French Drain Not Flowing: Groundwater vs. Surface Water Diagnosis

A homeowner dug a 60×12 ft trench with good pitch to connect to an existing French drain, but water won't flow the desired direction. High water table and possible springs complicate things.

Why Pitch Alone Fails

If the trench bottom is below the water table, water enters from all sides—no gradient drives flow. A French drain only works when its outlet is above the seasonal high water table.

Diagnosis Steps

1. Identify the outlet elevation of the existing French drain. Where does it daylight?
2. Check if the outlet is submerged or discharging into saturated soil.
3. Test: Dig a test hole at the low end. If water rises to the same level as the problem area, you're in the water table.

Solutions for High Groundwater

• Herringbone/grid layout: Multiple parallel laterals (10–20 ft spacing) feeding a main, not a single trench.
• Sump pump: If no gravity outlet exists, pump from a collection basin.
• Intercept higher: Catch water upslope before it reaches the saturated zone.
• Verify existing drain: It may be clogged, crushed, or undersized for added flow.

Pitch Specs

Minimum ¼ inch per foot (2%). On bare soil, friction is higher than in pipe—increase to ⅜–½ inch/foot if possible. Use a string line between fixed high/low points to maintain grade.

Single trenches rarely solve groundwater. You need a system, not a pipe.

Expert: Mark Harshman | www.prestoexperts.com/expert/markland