layers hold it back, following the filter rule in Inlet Wall: each layer’s 15%-passing size no more than five times the 85%-passing size of the layer it protects (Terzaghi’s filter rule, recalled).
The Pipe Has Plenty Of Capacity
For a smooth plastic pipe flowing full under gravity, Manning’s equation gives Q = (1 ÷ n) × A × R^(2/3) × S^(1/2). Take n = 0.013 (a typical value for smooth plastic pipe, recalled), a 50 mm bore (A = π ÷ 4 × 0.05² = 1.96×10⁻³ m², hydraulic radius R = d ÷ 4 = 0.0125 m), and the slope of 0.5% (S = 0.005), the minimum this guide uses for drainage (Ditch Layout):
Q = (1 ÷ 0.013) × 1.96×10⁻³ × 0.0125^(2/3) × 0.005^(1/2) = 76.9 × 1.96×10⁻³ × 0.0539 × 0.0707 = 5.7×10⁻⁴ m³/s = 0.57 L/s = 34 L/min
A seep that yields 10 L/min (the Spring Capture guide’s 20 L in 2 minutes) uses under a third of that. The fall: 0.5% is 5 mm per metre, so over a 20 m run the far end of the pipe is 20 × 0.005 = 0.10 m higher than the chamber end.
Even A Short Drain Moves Several Cubic Metres Of Earth And Gravel
Illustrative figures for a drain 20 m long, 0.3 m wide and 0.8 m deep, with a clay cap 0.2 m thick (assumed sizes):
- Excavation = 20 × 0.3 × 0.8 = 4.8 m³
- Gravel layer 0.6 m deep (pipe and gravel) = 20 × 0.3 × 0.6 = 3.6 m³
- Fabric per metre of trench, lining the bottom and sides and folding over the top = 0.3 + 2 × 0.6 + 0.3 overlap = 1.8 m, so 36 m² for 20 m
- Pipe: 20 m, with the slots or holes cut to suit (Slot Cutting)
What You Need
- Perforated pipe, about 50 mm, enough for the run
- Gravel, 10-20 mm, washed
- Geotextile or graded layers — salvaged fabric, or the graded gravel of the inlet wall
- Clay for the cap
- A level and string line, or a water tube level — to hold the fall (Water Tube Level)
- A sealed chamber (Collection Chambers)
Method: Chamber First, Dig From Below, Lay To Fall
Step 1: Build the chamber at the downhill end first, so the outlet level is fixed.
Step 2: Mark the line across the slope, and set stakes with the fall: 5 mm per metre rising from the chamber.
Step 3: Dig from the chamber end uphill, to the water-bearing layer, so that any water that enters drains away as you go. Shore if deeper than 1.2 m.
Step 4: Line the trench with fabric. Lay gravel 5-10 cm deep, set the pipe on it to the fall, check the fall by level, and surround the pipe with gravel.
Step 5: Fold the fabric over the gravel (or add the finer graded layers), and cap with 0.2 m of rammed clay, mounded slightly.
Step 6: Connect to the chamber, with the joint sealed and the pipe end protected by a screen.
Step 7: Check the yield from the drain’s end, and compare it with the pilot-trench estimate (Seep Springs).
Common Mistakes
- A trench that does not reach the base of the layer. The lower water passes underneath.
- Gravel without fabric. Fines clog it within a season.
- No fall. The pipe silts and water stands.
- A thin or missing clay cap. The drain collects surface water too.
- Placing drains in a pattern that overlaps. Neighbouring drains share the same water.
Safety
Wet trenches collapse
Slope or shore any trench deeper than 1.2 m, keep the spoil at least 0.6 m back, and do not work alone or below an unsupported bank. Do not use a shallow collection system as a drinking source until it has been fenced, capped and tested: a drain close to the surface is easily polluted (Fencing).
What’s Next
- Learn next: Gallery Trenches — deeper and longer collectors in hillsides
- Combine with: Uphill Diversion — keeping surface water out of the drain