French Drain – Green Stormwater Demo Site

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.

The Problem:

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.

The Solution:

Location Considerations

French drains are most effective in areas with identifiable water collection problems. Common applications include the edges of parking areas, along the base of slopes, at the perimeter of buildings, and in low spots in the lawn where water tends to pool. At the demonstration site, the French drain is positioned between the pervious paver area and the lawn, intercepting subsurface flow from the pavers.

Slope and Grading

The trench and pipe must slope consistently toward the discharge point to ensure gravity-driven flow. A minimum slope of 1% (approximately 1/8 inch per foot) is recommended. The discharge point should be at a lower elevation than the inlet area and directed toward a pervious surface or another stormwater practice.

See the in Action

Water enters the trench through the surrounding soil and the gravel fill at the surface. It flows through the gravel into the perforated pipe, which carries it by gravity to the discharge point. The geotextile fabric lining prevents soil from migrating into the gravel and clogging the system, while the stone provides structural support and additional water storage capacity.

Practice Type: Subsurface water interception and conveyance

Trench Dimensions: 18 in. deep x 18 in. wide

Pipe: 4-inch perforated pipe (perforated on underside only)

Stone: No. 2 stone encasing pipe; No. 4 river gravel top layer

Best For: Areas with persistent pooling, soggy lawns, or surface flow near structures

Maintenance

Task

Frequency

Details

Surface inspection

After heavy rain events

Check for ponding or slow drainage that may indicate clogging

Gravel surface

Annually

Top off river gravel if it has settled or been displaced; maintain even surface

Inlet and outlet check

Twice yearly

Ensure both ends are clear of sediment, roots, and debris

Vegetation management

Ongoing

Prevent roots from growing into the trench area; keep turf trimmed along edges

System flush

As needed

If drainage performance declines, flush pipe with water from a garden hose at the upstream end

The Basics

Try It At Home

For Professionals